IODINE RETENTION ON STAINLESS STEEL

SAMPLING LINES

Tutun Nugraha

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Graduate Department of C hemical Engineering and Applied C hemistry University o f Toronto

O Copyright by Tutun Nugraha 1997

National Library Bibliothèque nationale du Canada

Acquisitions and Acquisitions et Bibliographic Services services bibliographiques

395 Wellington Street 395. rue Wellington Ottawa ON K1A ON4 OttawaON K1A ON4 Canada Canada

The author has granted a non- exclusive licence allowing the National Library of Canada to reproduce, loan, distribute or sell copies of this thesis in microfom, paper or electronic formats.

The author retains ownership of the copyright in this thesis. Neither the thesis nor substantial extracts fiom it may be printed or otherwise reproduced without the author's permission.

L'auteur a accordé une licence non exclusive permettant à la Bibliothèque nationale du Canada de reproduire, prêter, distribuer ou vendre des copies de cette thèse sous la forme de rnicrofiche/nlm, de reproduction sur papier ou sur format électronique.

L'auteur conserve la propriété du droit d'auteur qui protège cette thèse. Ni la thèse ni des extraits substantiels de celle-ci ne doivent être imprimés ou autrement reproduits sans son autorisation.

IODINE RETENTION ON STAINLESS STEEL SAMPLING LINES

Tutun Nugraha

1997 M.A.Sc.

Department of Chemical Engineering and Applied Chemistn

University of Toronto

ABSTRACT

in this study. deposition of Ir., on stainless steel tubing was investigatcd using

radiochernical techniques. The purpose ivas to study the mechanisms of L,,, deposition on

stainless steel sampling linès. 1: drposits through both phpsical and chernical adsorption.

Pliysiçal adsorption predorninatrs at 1' concentration bslou 10'" rnol;L. A t concentration

higher ihan IO-'' mo1,'L. slou. chemisorption occurs for low humidit). (<25%). At high

rslati\.s humidit!. (>759b). rapid chemisorption ivith p i t t i n corrosion occurs. I I is

hrlisi-cd to rract at defect sites to form hygroscopic Fe12 which attracts more watrr

proprigating the reaction. Undsr soms conditions ihs propagation is inhibitcd resulting in

an apparent masimurn surface loading. At hiyli iodinr concentration. high rc1atii.e

humidit!. and tube tenipsratures of -!O0 or 60°C. no such inhibition occurs resuiting in

rapid and rontinuous iodine adsorption.

Despite the espectation that SS-3 16L should perform bsttsr than SS-304L. therr

rippsars to b r no direrencr betwren the two types of steel rsccpt. perhaps. at iodine

concentrations brlow 1 O-' rnol/L.

Acknowledgmen t

1 rvould like to express my gratitude and appreciation to :

Prof. Greg I. Evans for his supervision. encouragement and rspccially his patience throughout the course of the a-ork. 1 am tmly gnteful and indrbted to him. 1 ha\.s leamrd so rnuch.

C'.\'IDE o\iner group. Ontario Hydro and Atomic Energ- Canada Limited (AECL) for thcir gensrous financial assistance.

bl!. Colleagues : .Anandhi. Angela. Christine. Ed Panvan. Evon. Fariborz. Jim. Juliette, Kai. Lu, Mark. N a u d Phillip. Sandu. Sevana. Sithal. Sonia. Sophia. Ton' and Tinku: and sspecially to Chris Deir and Paul who have hriprd me to get started uith the esperiments. 1 am very happy and honored to ha\-e had the opportunit). to share al1 the moment togrther.

bls. Jackir Briscoe. who is veq. helpful and kind everytime 1 visit her at her office.

Mr. Tjuk Rahardjo for his invaluahle hrlp in regards to administrational matter at rhs Indonesia Nnsional Atornic Energ Agency ( BATAN ) in Jakarta. His hslp is - 2reatl'- appreciated. He has ahva)-s bsen thsre al1 the time 1 nerd assistance.

4f:- parents. n-hosr Io\-s and support. through al1 this tirne. has really bccn a source of strsngth and inspiration. The same goes to my sistcrs and brother. 1 n-ish you good luck ~vith ail your work. 1 know that distance kreps us so far apart: but distance onl>- niakes me realize that 1 love !ou al1 so much. 1 miss >.ou ...

Be!-ond that. al1 praise be for God. Most Ivlerciful. Most Coinpassionate.

Table of Content :

--\ bs trac t

(Jement Ac know led,

Table of content

1 . Introduction

1.1. Gensral O~en- iew

1 -2 The Role of Filtsred Air Qischar~e Systsm ( FADS ) and

Post &xidstlt Radiation Monitoring System ( PARMS) -

1 .j .An Oveniew of the Study

2. Theoretical Considerations

2.1 Iodins hlass Transfer

2.2 Iodins Deposition Velocity

2.3 Gcis Phase ;CIass Transfer. k,

2.3.1 The E ffects of Temperature on k,

2.4 Iodins Interaction with Steel

2.4.1 Ph>.sical adsorption

1.4.2 Pitting corrosion of Steel by Iodint.

2.5 Estimation of Iodine Transmission Fraction

3. Litsrrtturc Sun-ey

3.1 Iodine Species Distribution

2.3 Tube Inside Diameter and Length

3.3 Volurnetric Flow Rate

3 -4 Iodine Gas Phase Concentration

3.5 lodiiie Interactions w-ith the Steel Surface

3 3 . 1 Effect of Temprrature and Relative Humidity

3.6 Steel Type : SS-304 and SS-3 16

3.7 Other effects

3.7.1 Fittings and Bends

7.7.2 Clraning the sarnpling Lines

3.8 Tests at Ontario Hydro

3.9 Surnrnary

4. Esperimental

-1.1 Esperimental Apparatus

4.1. i Gsneral Features

4.1 -2 Msasurement of Gas Temperature and Relative Hurnidit'

4.1.3 Temperature Control of the Steel Tubing

4.2 Gaseous I2 Generation

4.2 The Stainless Steel Samplcs

4.4 Esperimrntal Parameters

4.5 Esprriniental Procedure

4.6 CV Spsctrophotornetsry

5 . RESL'LTS AND DISCLTSSION

5. I Yririsblss Studied

5.2 Deposition Parameters

5.7.1 The Deposition Velocity (k,)

5 - 2 2 Psrudo-saturation Surface Concentration ( c,")

5.2.3 Steady siate and Surface Pseudo-saturation

5.3 The EtTects of Operating Conditions on Deposition Parameters

5.5.1 High Iodine Concentration

5.3.2 Medium Gas Phase Concentration

5 - 3 3 Low Iodine Gas Phase Concentration

5.3.4 The overall effects of Gas Phase Concentration

5.3.5 Summap of the Effects of Operating Conditions on k, and C,"

5.4 Desorption

5.4. I Desorption Cndsr Different Range of Humidity

5.4.1 Desorption and Iodincr Reloading

5-4.3 Volatilization of Iodine from Fei? Solution

5.5 Iodine Rrtrntion on Compression Fittings

5 -6 Possible bkchanisms

5.6.1 Physical Adsorption

5.6.3 Slow Chernisorption

5 -6.3 Iodine Chemisorp~ion with Corrosion

5.7 Applications to Gas Sampling Lines

6. Conclusions and Recommendations

6.1 Conclussions

6.2 Recommendations

S. Nomenclature

Appendices :

.-\ppcndis .A Data Plots

Appcndis B Tabulated rssults : adsorption

--ippendis C Tabulateci results : dssorption

Appendis D Classifications of Opsrating Variables and Rrproducibility

of Resul ts

List of Fi, sures :

Figure 2.1 Transfer of iodine from the gas phase to the tube surface

Figure 1.1 Pitting corrosion of stainlsss steel by iodine

Figure 3.1 Initial iodine transmission as a function of flow rate

cure 4.1 Esprrimental apparatus C

gure 5.1 Variation of k, xith gas phase concrntration for SS-3 16L

gure 5.2 Teniporal variation of iodine surface loading

cure 5.3 blasirnum iodine surface loading as a îùnction of gas L

phase concentration

Figure S.4a and 5.4b Deposition velocity at High [LI2

Figure 5.5 Iodine surface ioading at 40°C. high gas phase concrntration

and high relative humidity

Figure 5.6 lodine surface concentration high [IJg and low relatiw humidit).

Figure 5.7a and 5.7b Deposition velocity at medium concrntration

Figure H a and 5.8b Deposition Vclocit). for SS-3 16L and SS-3041.

as a function of Temperature

Figure 5.9 Variation of dsposition vrlocity with gas concrntration SS-304L

Figure 5.10 Variation of distribution coeficient with sas concentration

at low relative humidity

Figure 5.1 1 Variation of distribution cosftkient with gas concentration at

high relative hurnidity

Figure 5-12 Surhcs loading as a function of time : rsperinirntal and calculated

( [LI- = 8s 1 o - ~ o P L . T=90°C. high relative humidity )

Figure 5.13 Surîàce loading as a function of tirne at high humidity:

esperiniental and squation 3.3 ( [I3]g = I 0- rnoli'L. T = 23°C.

high relative humidity )

Figure 5.15 Dssorption under diffcrenr relative humiditirs and tube temperatures

( = IO-' mol/L. T = 40°C. relative humidity as indicatrd in figure)

Figure 5.16 lodine drsorptioii under different relative humidity at 90°C. I I Iodine loading involvrd [ I & = 3.5~10' moliL. low relative

humidity and 90°C

Figure 5.17 Iodine desorption and reloading ([I& > 10" mol/L. high relative

humidity. 23°C).

vii

Figure 5.18 Apparatus to study iodine volatilization from FeIl ,,,, 79

Figure 5.19 Volatilization of iodine from Fe[: solution 79

Figure 5-20 lodine deposition with time : SS-3 16L tube with stainless steel

titting at the middle ( [I& = 3.5E-8 moI/L. T = 23 C. relative

humidity = 85%). S 1

Figure 5.2 1 Distribution of iodinc activity dong the tubing 82

Figure 5.27 Iodine deposition rate (N,) as a tùnction of =as concentration

(SS-3 16L) 84

Figure 5.23 Iodine deposition rate (N,) as a hnction of -as concentration

(SS-304L) 85

Figure 5.13 Variation of iodine transmission fraction n-ith time 95

List of Tables :

Table 2. I Effect of temperature on k, 3 1,

Table 3.1 lodine spscies distribution in PWRs and BWRs 19

Table 3.2 Effcct of tube inside diametsr on transmission fraction 20

Table 3.3 Effect of flon- rate on drposition vrlocity (SS-3 16L) 33 -- Table 3.4 Transmission fraction at high tlow rate 23

Table 3.5 Iodine deposition on Fe. Cr. and Ni at temperature around 388 K

(115 O C ) 27

Table 3.6 Xletal composition of SS-3O4L and SS-3 16L 30

Tablr 3.7 Iodine drposition velocity on SS-3 16L and SS-304L samplinp iines 3 i

Table 3.8 Pickering NGS Unit 4 - cshaust stack 35

Table 3.9 Pickering NGS Unit 1 - eshausi s t d 35

Table 3.10 Pickering NGS PARiMS - low activity monitor 35

Table 3.1 1 Pickering NGS PARMS - high activity monitor 35

Table 4.1 Steel sample specifications 43

Table 4.2 Esperimental parameters 45

Table 5. i Range of variables Invcstigatrd 52

Tabie 5.2 SS-3OJL medium [IZ], 64

Table 5.3 SS-3 16L medium [12]., - Low %RH 65

Table 5.4 SS-304L low [I,], - High %RH 67

Table 5 -1; SS-3OJL Io\$- [L,], - - - Low %RH 67

Table 5.6 Espectrd iodine deposition parameter values for srainless steel lines 94

Tiiblc 5.7 Estimatsd iodine transmission tiaction based on equation 2.16 95

I . INTRODUCTION

1.1. General Overview

In the event of a core disn lptivs accident in a nuclsar generatin! station. some fission

products ma). be relrased from the fuel into the containment artta. Among thsse fission

produçts. radioiodinti is one of the most important to consider in reactor safety studirs.

Radioiodine causes grear concrrn dur to its high tission yield. its potential \.olatility

and its radiobiological hazard. Soms of the ndioiodins released from the core ma>- becorne

airborne and escape to the environment. One of the most likely pathway of release from a

multi-unit CANDU (CANadian Deteurium Uranium) station is controlled venting through the

Filterrd Air oischarge S>.stem (FADS). Through its filtering capabilities. the FL\DS pla!.s a -

\.en. important role in minimizing the arnount of radioactive tission products releasrd to the

environment. The concentration of sas phase iodinç in the discharged air can be measured

~ising the Eost Accident Erlease Monitoring $ + s t e m (PARMS). The PAR41S requires thal the

air srirnples hè passed through a considerable length of stainless steel tubin, c7 to a rtlmote

location whrrt: the dçsired nieasurèment can be safely made. .A signitïcant loss of iodint:

ca~ised b! adsorption on the sarnpltt line surfaces \ \ d l greatly diston this measurement. The

acçuraq of the measurrmrnt is very important since it will be usrd to estimate the magnitude

of an' radioactiw release. rhis information is needed to estimate the radiation dose to the

population and. thereby. to determine appropriate off-site actions to protect the public.

1.2 The Role of Eiltered Air Discharge xvstern (FADS) and Post Accident Radiation

.Monitoring Svstem (PAARMS) -

Should radioiodine escapes from the hel. the last barrirr beforr its releasc to the

entironment is the containment structure. The containment systern of multi-unit CANDL:

reaçtors is maintained at sub-atmospheric pressure to prevent an). uncontrollrd radioitctiw

emission to the outside air. Provided that this containment structure remains intact follou ing

an\. tùcl tàilure. a major portion of the radioiodine will be safely kspt within the containment

building. or rlse be tnpped b). the FADS. hrnce minimizing the environmental impact of the

incident.

The purpose of the FADS is to provide a filtrred and well defined path tor the

controllsd relrase of an>- air vented to the outsids atmosphere. The FADS is not required undrr

nornial upsrating conditions. During normal operation. the FADS rtsmriins isolated from the

reciçtor containrnrnt. It will bs run followint a substantial release of radioactivity into the

containment area. Ewn so. it \ d l only be opsrated when the pressure within the containnient

huilding is npproaching atmospheric pressure. Once the required sub-atniospheriç pressure is

xhievrci. the FXDS x i 1 1 bs tumed off. I t will bc tumed-on and off subsequently to inaintain

the required low pressure insidt: the containment building.

To ensure high removal of fission products. the FADS has in place a series of tlitrring

s'stcms. For iodine removal. the systern utilizes TEDA impregnated charcoal. This tilter does

have limits in its prrfomance. Several factors that can affect the iodine tiltering performance

cire the thermal stability of the oganic charcoal irnpregnant (TEDA). the humidity of the

intlucnt gas as u-el1 as the flow rate of the sas [ I l . Additionally. a small portion of iodine

which is already trapped in the charcoal bed ma)- desorb. The arnount of desorption d i

dcpend on the total iodine invenrop on the filtsr [7].

Considering thrss limitations. it is necessar?- to monitor the amount of radioacti\.it>

that escapes to the atmosphrre. The on-site measurement of radioactive rrlease is crucial in

estimaring the nmount of radiation dosage that ma>- be recri\.sd b > the population especially b>

those within several kms of the reactor site. Additionally. the measurement results ma) also

determine whcthrr an) off-site actions are required to protrct the safety of the public.

Thercforc. it is crucial that the measurement be performed to a known degree ofacçuracy.

The activity in the rcleased air is rvaluated by the PARMS by sarnpling of the gas prior

to the discharge point. .\ sarnpling port diverts some of the -as tlow and cames it to a location

~vhcre rneasurement can bs safely made. .A detsction systsm is thzn used to dstsct and measure

al1 of the major fission products that may bt: present in the gas stream. To c;in?. the gas stream

to the merisurement point. the PARMS utilizes a length of stainlsss steel tubing. Steel type

204L. is used in most of the esisting systsrns. Houxver. the 304L steel has recently bren

rcplared b>- SS-3 16L at one of the plants.

Somr specirs of gasrous iodine. such as L,,,. adliere to man!. materials. Stainless steel

is not immune to this problem. -4s a result somc of the radioiodinr passing through the

stainless steel tubing will deposit on the tubing surface before reaching the detçction point.

Bccause of this problem. the operator may be receiving misleading information regarding the

amount of iodine being released from the reactor. The value reported ma). be lo~ver than the

rictual mount of an!. release.

1.3 An Oventiew of the Study

Iodine relsased from the containment ma- be in many foms incliiding I,,,, . CHJ,,, and

other organic iodides. RI,,, - . However. of these. only II,,, is readill- adsorbed by steel. The

objective of the study \vas to determine the efkcts of oprrating conditions on the drposition of

iodinr ont0 stainless steel tubing. The rate and estent of 12, .<, deposition on two types of steel

tubing was yuanrified and these results w r e applird to estimating linr loses in ssisting

PARMS systsms. Possible rnechanisms responsible for the retention were also proposrd.

The operating conditions considerrd in this investigation included gas phase iodint.

concentration. relative humidity and tube surface temperature. Initially. it \vas anticipated that

the rats of gris phase mass transfër would bt: dominant in determining the overall rate of iodine

adsorption. .As the projrct progressed. howevtr. it became apparent that. given the proprr

conditions. the rate of surface rractions playrd a more important rolr. The operating conditions

utire esprctrd to affect the rate of iodine deposition mainly through thsir sffects on the

interaction betiveen iodine and metal constituents of the steel

The. esperinients werc pcrfomed by passing a @as stream containing IZ,., lahrltid with

1- 13 1. through 25.1 cm long. 1 .'4" OD. stainless steel tubing. The rate and estent of iodinr:

rcttintion \vertt obtainsd by rrcording the gamma rmission of any 1-13 1 drpositrd on the

tubing. Two types of steel were esamined in this study. SS-3 16L and SS-3OlL. I t &.as bçlie\.ed

that. duc to irs mrtal composition. SS-3 16L would perforrn bettrr in the presencr of a reacti\t

çomponent such as molecular iodine. leading to lower iodine retention. Additionally. somr

rsperiments wrre also conducted on iodine deposition around stainless steel fittings. Iodine

retrntion \vas rspected to be high around the fittings.

In the bcginning of this thesis. the published works of others that are relevant to the

currrnt projrct are summarized. Thcse include some data on actual linc testings pcrfornied b!,

Ontario H>.dro as wsll as somr general characteristics of iodine interaction with stainlrss steel.

The theorirs that are relevant to the process of iodine deposition are then presented. Follouing

the theorirs. the esperimental procedure used in the study is describecl. The discussion ut'

rrsults is then prrsentrd. The discussion focuses on the obsrned trends of dspositinn under

\arious opsrating conditions as ~vell as some possible mechanisms that c m esplain the

bcihavior. The cornpiete log of the esperimental results c m be found in the appendices. The

releiance of the u-ork to the gsnerat understanding of iodinekteel interactions and its

applications ro PXRMS conclude the report. Based on the resul ts. some recomniendations for

tùrther studies are also givsn.

2. THEORETICAL CONSIDERATIONS

Iodine retention on the steel surface involves several distinct steps. The process begins

nith iodine being transferred from the pas phase to the tube surface. Once the iodine is on the

surface it ma: be physically adsorbed and. considering the reactive nature of molrcular iodine.

it ma)- thrn react with the metal constiturnts of the steel. Somc of the iodine that is alrcady

drpositrd on the surface ma- brcome resuspended and be transferred back to the gas Stream.

Thsse different phenornena udl determine the rate and estent of iodine retention on steel

sampling linrs.

In this chaptrr. some theoriss that are relevant to the above processes are disçiissed. The

cquations that can b r iised to estimate the performance of sampling lines are also drscribrd.

2.1 lodine Mass Transfer

The rate of iodine deposition will deprnd on two procrssrs : deposition and desorption.

I t i s therefore nrccssary to clearly detinr the terms rrlated to thesr two phenomrna. The

parameter oftcn uscd to describs the rats of iodins deposition is the deposition \.elocit)-. k,

(cm. s ) . \ihich can be detined as follows :

N- = the iodine deposition rats (rnolkm2.s)

C, = gas phase iodine concentration (mol/cm3).

The iodine on the surface c m also becoms resuspendsd and bs transferred back into the

cas srream. The constant that describes the rate of desorption is defined as folIows : C

L = desorpiion rate constant (s"

N, = desorption rate (mol!cm-.s)

C, = iodine surface concentration (rnolkm-1.

Considering the abovs dehition. the overall mass balance of iodine owr the tube

surface crin thsn be set up :

In Eqn. 5.3 it is implicitly assumed that the net iodine deposition rate is first ordsr with respect

to Cz as \\-el1 as C,. Furthrrrnore. This equation implies that afttrr a certain period of iodine

loading. steady stritti will bs achicved. During such a condition. the rate of deposition would be

qua1 to the rate ofdesorption. In the initial stage ofdeposition the iodinr surface ccincentration

sliould be vrry Io\\-. Consequrntly. the drsorptioii term during this period will br nqligiblr.

17herttfore. the initial deposition velocity can br rstimated using the follouing equation.

Eqn. 2.4 is used in this studp to estimate the \-aiue of the initial deposition velocity.

2.2 Iodine Deposition Velocity

The procsss of deposition invol\-es two distinct strps : pas phase mass transfcr and

iodini: interaction u'ith the steel surface. Consequrntly. the value of lid will depend on the two

trrms thnt are associated with thesr two separate steps. The rate at which iodint is transferrtd

h m the gas phase to the sas-surface interphase is drnoted b>- k2 while the rate at which the

siirtke c m receive or interact with the iodine is denoted by k,. blathematically. the relation

bcitween these parameters can bct u~itten as follo\v:

The proçess of deposition is illustratrd in the following diagram.

- Gas Stream -&

Gas Phase Boundary Layer

Gas-Steel Interface

Steel Surface

Figure 2.1 Transfer of Iodine frorn the Gas Phase to the Tube Surface

Equation 7.5 above signifies that k, cannot escced E;? or k,. Furthçrmorç. rither kz or k,

ma) be the term that will limit the overall drposition vr1ocit)-. For esample. if k, is

substantially lürger tlian k, . then k, will be the factor limiting the overall transfrr of iodine md.

conscquently. will dcterminc the value of k,. If the limiting strp can be idrntified. effort to

limit iodine loss a l o n the sarnpling lins c m be accomplished by modifj-ing the operating

conditions so that this limiting step is funher suppressed. Due to the importance of k,: and k, in

dctrrmining the o\.erall rate of iodine deposition. the following sections are dedicated to

discussing the two parameters.

2.3 Cas Phase :Mass Transfer, k,

The valus of k2 can be esrirnated by using standard correlations based on the analog!.

I\-ith heat transfrr. The dirnrnsionless Shenvood number. Sh,. is usrd in the estimation.

The Shenvood number can be estimated by using correlations for intemal flow through

a tube. The comrlation that is applicable will drpcnd o n the type of tlow within the tube. -Two

correlations to estimate the Shenvood number are presented belon-.

i ) Fully developed larninar flow [3] :

Sh,, = 4.36 (2-7)

i i Combination of full! developed tlow with entrance effect by Sieder and Tate [3] :

Equation 1.8 is rccommended bu Whitakrr if the condition belou. is satistied

For the conditions relri-ant to the esperiment. the trrm on the left is rqual to 0.83. Thus. full>.

&vsloprd laminar condition encompasses much of the tube and. hence. rquation 2.7 should be

Re,, is the corresponding Rqmolds number for tlow through the tube :

Sc is the Schmidt number:

D = tubs diametsr (cm )

p = drnsity ( g c m ' ) 7

\ * = kinematic \.iscosity (cm-%)

V = C cas vrlocity (cniis)

IL = viscosity of the gas (k~,/m.s)

,LI, = viscosity of the gas near the wall (kg i rns)

The itiscosity of the gas near the tube wail. p,. can be estimated by the correlation of

T = trmprrature in Kelvin

il To obtain kinematic viscosity from r* = -. the dynamic \-iscosity. u . c m bs sstimatsd

?

bj. Eqn. 2.6 above while the drnsity of air crin be estimatsd by using the ideal gas law. For

comparison. sonir plottrd values of kinemûtics ~kcosity by Fos et-al. c m be ussd [4].

The estimation of b i n q diffusion coefficient. D,,. is more cornplicated compared to

an!. other variables incolved. There are several mrthods a\-ailable to estimate D,,,. The

difticult!. is that some of the data required are not available in the literature. For the purpose of

this stiidy. a simple correlation by Fuller. Schettlrr and Giddings tvas usrd [jl. The squation is

ris tollow :

D ,, = binary di ttùsion coefficient (cm2:s)

M., and hl,, = molecular weight ofcomponsnt .A and B

T = bulk temperature in Kelvin

P = pressure in atmosphere

5 , and ZvU = the atomic diffusion volumes of component .\ and B

2.3.1 The Effects of Temperature on k,

In the current investigation. tube surface ternperature is one of the main variables being

studied. The variation in temperature is espected to have some impact on the rate of gas phase

mass transfer. by altering the values of p,. v. D,,. These parameters. however. are only weakly

affectrd by variations in temperature within the rangs relevant to the current iiivestigation.

Hence. the rate ofgas phase mass transfer can be esprcted to be affecred only marginal!!.. The

follouing table show the impact of increasing @as temperature on the value of k-. For the

purpose of cakulating viscosity. a mean value of tube surface tcmpçrature and temperature of

the sas was used.

Table 2.1 Effect of tsmoerature on k,

The combination of temprrature and relative humidity on k, is more important to

çonsider and this is discussed in chaptrr 3.

2.4 Iodine Interaction with Steel

C'nlike kz. i t is vrry difficult to estirnate k, based on an). mathematical expression. There

are various phenomcna in\.olvrd in the determination of k, such as temperature and relative

Iiurnidit>- dong u-ith their impact on the growtli and disappearance of water tilms or water

droplets on the surface during an esperiment. '4s described in chapter 3. it is possible that k,

plays a very important role in determining the overall kd.

Ln the sections below. some possible interactions between iodinc and steel surface are

discusscd. The phenornena described should assist the interprcitation of the data obtaincd from

the esperiments.

2.4.1 Ph?.sical adsorption

r\fier iodine is transfrrred to the steel surface. thrre are tu-O possible types of interaction

brineen the iodine molecules and the steel surface. The tirst is physical adsorption of the

molecular iodine. Ph>.sical adsorption in\.olves weak or indirect bonding brtween the iodinc

molecules and the steel surface. Given that the bond is not to any sprcific atom on the surface.

ph>.sical adsorption is not usually limited by the nurnber of reaction sites on the surface.

Ho\vs\.er. as iodine layers nrar the surface increase. the bonding force becomcs progressivel!.

\\-caker. dur to the increased distance from the surface. In fact. physical adsorption b q m d

nionolayer coi-erage doss not usually occur unlrss tlir partial pressure of the adsorbate is close

tu its vapour pressure. Xfter a certain pcriod of loading. a balance brtwren the attractive force

of tlir surface and the repulsive h r c r crsatrd by the concentration gradient brtween the surface

and thc bulk will be achievcd. Hencr. the surface wi l l achieve a steady state. .As a result. the

deposition 1-elocity is initially high but drclines as deposition procerds. reaching zero at steady

statr. Some authors haw reported that monolayer coveragr of a steel surface is on the ordcr of

10'' mol.'crn2 [6]. Deir. however. obsen-sd maximum loadings up to 10.' mol;cm2 [7 j. This

suggests that liundreds of monolay- adsorbed ont0 the steel surface or that the deposition was

not dur to physical adsorption alone.

The second process is iodine adsorption that involves chernical reactions. The process is

dso comrnonly h o w n as chernisorption. The process of chrrnisorption is usually precrded b!

physical adsorption. Se\.eral authors such as Deir and Tsukaue et-al [7.8.91 have proposed

interactions which involve pitting corrosion attack of steel by iodins. .A scherns involving

corrosion reactions were suggested since. as described in chaptsr 3. direct rsaction bstween I2

and Fe will not occur to an- appreciable estent at temperatures lower than 100°C [6]. The

possible chernical reactions \vil1 be rlaborated in the following subsections.

2.4.2 Pitting corrosion of Steel by Iodine

Pitting is a highly localizrd corrosion attack on a metal surface whik lttaving most ot'

the metal surfacr intact. The attack ma) begin on a site rvhere the protsctive oside tilm has

k e n damagrd. Certain anions such as halides. most notably chlorides. are known to bc capable

of damaging the oside layer [ I O . 1 11. Corrosion pitting due to bromide. tluoride as wcll as

iodide has also becn obsen.ed [ W . 1 1 1. Other important factors that ma). increase the

probability of pitting attack arc impurities such as the inclusion of rnmyanrse sulfidc [ 9 ] .

temperature and relative humidity 18.9.1 11. Addition of alloying mctal such as molybdcnum

into the siainless stcc1 has been known to reduce pitting prohabilit!..

In relation to iodinr rsteniion dong stainless steel sampling lines. the involvement of

pitting corrosion ma' enable the steel surface to accumulate a higher amount of iodinr than i T

merrly physical adsorption \vers occurring. It is likelp. for example. that the high icdine

accumulation observed by Deir involved a combination of physical adsorption and

chemisorption.

The chernical reactions involved in the process are rlrctrochemical in nature. The

presence of water is nrcessary for the reaction to occur. Deir has suggrsted sorne of the kcy

reactions that ma? be involved in steel attack by iodine [7]. These reactions are described

Cornbining the two equritions yirlds :

3 - Fe,,, - Ligi ~f Fe- ,;,,, - 21- (-0.976 V ) ( R 2.3)

The resulting ~ e ' - ions ma). react n-ith 0: diffusing from the pas phase under the prrsence of

a-ater giving the final corrosion products Fe201 :

1,

4Fe- ,,,, - 0: - (4-7s)Hz0 -t 1Fe20,.sH,0,,, - 8H- ( R 2.4)

The iodids ions will also react with the rnolecular iodine resulting in the formation of triiodide

ions.

I',,, - 1‘ * 1,' ( R 2.5)

rriiodidr ions are very reactivr and i r ma)- contributc to funhsr corrosion of the steel [7]. The

tcillowing diagrrini illustrates one possible schsms of stainless steel attack by rnolecular iodine

ditlùsing from the gas phase.

Figure 2.2 Pitting Corrosion of Stainless Steel by lodinr: [ 5 ]

2.5 Estimation of Iodine Transmission Fraction

C'pon obtaining the data for rate and estent of iodine retention undrr various

csprrimentai conditions. i r is important that thrse data bc related to the performance of rsisting

gaseous iodinr sarnpling lines. For the purpose of cstimating the transmission fraciion. a simple

correlation that inwlvss drposition wlocity c m be used. The rquation is based on the rate of

change of iodinr niolar flou. rate through the steel tube. It assumes that the deposition rate is

proportional to the iodine concentration. Dur to tlirsr assumptions. the equation ma'. onl! he

applicable ovrr a short period of time which will depend on srveral factors. One of the most

iniportant factors is the iodine -as phase concentration. For esample [El. during an accident

u-ith Cz nsar 1 O*" rnol/L and a relatively high deposition velocit>- of 0.1 cmis. the time required

to reach saturation would be up to several days. A sirnilar time framr is required until

desorption rate becomes significant. However. it c m be expected that the actual kd during an

accident u-ould bt: l o w r than 0.1 cm's. This u-il1 incrrass the period of applicability of the

equations. The cquation is as follows.

F x LK', = -C', x ,7 x D x k , x c l ï (2.14)

Ssparating the variable and integrating :

1 -n x D x X-,

F 1 dl- , ,: 1, ()

rodinetransmission traction ( ! )= = esp( -- F

1 (2.16) c S ,J

The equation çan also be written by usiny gas Iinrar veloçiry (V) to replace gas \.olumrtric tlow

rate ( F 1. yislding :

C * .c , - I X X - ~ X L Iodinr transmission liaction (t) = - = csp( c .- 1 - x D

)

C,,, = iodins pas phase concentration at inlrt (mol/cm2)

C,, = iodins gas phase concentration at outlrt (molicrn')

k, = dcposition vrlocity (cm&)

F = volumetric t h v rate (cm%)

L = line length ( c m )

D = tube insids diameter ( cm)

V = gas linear \'elocity (cm;'?;)

.As statsd previously. the equation nrglects the rate of drsorption as ti-el1 as the declininp

\.aliir of k, \\-ith rime dur to saturation. Both factors will improve transmission fraction.

Thrrrforc. the rquation is likely consrrvative and it should owrestimate the loss over the long

term. Ho~vevsr. the squation also does not consider any increase in k, with increasing surface

loading as a result of corrosion or othrr surface reactions. Hence it is possible that this

cquation wi11 underestimate the loss under some conditions.

3. LITERATURE SURVEY

Estensive studiss have been carrird out to investigate the performance of radioiodine

monitoring systems in nuclear reactors. These studies include on-site tests at nuclear stations and

csprrirnents perfomxd in laboratorics. Some of the investigations which have been directed

toward assessin- and improving the rsliability of csisting radioiodinr sarnplinr s!.stems. as well

as improving the current understanding of iodinr interaction with stainless steel surface. are also

discussed in this chapter.

There are w-ious parameters that can be used to charactrrize the performance of

radioiodinc sampling lines. In the sections to follou.. the rfkcts of these parameters on the rate

and estent of iodine retrntion are described. The parameters are discussed in the following order

: iodine speçies distribution. tube diarneter and Isngtli. volumctriç tlow rate. iodine gas pliase

ccinçentra<ion. interaction of iodinr and steel along with the effeçts of temperature and rrlati\.c

humidity. the type of material used in the sampling line and several othrr factors such as tittings.

bends and clctaning of the samplc line.

3.1 Iodine Species Distribution

lodine spscies distri bution in reactor eftluent is of inrcrest because iodine transport

brhavior through sampling lines is highly dependent on the iodine forms. An average iodinr

speçics distribution from threr Pressurized Nater Beactors (PWRs) and three Boiling Kater

Rcactors ( BWRs) is given in table 3. l (1 31. These values will not necessarilp be applicable to -

an'. speçific plant as the distribution and quantity will differ amongst plants. plant areas and

diffcrent operating modes.

Table 3. I Iodine species distribution in PWRs and BWRs

.-\n~ong thcss iodine species. 1, is the most reactiw and. as a conscqucncs. it prcscints the

biggsst challenge to design enginrers. due to its hieh retention on the surface of sampling linrs.

Cnrcin et-al. reportrd that the deposition velocity (k,) of HO1 is less than 50h of that for 1' while

the dcposition \.rlucit!- of CH3[ is less than O. 1 Oh of that of 1' [Hl. Glismeyrr and Schmrl also

o h s r n d sirnilar behavior for 1:. HO1 and CH3[ [ l j ] . Tests performed by Ontario H>.dro duriny

the yrar 1992 and 1993. at Bruce NGS-B stack monitor and Pickering PL\RM sustem. showtd a

similar trend. Nrgligible retention of CH+ was found rven though signiticant retention of 1' \vas

observed within these lines [15.16]. Due to its greater retention. much more attention has been

püid to invsstigating 1: transport behavior through stainless steel sampling lines.

The current research drais only wirh I?. In the following sections. only lirerature data that

pertain to the transport behavior of 1: arc prrsented. Data regarding the otlirr iodinc specitis are

includcd only if the! are reIe~ant to the discussion.

BWR ( O h ) I

28 Radioiodine Species Molecular Iodine ( I7 ) -

Hypoiodous acid (HOI) Orrranic Iodides (CH:I. etc.)

3.2 Tube Inside Diameter and Length

The impact of tube diameter on retention has usually been studird in conjunction with

tlow rate. For this rrason. the discussion in this section will br closrly related to the discussion

on the sffect of flow rate within the nest section.

PWR (%)

27 40 3 1

l

20 l

40

Esperimsnts have

lengths. The lrngth of the

diameter \vas varied tiom

performed \\.-ith stainless

pressnted.

t'nrein st.al. [14]

been performed with stainless steel tubes with different sizes and

tube vciried fiom as shon as 15 cm to as long as 78 m. u-hile the tube

0.457 cm to close to 2-54 cm (1"). Somr sspsrirnrnts have also brrn

steel coupons. In this section. only results with steel t u b i n are

performed rsprrimrnts at trmperatures bstween 25°C to 30°C with

relati\.e huniiditiss between 25% to 70°/0. The steel used mas either SS-304 or SS-3 16. There is

no sprci tic msntioning which steel type uas used in an'. particular lins. The diftkulties with the

data is that the tube diarneter was varird together n-ith gas volumetric tloii. rate. Both parameters

would play thrir rolcs in detcrmining the transmission fraction of iodinr. .As a result. it is

difticult to dctenninr whsthrr the tube diameter or the How rate was the predominant rffect.

Table 3.2 Efkct ci t- tube inside diameter on transmission fraction 1 4

Line #

1 3 l -

3 3

Bascd on the data above. the imariation in tube diameters and îlow rates dors not appear to

influence the rate of mass transfer of iodine to the tube surface. No signiticant changes in

deposition velocit?. was obseneed. This is sornewhat surprising since a drcrease in gas linrar

~ d o c i t y accompanied by a lower value of tube diameter. such as shown in lins 4 in table 2.2.

Flow rate (L/min)

84.96 84.96 56.64 56.64

Inside Diametcr 3.22 cm 2.22 cm 1.91 cm

Length (mi

30.48 1 5.24 43.93

Velocity ( m w 3 -66 3.66 3.30 3.30 1 cm 1 42.93

(.W) ratio (cm-' )

1.80 1.80 2.10 3.1 O

Transmission Fraction

75% 7 8 2 3 "O

629'0

k, (cm/s) 0.020 0.032 0.054 0.0 18

should decrease the Reynolds number which should reduce the gas phase mass transfer

coefticient. It is likely that. given the low values of k,. gas phase side mass transfer was not the

limiting factor. or the change in tlow conditions a-as not suficient to alter the deposition

\.slocity.

The transmission fraction. howver. suff'ers a substantial decline in linc no. 4. Cornparcd

to the othsr linrs. this line has the smallest tube size (0.64 cm) as well as the lou.est lincar @as

\.elocit>. (0.88 cnù's ). The reduction in transmission fraction is espectcd sincr the tubs sizr and

cas linsar \.elocity wcre reduced. Smaller tube diameter means highrr surface area to volume - ratio whilr l o w r linear vslocity means higher residrnce tirnr. Both will enhancr the opportunit'.

t j r interaction bstwren iodins and the steel surface.

Deir psrformrd ssperiments under similar conditions as the ones in line no. 4 and

obtained higher values of k, ranging from 0.07 cmis to I c d s L17.71. One possible diffrrencc is

the gas concentration ivhich ma!- have bern rnuch Ion-er in the ssperiments of Cnrrin et.al.

htonunatr l>. . gas concentration \vas not reportrd b!. Unrein et-al..

3.3 Volurnetric Flow Rate

Flow rate is esprctrd to influence 1: retention by afkcting the rats of inass transkr. Gas

phase sidr m a s transfrr m e - play a vcry important role in determining the rate of 1' drposition

since prior to an). interaction between 1: and the steel surface. the I2 must first bc transferred io

the surface from the @as phase. The gas tlow rate will also affect 1: retention by changing the

rrsidencci time. This will be discussed in the section on interaction of I 2 with a stainless steel

surtace.

In somr sxperirnents performed at low tlow rate by Deir. it was shown thal

\vas incrctasrd by two. thsre \vas a possible impact on the deposition velocity

~vhen the tlou

[71. The tube

diametsr \\as 0.64 cm and the lrngth \vas 15 cm. Ewn though the ssperiment was psrformrd in

the lamina region. cornpared to results from othrr researchrrs. the deposition velocity \vas found

to bc: v t ry high. End dfects ma? be dominant in this case. The values rire slightly highcr than 1

Table 3.3 Effect of tlow rate on deposition velocity SS-3 16L) 171

- - - - - - - - - -

Esperiments with sirnilx tube size and tlow rate have been performrd by Genco r.t.al.

[ 1 9]. Genco et-al. carried out the ssperiments at temperatures abow 1 50°C. Such temperature is

too high to bct applicable to a radioiodinr sampling line. Most sampling lines operats at

teniprrature between ambirnt to jO°C [ l j ] . The ctsprrimcnt L i a s originally intended to stud)

iodi ne deposition on the prima?- heat transport s y m .

Thri tube usrd was 137 cm long. This lrngth is suftïcient to supprrss the end effctct on the

u\-erall deposition \.slocit>.. Additionally. the tube had bern pretilmed by rsposing it for 1000

hours to \vater at 288°C recirculated at a flow rate of 8 ml's. A t the above conditions. the

obsened value of k, was rnuch lower. rmging brtween 104 cm/s under dry conditions to 10.'

cm/s whrn the air was mised with stram (10% d o ) . A 40 m long sampling line operated with

tnis deposition 1-elocity would. in theor).. have a transmission fraction ofapprosimately 9096.

Flow Rate (L/min)

Temp (OC) %RH Mg ( m o w k, (cm/s)

Edson et-al. carrird out somr ssperiments at a high tlow rate of 187 L/rnin (linear

\.slocity = 835 c ~ s ) with an iodine gas phase concentration in the order of 5 s 10-"' mobL [18].

This tloiv rate is approsimatelp 2 to 5 times higher than the tlow rate generally used by othcr

researchers. The tubs size was 2.18 cm \\-hile the Içngth u a s 45.72 m. Provided that deposition

vrlocity is on the order of 1 O-? cm/s or lower. at this combination of tube diarneter and pas linear

vrlocity. a high transmission fraction would be espected. A relatke humidity of 5 0 ° / o and

temperature of 30°C wero chosen as there \vas no auilable litsrature value for the prrdicted

trniperature and relative humidity during postulated accident conditions. The values of kd

tabulatsd bslow are calculated based on the obsrnrd transmission fraction.

Table 3 -4 : Transmission fraction at hioh tlow rate 11 81

It should bç notrd that the loading aftçr four hours would have been less than 10" molicrn'. It is

slioii-n in this thesis that at thesr loti loadings. deposition due to chrrnisorption is not usually

signitiçant. .-Inother observation made by Edson rt.al. ivas that the transinissiun tiactiûn \ras

loti-cr during the initiai period comparrd to the value obtained at the end of the 4 hour

csperiment. No esplmation was offerrd for the phenornenon. Deir mentioned that aHer a psriod

of drposition. the rate of deposition drclinss and rventually a maximum loading can be reached

171. This sugsests that there is a limit in the number of sites availabic: for iodinr adsorption or

that the systsm approaches stead>- state. As a result. the deposition velocity may decrrase as

iodine deposition proceeds.

SS-304 SS-304

Flow Rate (L/min)

187 187

'/,RH 50 50

kd (cm/s)

1.6s10-- 5.1 s IO-'

T (OC) 30 30

Transmission factor

63% or better at 15 minutes 95% or better at J hours

Edson et-al. also includrd a plot of iodinr transmission as a function of tlou- rate as

reportrd hy Widnsr et-al.. The data drmonstratrs the sensitivity of iodins transmission fraction

cspeciall> at vsry low flow rates. The basis of the data was a stainless steel line with 0.64 cm

(0.25") inside diametsr. 16 m long and a deposition velocity 01'0.023 cm js .

01% + O 5 10 15 20 25 30

Flow rate (Umin)

Figure 3.1 : Initial Iodint: Transmission as a Function of Flow Rats [ 1 81

Based on the data prrsentcd in the last tu-O sections. i t ma!. be possible to attain a value of

~sition wlocity in ~ h r order of IO-' cm/s or lower. If this can bc consistently maintained. i i is

ihen possible to achirve transmission fraction of 809.; or higher by selrcting a propcr

conibination of flow rate and tube diameter. Basrd on the data discussed pre\-iously. small

iubing such as 0.64 cni and low linear gas velocity should be avoided. An sxaniplr of such

situation c m be found at hiyh activity monitoring sysrrm at the Pickering NGS PARMS [Mi.

The design flow rate usrd is 1.7 L.;min (88 cm& which is \wy Ion.. Based on some tests

perfurmed by Ontario Hydro. the transmission fraction obtained is betwren 1% to 18%. Ontario

tlydm trsts rrsiilts will be discussed furthrr in a srparate section.

7'hr valus of deposition veloci ty. hoivever. ma- be a strongcr hnction of othrr operating

conditions such as I I gas phase concentration. tempenture and relative humidity. Thrsc oprratinp

conditions nia? be more dit'ficult to control sincr thry drprnd on the atmosphrric conditions

\vitliin the containment building. Thesr parnmrtrrs ma' affect the deposition velocity by altering

the naiure of the interaction betwern iodine and the steel. This is discussed in the foIlo\i-ing

sections.

3.4 lodine Gas Phase Concentration

Therr ha\-e bcrn few rrsearchsrs who haive investigated iodine retention with gas phase

iodine concentration as ont: of the main parameters. One of the niost extensive data sets \\-as

ohtainsd by Deir [7.17]. The esperimcnt was divided into two catsgories. low Bas phase

concentration and high gas phase concentration. The low gas phase concentration esprrimttnt

rangsd betwecn 10" mol/L and 4~10 ' " mol/L whilc the high gas phase concentration ranged

from 1 O-' moPL tu 8s 1 O-' mol/L. The t~ibe test sampleç wrre 1 j cm long pieces of SS-3 16L

tubing n-ith a 0.64 cm outside diameter.

Bassd on the rrsulrs of Deir. i t appears that deposition wlocity is less sensiti\.r to

1-ariaiion in -as phase concentration than to a combination of temperature and relative humidity.

Gcnerally. the values are on the ordcr of 0.1 cm's or hiphsr. At high gas phase concentrations.

tciiiperatures hiphtir than 70°C and ION- humidity. the deposition velocity decreased to bctnven

2s 1 O" cm/"; and 1 O-' cm/s.

An additional observation made by Deir is that at high gas concentrations. saturation

valurs for the iodine surface concentration sxist. He suggests that there may br a limited number

of adsorption sites such that the deposition velocity is high in the begiming and continuously

declines as the espsriment proceeds. Eventually. the amount of deposited iodinr reachrd its

masimum value. This ma! explain the phenomena obsen-ed by Edson and Duce [ IS] . The!.

found that dunng the tïrst 15 minutes. the transmission factor w s as low as 63%. At the end o f4

hours. howsver, the transmission fraction kvas raised to 957'0 or higher.

Slorris and Nichols perfomed some esprrimenrs under sirnilar conditions [?O]. At a hiph

cas phase concentration. 20°C and [ou. humidity thçy obtained a dsposition velocity of 0.12 çm;s - \\.hich is similar to values obtained by Deir. .At a low \-as phase concentration the! found that

deposition i*sloçity dscreascd to 0.048 cnts. A similar trend was obsrn-ed at an r1svatt.d

temprratiire of 1 50°C. At this temperature. the high gas phase concentration yielded a deposition

velocity of 3 . 6 ~ 1 O-' c m s u-hile at the low pas phase concentration. the deposition \-slocity \\as

rrduced to 9.1 s 1 0%m/s. .-\ccordiny to the data obtainrd b). Morris et.al.. lowering the pas phase

concentration ma! also lower the deposition velocity. Results by Deir. howwer. point out that

thc sA;.çt of temperature and relative humidity ma! be more significant.

3.5 Iodinc Interactions with the Steel Surface

In this section. the nature of iodine interaction with stainless steel surface along with its

passible consequsnces on the rate and estent of iodine retention are discussed. Since the

phenomena ma?- bc strongly affected bu temperature and relative humidity. these parameters are

also considercd in the current section.

Aftcr the iodine is transferrrd from the gas phase to the tube surface. it ma? be physicall!.

adsorbed to the steel surface and it rnay undergo chemical transformation due to reactions that

occur wirh the metal constituents of the stainless steel. Iodine that undergoes these reactions ma'

br irre\-ersibl!. bound to the metal surface. Brlow is some data ohtained from esposing Cr. Ni

and Fe to gasrous iodine. The threc metals are the major constituents of stainlrss steel. The

absence or presence of desorption ma? indicatc the nature of iodinr interaction with the steel.

Table j.5 Iodine ds~osition on Fe. Cr. and Ni at temperature around 388 K ( I 15 OC) [ B I :

Cr

1 1 1 1 0.45 m g h i - was irreversibly bound 1

Ni Fc:

Based on thrse data. desorption from Cr and Ni could not be inducrd. This suggests tlint

the iodine ma). be cherniçally bound to these metals in steel. Dcsorption of iodine frorn the iron

surtàce. on the other hand. could be induced though approsimatrly 7.jO.o of the amount depositcd

codd not bt. desorhed. I t is possible that soms of the iodine ma). have reacted with the iron \\.hile

sonie \\-as mercly physically adsorbed ont0 the mrtal surface. Given the much higher loadin, '1s as

conipared to Xi and Cr. the results suggest that Fe should br the largest sink for iodine among the

metal constituents of steel.

Chernical reactions betwern iodine and stainless steel \vil l produce inetal iodides. The

furmation ot' iodidr niolccules on the steel surface has been obsen-ed by Tyler et-al. after

esposing the steel surîàce to gas containing a misture of C02~CH,I [11.21]. These investigations.

howver. did not suggest an!. speci fic chernical reactions that could producr the iodide. .A breîàh

r.t.al. also found that the dcposited iodine is chemically bound to the steel surtàce [3]. Heilmann

and Funke found that direct rlemental reactions between 1: and stainlcss steel could only occur at

Hours of Exposure 18 18 24

Iodine mglcm'

1 O-! No desorption in damp air observed 0.116

6 No desorption obscned Desorption in d q . air \vas obsen.t.d.

temperatures highrr than 200°C [I-I]. Data presentrd in table 2.5 was obtained at lowrr

tsmprrature. Thcrefore. such direct reactions could not have brsn responsible for the value

obtaincd. Sr\-eral authors such as Deir [7 ] and Tsukauc et-al. [8.9] have sugpssted a mrchanisrn

involving corrosion reactions. Fukuda et-al. also investigatcd corrosion reactions of steel by

iodinr cscrpt that theu carried out espsriments at very high temperature [3]. Interestingl?..

Fukuda st.al. also found that irradiation would make the steel more prone to corrosion attack.

The reason is that the esposure to radiation could damage the oside l a y that initially protscts

the stainless steel.

3.5.1 Effect of Temperature and Relative Humidity

1 corrosion reaction is electrochemical in nature and would require the pressncs of water

on the surface of the iubing. It should thrrrfors be espected that hightir gas water content if-ill

assist thc process. Furthsrmore. it is also possible that increasing the temperature \vil1 also

accslsrarc the corrosion procsss. Deir [7j found that increasing the tube temperaturc to 40°C at

Iiigh tiumidit> incrcasrd the rate of iodins rctention. Hou-cvrr. when tlir temperature \vas raised

tu ü b o w 70°C. the rate of retention was substantialll- lowrred. Deir suggssted that at this

condition the availability of water on the tubs surface \vas significantI>. reducrd and. as a result.

the rsaction couId not proceed.

Grnco et.al and Abretàh et-al. also found that the rate of iodine retention \vas increassd in

an air-steam en\.ironment [19. 333. Lee and Jester also carried out some esprriments under

diFferent relative humidiiies and ambirnt temperature[26]. At higher relative humidity. the rate of

retention or deposition velocit). \vas higher. Morris and Nichols also obsen-ed a similar increase

of iodinr deposition at high relative humidity [20].

The above rrlationship brtween temperature or relative humidity and the deposition

\.clocit!. supporrs the theory of increasing iodinr retention dus to corrosion attack of the stainlsss

steel. Tsukautt er.al. have inwstigated such attack on stainless steel coupons (91. In thess

rsperiments. the stainless steel coupons \ivre rsposrd to a gas tlow containing l2 undrr high

humidit!. çonditions. The! obsened that pitting corrosion occurred on the steel surface. The

corrosion attack began on several active sites. From these initiation points. the reactions only

propagatcd somr of the time. A delay period. or incubation time. \vas obssn-rd tirfore the

corrosion reaction proczsdsd.

Soms factors that can at'tect the propagation of corrosion are the rsistrncr and growth ot'

w t c r droplcts or films on the mctal surfaces. diffusion of iodine into the droplets or tilms and

evaporatioii or resusprnsion of iodine from this small arnount of \vater. Therefore. the operating

conditions such as temperature. relati\.e humidity. tlow rate and iodinr gas phase concentration

can influence iodine retention b>. affectinp the above phenomena. .-'dditionall>.. temperature and

relative hurnidit!. ma!. also influence iodine retention rate b>- incrrasing the ratr of an! corrosion

rcaçtions that ma?- occur. The impact of thrse comples phrnomena on iodinr rrtention ratr have

not bezn studied thoroughl~..

Tsukautt et.al. also obscrwd that one of the consequencrs of the corrosion reactions \\.as

the formation of triiodide molecules (I;-) [8.9]. Tt-iiodide could be formed by the reaction

brtwern 1, diffusing from the gas phase with the 1- created on the steel surface throiigh the

cliemisorption procrss. The presence of water is required for the reaction to occur. Triiodide is

corrosive in nature. Thus it may increasct the rate of corrosion of the steel. Highrr temprraturs

and higher re1atii.e hurnidity ma? serve to accelcrate the reactions as well. Vrn. high

trmpsntüres. howrvcr. may reduce the arnount of surîàce water and consequently the estent of

the chernical reactions.

Corrosion attack by iodine ma! introducs further consequences. Elemental iodine in the

water phase that attacks steel surfaces should be converted into non-volatile iodides (1-1. These

iodides arc highly soluble in watcr and immediately dissolve. As a resdt the samr steel surface is

ai-ailablc for tùrthcr attack by iodinr. Thcreforr. it is possible that the sequrncr of the corrosion

rsactions \vil1 cause the steel to be capable of adsorbing a higher arnount of iodine than if simp!e

physical adsorption \vas occurring on the surface.

3.6 Steel Type : SS-304 and SS-316

During 1989 Ontario Hydro invcstipatrd the nrcessity of replacing the csisting SS-301

sarnpling linr with SS-316L [17 ] . It \vas cspected that iodine retention could br reduccd by this

change.

Table 3.6 Metal comric.isirion of SS-301L and SS-3 16L 1281(a) :

Both types of steel have hiph chromium content. I t is generally known that the existence

of CrO, on the steel surface provides the steel with its stainless qualitp and. therefore. protects

the main body of the steel against any chernical attack from the outside environment. The main

304L 316L ( a ) Composition ma? vary depending on the manufacturer.

Weight Percentage C ' si Mn

1.2 0.8

0.014 0.015

0.00 0.66

P 0.036 0.032

S 0.007 0.001

Cr

18 17

Yi 10 13

k1 O

2.1

Fe Balance Balance

difference betwecn SS-316 and SS-304L is the high molybdenum content in SS-36L. High

molybdrnum content in SS-3 16L adds to its superiority against pitting as w l l as çrsi-icr

corrosion attack. Thsre haïe been a numbcr of studirs showinp the pood corrosion resistance

propsny of steel with high molybdenum content [29. 30. 3 1 1.

Despitr the supcrior propcny of SS-3 l6L. thrrr is no data to support its bettsr

pt.rfomiançe when the material is used in a radioiodine sampling system. Lrr and Jsstrr

pert'ormed esperiments with both types of steel. under ~.arious le\-sls of relati1.t: hurnidit>- at

ambient temperature.

Table 3.7 lodins drposition velocitv on SS-3 16L and SS-3OJL sam~ling linrs 1311 :

1 k, (cm/s), Temperature ambient

Based on the results above. SS-3O4L performed brtter than SS-3 16L at al1 the relative

Ii~irniditirs ssaminrd. This is contrary to what is espected. A rationalization of the phçnonirna

Flow Rate (Llmin)

SS-316L - # 1 1 65.1

could relate to the Ion. tcmperature at which the esperiment u-as esecuted. At ambisnt

NOTE : high [LI, = 6.25s l o ' h d ! ~ . high ilon. : 65.1 L!min (most tests). arnbient temperature 2.29 cm ID (0.90" ID). L = 6.096 m. (surtà~e:i.olume).,,~, = 1.78 cm-'

SS-316L - #2 SS-SO-IL

temperature. it apptrars that both SS-304L and SS-3 16L are able to rnaintain thrir stainless

%RH > 80%

4.5s 1 O-- I s IO- -

1 .Ss 1 O-?

%RH < 20% 50% < %RH < 70%

65.1 65.1

qualit>- such tliat onl!. a rrlativrly low amount of iodinc can be dcpositrd. This. howevrr. does

8s 1 O-' 2s 1 O-' 6s 1 O"

not ruls out the possibility that corrosion reactions take place to some extent. The inability of the

3 -8s 1 O-' 5s 1 O-'

1 -2s 1 O-'

corrosion to propagate may have prevented the steel from bccoming a large iodinr adsorber.

The sffects of temperature is crucial since man) iodine sampling linrs have trace hsating

systcms. The purpose of the heating is to prevent freezing of the lincs. as well as to prevsnt watrr

condensation alon- the lines [KI- Most sampling lines operates at a temperature betu-sen

ambient and 50°C. The trace hrating ma). in fact havr a negative impact on the amount of iodinr

losses. Higher temperature ma! increase steel suscrptibi1it~- to corrosion attack. Cnder thrse

conditions. it is possible that a higher content of molybdenum would increase the protection

against corrosion attack. and hencr SS-3 16L would have lower retention than SS-301L.

Tsukaur et-al. havr carrird out sorne esperiments at 60°C and relative humidil!.

üpproaching 100% [9J. Their rrsults showed that even at this elevatcd temperature. there is no

clear difference in the rate of pitting corrosion propagation bet\i.ren SS-304L and SS-3 I6L. In

some instances. SS-3 l 6 L appeared tu bi- more susceptible to corrosion attack as compared to SS-

304L. The nscrssity of hrther research to justif\- the replacement of SS-304L ivith SS-3 l 6 L is

no tcd.

3.7 Other effects

Several investifators have indicated that there could be several 0 t h hctors tvhich would

affect the rats and estent of iodine retention on steel sampling lincs. Thrse factors includr the

nurnber of fittings and bends in the sanipling system as well as pretreatmsnt of the steel tubing

prior to installation.

3.7.1 Fittings and Bends

Fittings and bends are locations alone the sarnpling line where the tlow configuration

might change. A highrr levA of turbulence ma! be inducrd around these rrgions. .As a rrsult.

deposition of iodinc ma? be increased due to a higher mass transkr rate. Several authors such as

Gcnco st.al [19] and Edson etal. [6] haw observed higher level of iodine retention around

compression fittings. Genco et .al. furthrr suggestsd that the main cause of this increased iodine

rçiention Kas damage of the oside films as a rrsult of cutting of the rubiny. Gçnco et-al. also

adcicd that stress caused by compression tittings ma? increase iodine retention. To test this

hypothesrs. Genco et .al. placed screw-type hose clamps at two locations along the tubing. The!

obsrtn.rd higher iodinr retrntion at the points wherr the clamps were tightenrd. Visual

esaminations also re\.ealed a localized black drposit beneath the clamp thus confirming their

tiypotheses. Therrfore. it may bc crucial that durinp the design process attention is given to the

nuniber of fittinys and bcnds between the sarnpling and the mrasuremsnt points.

3.7.1 Cleaning the sarnpling Lines

Cleaning of the sarnpling linss may affect the estent of iodine retention. Soms

containinants that rsist inside the tubing ma). interact with the iodine from the gas phasc: rlius

causing higher retention rate. Lee and Jester found that when the tube \vas used as received. there

w r e somr points u-here iodine deposition \vas substantially higher than any other region within

the tubing [26] . Upon cleaning. this localized high iodine deposition disappeared.

Edson et-al. howsver obtain difirent resuit [18]. They found that cleaning had littls to no

dfect on the mount of iodine retention. The differences in result ma) suggrst that the

contaminant ma). br specitic to tubing from a particular tubing manufacturers.

The operating histoc. of a sampling line may also afkct its performance. Sonie forrign

o bjeçts such 3s c harcoal tiltcr part ides or an!. ti ltrr rnatsrial fragments may be entrainrd during

an)- rrgular tests or dunng normal operations. This ma! cause high localized iodine retention.

Regular cleaning of the line is recommcnded.

3.8 Tests at Ontario Hydro

.-1 signifcant number of tests have bern perforrnrd b~ the Ontario H>.dro at al1 three of

its nuclear po\t.er stations. blost tests in\.ol\.ed threr species. namely uranine paniculates.

molrcular iodine vapor and mrthyl iodidr. The results from thesr trsts have been mised. Some

tests showd excellent iodine transmission factors but some did not. During the tests. temperatlire

and relati~e humidity were not controlled or measured. Ambient temperature and relative

humidity must thersfore be assumed.

Pan of thesr tests wsre prrfonned at the eshaust stack monitoring systrm. These are the

stacks through u-hich an!. radioactive gases and paniculates are relsased dunng normal operation.

!n an accident conditions. any release should occur through the EFAD systems. Thereforii. some

of the trsts w r e also pcrfomed for the P.4RMS monitoring systems. Tests performed using 1 2 ( - ,

at tlic Pickering NGS during 1990 to 1993 are listed in tables 3.8 to 3.1 1 ..

Table 3.8 : Pickering NGS Unit 4 - eshaust stack 1321

Table 5.9 : Pickering NGS Unit 1 - eshaust stack [3;1

Iodine Rrcovery ( 3'0)

107+ 16 1 2 6 i 19

Test date February 1990

Table 3.10 : Pickerin~ NGS PARMS - low activity monitor cl61

Flow rate -

Test date June 17 - 21 1991

1 Test date 1 Flow rate 1 Iodine Recoverv (%) 1

Flow rats I Iodins Recovery ( 9.0 )

1 Sarnplr linr SS-3 16. ID : 1.57 cm. lrngth : 9.114 m. surface to area ratio : 2.54 cm-' I

65 Limin

July S. 1992

Ssptsmber 22. 1992

Tahle 3.1 1 : Pickerinc NGS P.4RMS - high activitv monitor r161

1

19. 67. 56

5 1 L,"min

51 Llmin

71. 82. 85. 78. 75.67

30. 69. 89, 85.93

Test date July 13-1 5. 1992

Septeniber 3 - 2 4 . 1992

Sarnple line : SS-3 16. ID : 0.457 cm . Iength : 9.144 m. surface to area ratio : 8.75 cm-'

Sov. 30. 1992 ( incrsase tlow rate )

February 23-29. 1993 ( rc.duc.s surface to wlums

ratio by ;C)",o)

The lou-est transmission factor was obtained from the PARMS-high-radiation-monitoring

sampling s).stcm. I t \vas hypothesized the low iodine recovery may have bren caused by the

small tubs inside diameter or the low flow rate or the combination of the two factors.

Flow rate 2.5 L/'min 2.5 L:'min

12- 1 3 Lmin

2.5 L:'min

Iodine Recovery ( O h )

2. 5 . 9. 2. 1 4. 18. 11 . 8 .3 Retest of prsvious

resul ts 43. 35. 13. 6.4

<1. < I . <1.2.2

unesplainsd declinc of iodins recovsry

To test this hypothrsis. in the espcriment perfomrd on November 30. 1993. the tlow

rate uas increased tiom 2. j L,'min to 12 Lhnin. thus increasin the @as linear vrlocit). The

rrsults showed that. initiaIl'. the iodine recovep- \vas improved to as high as -lY%. The recovsry.

ho\ve\.èr. continuously declined in the subsrquent trsts to 4% at the end. No esplmation was

ci\.sn for the phenomrna. III the nest tests scries. the inside diarneter of the line was increased b

from 0.457 cm" to 0.74 cm-' by using the PXRMS a u s i l i q . sarnpling lines. The surface to

volume ratio was drcreasrd by 39?6. Dunng this trsts. the air tlow rats \vas kept at a low \ d u e

of 7.5 L min. The outcome did not show an)- improvement. The iodine recovery remainrd below

2 O . 0 . Similar tests results wsre also obtained from Bruce KGS eshaust stack monitoring as u d l as

PARhIS systsms.

3.9 Surnmat-y

Based on the available literature. i t is clear that thrrr is a relationship bctween the amount

of iodinr retention on stainless steel sarnpling lines and the operating parameters which include

tube insidr diameter. Iength of the sampling line. the number of bsnds and fttings in tlic systtini.

the gas tlow rate. temperature. relative humidit!. and iodinr gas phase concentration.

Thsoretical1~-. iodine transmission fraction could br: cstimated by using correction factors

hüscd on these parameters d o n g with the availabls smpirical data. Despite the large body of data.

there are still diffïculties in applying the rsperimental results to a full scais systern n.ith

contidence. The main problrm is that the underlying phrnomena which is responsible for iodine

retention is still not vepr well understood.

.According to the data presrnted. an? surface reactions that ma). occur could in f c t play a

\.rrv important rois in determining the rate and estent of iodine retention. Two w-iables that can

be identified as very important in determining the surfacr reactions are temperature and relative

humidity. This raisrs the question as to whethrr gas phase mass transfsr or surtàcs reactions \\-il1

have the most dominant effect on the rats and sstent of iodine retention. For this reason. the

çurrent research effort \vas also directed toward answring this question.

Deposition i-elocity has been ussd estensi\-cl'. to describe the iodine retention rate. It is

important to point out that deposition ~.elocity ma). change with time. Deposition vrlocity ma!.

decrease as iodine deposition proceeds. due to depletion of the adsorption sites or the inhibition

of an) surface reactions. Furthermore. It is possible that dçposition velocity is a fiinction of

iodint. gas phasr concentration. The -as phase iodinr concentration ma). aIso Vary with time

during an accident. This adds to the already complrs situation since @as phasr concentration is

the variable to bs measured.

In the esistinp literature. there is inadequatr data to resolw the dii'î'errncr. if an!-. brtn-sen

the iodine retention behavior of SS-304L and SS-316L. Despite the espeçtation that SS-2 16L

should perfomi better. there appsars to br no difkrencç heiween the two types of steel. In faci.

soms results suggcst that SS-304 will psrform better than SS-3 16L. Thsrdore. in addition to

inwsti&ng the effrcts of operatine conditions such as temperature and relative humidity. the

purpose of the current research \vas also to study the diffrrcnces between SS-3OJL and SS-3 16L

at various temperaturcs. relative humidities and iodine gas phase concentrations. It was hoped

that the results would lead to a better understanding of the underlying mechanisms that are

responsiblr for iodine retention on stainless steel sarnpling lines.

4. Experirnental

The technique to determine the concentration of iodine dspositrd on the steel surface

131 131 relied on the labeling of stable iodine with radioactive 1 1 was chosen duc to its relatiwly

shon half lire of 8.040 days [;A]. The method allowcd the detection of iodine surface

concentration as low as 10-" rnolicm' which is l e s thûn one hundredth of a rnonolayer. The

dstection limit deprnded on the spccitic actkity of radioiodine being used in the csperiment

131 which describes the ratio of radioactive 1 to stable iodine. Typically the speci tic activit).

ussd Kas betwesn 1050 10' Bqmol.

.A similar technique was usrd to determine gas phase concentration. This measurcment

\\as perforrnsd by passing the air stream through TEDA irnpregnated charcoal filtrrs

çontnined within 0.64 cm ( I '4") tyson tubing. The charcoal tilters u-ere subsrqurrntly countsd

on an LKB gamma counter containing a 3s3 iveil type sodium iodide detector. The drtector

131 has an efticisncy of 5S0'o for the 365.4 keV gamma ra)- tmitted b>. 1. By using the knou-n

specific cictivity. the gas phase iodinr concentration \vas calcuiatrd.

1.1 Esperimentai Apparatus

The arrangement of the apparatus is show in tigure 4.1. The apparatus as well as the

niethodology Iras originally designed by Christopher Deir at the University of Toronto [7].

Some modifications were made to it for the current research effort.

Gas

Prob t -1

4 one way nive

u t h r e e waÿ vd~- t Ia Source

Gamma Detector -2

Unit

L

Figure 4.1 Esperimental apparatus

-4s the apparatus allowed concurrent tests of two steel specimcns. duplicatr in

mcasurernrnt !vas obtained in ew.sp. espsrimsnt. In spite of this. in some cases it u s

necessrin. to repeat an ssperirnent to contirm the trend that \vas obtained. One of the main

difticultics in the design of the equipment \vas the hc t that iodins is ver\- rcactiwc Iodine

\vould deposit on most matrrials. To prevent excessive loss of iodinr. tefion ua s chosen as the

niain materiai for part of the apparatus in contact with iodine.

The miount of iodine retained within the steel tubing was determined usine two 2s'

sodium iodide scintillation detectors. The signal from the detectors was sent to a 386 IBM

compatible personal cornputer rquipped with two APTEC Multichannel Analyzers ( MCArds)

eaçh haLing a 1 O2-l charme1 capability. .An APTEC detection prograrn called Supenisor was

ussd to collect the data. The prograrn \vas set to collect a mcasurement every 150 to 600 s

dspcnding on the espected rate of retention. For a very slow procrss. such as desorption. a

600 s counting time was more suitable while for adsorption a 150 to 300 s counting time was

ussd.

4.1.1 General Features

The system allowed the indepcndent control of relative humidity. temperature of the

strci specimens. gas tlow rate as u-el1 as the gas phase iodine conçrntrations. The systsm also

allou-ed two steel specirnens to be tested concurrently. This was important in obtaining

rsliablr data since the results frorn the two specirnens could be used to ver@ one another.

The gas Stream entsring the systsrn was split into tlires strearns with independcnt tlou-

cuntrol systems. -4 small proportion of the gas stream \vas diverted to pick up molecular

iodint.. .-1 bubblsr was placed prior to the iodins generator vsssel. The purpose was to prewnt

the dèplstion of sater inside this vessel. More stable concentrations of iodinr could be

obtained this way.

The relative hurnidity of the gas could be set î'roni near 0% to as high as 100°/& Lou-

range hurnidity was achieved by using an air dehumidifier systern which \ a s placed at the

inlet before the air strearn was split. The dehumidification unit. which is not s h o w in the

diagram. consisted of dnerite to absorb the moisture and molecular sieve to prevcnt an).

paniculatrs from entering the system. .4 higher level of relative humidity could be achieved

b>- increasing the flow that passed through bubbler- 1. Any intermediate relative humidity

could be obtainrd by nltrring the ratio of the gas tlow that passed through the bubhler to the

one that bypassrd it. This stream thrn passed through a temperature and relative humidity

probe and finally \vas rnised with the strearn that carried the iodine. As mrntioned previously.

only a srnall proportion of the gas stream was divened to the iodins genentor vessel. This

stream. howver. wouid always contain some humidit:.. This made it impossible in practics to

rracli a relative humidity of 0%.

1.1.2 .Measurement of Cas Temperature and Relative Hurnidity

For the purpose of msasuriny pas temperature and relative humidity. tu-O probes w r e

inçorporated into the systrrn. InitiaIl! only one probe kvas used. The first probe u.as piaccd

prior to the mising point betwecn the main gas stream and the stream that carricd the iodinr.

This probe was never contaminated with radioiodine. During the later stages of the study. a

second probe was installed. This probe was placed atirr the steel specimsns. The purposr \vas

to vrriîj the rradings of the tïrst probe and to contirm that thrrr \\-as no signilicanr change in

the sas temperature and humidity during transport to the steel specimens. The second probe

d so al lowd the impact of the iodinri genrrator on relative humidit? to br determined. This

\vas rspecially important for esperiments under lou- humidity conditions. blonitorinp of the

pas phase temperature \vas ver). crucial since in sorns esperiments the stainless steel

specimsns were kept at a temperature as high as 90°C. Due to the very shon residence time

of the pas within the tube. only slight increases of up to ?OC were obsewrd.

4.1.3 Temperature Control of the Steel Tubing

One of the main variables being investigated was the temperature of the steel tubing.

Cleating of the steel sprcimens was carried out by placing the steel sample inside &ss tubing

u.hich \vas hrated using heating tapes. The control of the temperature was achirved b!.

attaching themocouples on the outsidr surface of the steel samples. The temprrature read by

the thsrmocouples uwe k d into PID temperature control systems which \vers used tu

manipulate the poxer output of the hrating tapes. The method allowed the temprrature of the

tubings to be maintained to within I 3°C of the set point.

In the beginning of the study. it was noticed that there was an asial temperature

gradient xithin the tube. This occurred due to cooling at both ends of the steel samples. 7'0

reditce this temperature gradient. modifications were made to the wa). in which the hrating

tapes w r e wapped around the glass tube: heating was slightly more concentrated at both

ends of rhr tiibrs. -4 srnailsr temperature gradient kaas obtained after this modification LW

applied. f l o w w r . despite this modification. a temperature ditference of up to 3°C \vas still

observrd especially during csprrimrnts at high temperature. To rnonitor the temperature

distribution. t ~ o more themocouples w r r installed on rach sample. one at the inlet and the

other at the outlst.

4.2 Gaseous I2 Generation

I I I this esperirnent. rnolecular iodine kvas generated by using a triiodide mrthod. The

process \vas achirved through rraction 2.5 below.

Ihq - 1- t a q ~ * Lq~ (R 2.5)

The fonj-ard and reverse rate constants of this reaction are 1 O-'() M-' .s-' and 1.3;s 1 O' hf ' . s-'

respectively [7]. As 1: left the solution to the gas phase. it uould be immediately replacrd b!.

the dissociation of 1;-. By using this mrthod it \\-as possible to obtain a relatively stable gas

phase iodinr concentration in the range from 1 O-' to 1 O- ' ' moI/L.

The solution was prepared by adding sublimed iodine crystals ro a Nd solution. To

this solution a sufficient arnount of radioiodine tracer was thrn added. The arnount of '"1

rrquired dependrd on the speciiïc activity required during any particular ssperiment.

Espsriments at 90°C and low humidity. for esample. required a higher specific activity as

conipared to rsperimsnts at -IO0C and high humidity.

4.3 The StainIess Steel Samples

Tu.0 t!-pes of steel tubing i\.sre being esmined. SS-2 16L and SS-304L. The

sptxitications of the steel specimrns are given in the following table.

Table 1 . 1 Steel sanlple specifications

No sprcific pretreatment was applied to the steel samples other than smoothing the

ends of the tubing sections and blowing air through the tubing to remove any dust particles or

steel fragments lefi during cutting of the tubing. The air blowing was performed for

Type

SS-3 16L

SS-304L

Outside

Diametcr

0.64 cm

Wall

Thickness

0.08 1 cm

Inside

Diameter

0.472 cm

Lcngth

25.4 cm

Pretreatmcn t

air blowing

u \vas approsimatel!. one half hour. Before an experiment was startrd the inside of the tubin,

inspected visually to maks sure that therr was no obstruction present.

In the brginning of the studu. the length of the steel samplrs \vas 6" ( 15-24 cm ). It \ras

found that iodine retention at rrgions around the inlet and outlet \vas signiticantl- higher thm

the average retention in the middle part of the tubings. I t was believed thar the higher iodine

retention \vas due to turbulence at the inlet region or damage of the oside Iayrr that made the

steel surface more prone to iodinr chernical attack. To reduce the impact of this iodine

distributiijn. the length of the tubing samples was increased as much as possible i w n the

spacti constrain u-ithin the fume hood. A 25.4 cm length was chosen. As a result of increasing

the tubin2 lcngth and collirnating the gamma detectors using lead. only iodine deposition on

the central 1 Scm was monitored for radioiodine acti~ity. This [vas the only pan of the samplrs

that could bs detected by the sodium iodidr: detectors. Subsequent tests rewalsd a tlat iodine

distribution nithin this region. In addition. any iodine that [vas retained by the tetlon tittings

wnnecting the steel samples \\as also not detected.

Somr tests w r r also psrfomied on glass tubing with similar dimensions as the

stüinless steel sarnples. Glass is considerably more inrrt cornparrd to stainlrss steel in regards

to L,,, deposition. Hence. it \vas sspected that iodine deposition on glass surfacc would bç

much lower. The purpose of such tests was to confirm that the measured iodine deposition

\vas due to iodine interaction with the steel surface rather than to an). other phenomena such

as deposition on the teflon fittings.

In addition to the tests performed on stainless steel tubing. some esperiments w r e

also carried out on stainless steel compression fittings. The purpose of these esperiments was

to study iodine deposition in the region near the fittings. These esperimsnts w r e motivated

by the çoncern that îïttings rnight contribute substantially to the total loss of iodine alon- the

sampling lines. The rsprriments were perforrned by inserting a stainless steel titting at the

middle part of the steel samplrs. The activity of radioiodinc dong the tubing including the

stainlrss steel fitting \vas rnonitored throughout the ssperiment.

4.4 Experimental Parameters

Table 4.2 belon- shows the esperimental conditions that w r e usrd in this study. The

temperatures @en are those of the steel surface while the relative humiditirs are the ones

assvciated with the gas Stream.

Tabis 4.1 Esperimental parameters

Temperature (OC) Relative Humidi l Iodine Gas Phase Concentration

Tests w r r prrformrd for e v e n possible combinations of the abow esperimental

settinps. The classitication of the iodine gas concentration into Ion.. medium and high \vas

arbitra-?. .As discussed in the litsrature survsy. research into the effect of iodine gas phase

concentration on the rate and estent of iodine retention has been scarce.

4.5 Esperirnental Procedure

To some estent the esperimrntal procedure that was used in the investigation has been

mentioncd in the precrding sections. The follou-ing description is given to compl imrnt the

carlier sections. by clarifving a few points and describing others that have nor bern discussed

prsviously .

Steel tubing sections that had bsen previously prepared for the rsperiment were

insrrtsd into the detcction stations. The tubes wrre connectsd by using tetlon tittings at both

ends. The tittings were then hand tightrned. The tubes were carefully placrd inro the dctection

systrrns so that a similar gcometry \vas maintained in each rsperiment. This \vas crucial to

snsure that the detecton collrcted gamma swnts that originated on1y from the relevant area of

the tubinp. -1s i t was esplainrd in section 4.3. although the length of the tubing was 25.4 cm.

the area thai was monitorcd for iodinr deposition was only the central 18 cm". Lpon proper

installation of the tube samples. the drtectors were put in place above the sarnplss. The

temperature controller was then set to bnng the samples to the set point temperature.

--Ipprosimately one hour Las required to allow the set point to br rrachrd and stabilizrd.

Once the temperature stabilized. the Supen-isor program \vas set to collect and save

the data ewry 150 to 600 S. Before an ssperimsnt began. the region of intrrrst of the , w n m a

dctrction systeni was calibrated. This calibration \vas necessary sincs some driftiny of the

region of interest had bcen observed in sevrral early esperimenrs. tspccially in espcriments at

higher tcmprratures. The temperature changes between the rsperiments caused the dritiing

problems to deteriorate making the calibration procedure a necessary part of the experiment.

The calibration waas performed using a short section of glass tubing that containrd

somr charcoal which \vas loaded with '"1. Three main gamma lines of " ' 1 were used in the

cnlibration process. i . ~ . at '84.3 keV. 364.5 k V and 637 keV. The region of interest was then

set to collect data corresponding to the iodine peak a< 344.5 keV which is the most dominant

camma emission associatrd with the drca!. of I3'I . b

.Ahsr the calibration procedure was complcted. the vacuum pump was turned on. The

zas tlow \vas adjusted to the set value of tlo\v rate and relative humidity. The temperature of

the gas Stream \vas ambient in al1 the csperirnrnts. The tlow rate used in this stud>- \\as 1.5

L min. Bsforc an rispcrirnent \vas started. the gas concentration was çhecked to ensurr that the

\.al ue \vas ni thin the inténded range.

The gas sampling process was carried out by inserting a shon section of tygon tubing

containing TEDA impregnatcd charcoai tiltcr into the gas sarnpling port. The gas tlou- \vas

allowed to pass through the tilter for a psriod of 30 seconds to 1 minutes. Longer timrs u-err

requircd to obtain an iodine activit). that was \\-el1 above the background value. The charcoal

tilter u . 3 ~ thsn countsd in the LKB Cornpugamma system. Cornhining the intorrnation on the

sprcitiç cictivity. the iodins activity collrcted in the tilter and the total volums of gas that

passed through the tubing. the iodinr gas phase concentration could then be calculated.

Once all of the initiation procedure \vas complrted. the Supen-isor program \\as set to

commence the data collection. During the îirst hour. bac kground radiation L i a s el-aluated.

Throuphout this period. the gas Stream was allowed to tlow in the by-pass mode. At the end of

this first hour. the by-pass line was closed. The gas began to pass through the steel specimens.

At this point the first gas sample was taken. Subsequent sampling of the gas \vas carrird out at

iiiterwls of one hour to 1.5 hours. Most esperirnent lasted for about four hours. Some

esperiments had to be terminated before four hours had elapsed to prevent accumulation of

0 had to tao high a radioiodine activit). on the tubing. This \\-as for safety reasons as the tubin,

bc tut at the end of the esperirnrnt in order to evaluate the efficisncy of the detectors.

When the temperature of the steel samples had sufficiently cooled. the steel samplrs

wsrr removcd from the apparatus. The tubing \vas cut into 3 cm sections. with the escrption

of the inlet part \\.hich \vas 5 cm and the outlet section which \vas 2.4 cm. The tubing sections

L\-ere subsequcntly countsd in the LKB Cornpugamma. By comparing the gamma actiiit!.

measurrd using the LKB with the final counts from the on-line drtectors. the efficiency of the

on-lins dçtrction systern was calculaisd. In most esprriments. the tificiency of the _ c~amrna

Iine at 364.5 keV ranged between 1 O'O to 1 .gO/O. This average value of effïcirncy accountrd for

the txiation at points along the steel tubings due to diffrring distances from the dstectors.

in somr esperiments. desorption of iodinr from the tube surface \vas carried out.

Drsorption is a much slower process compared to drposition. The Iength of the ssperimcnt

might last between 24 hours to three days. Due to time constraints. desorption uas not

performed at the end of cvrry esperiment. Furthemore. for some conditions such as at hi&

tenipsratures. low humidity and low gas phase concentration. v e p low iodinr

retention was obsen-ed. Desorption at the end of thesc rsperiments would ha\-e lousred the

amount of iodins on the tube surface rnaking it impossible to accuratel>- determine the

t.i'ficit.nc> of the detectors at the end of the esperiment.

4.6 UV Spectrophotornetery

To complement the results obtained from the procedure involving radioiodine. sorns

esperiments utilizing spcctrophotometry were also performed. The purpose of thrse

rxperirnents was to identifv an? products of the reactions ocçurring on the steel surface such

-i - ris Fe- and ri? The method kvas based on the technique developed by H a n q et-al. ~vhich

in\-011-ed the addition of 1.10 phrnantroline to an aqueous solution containing the ~ e ' - and

~ r ~ ' ions [ 3 1 . This procedure allowsd the concentration of ~e: ' and the total concentration of

Fe tu bs cieterrnined.

In ordrr to prrform the above analysis. a solution containing the reaction products

froni the insidr surface of the srrel tuhings was required. Steel samples were esposed to a !as

Stream contnining iodinc for 4 hours at ambient temperature. The inside of the tubing \vas thcn

rinsrd with 50po HNO, solution t'or approsimately 10 minutes. The solution was anal~.zrd

~ising L'arian Caq. 3 CV-WS spectrophotometer.

In addition to this. some esperiments were also carried to investigate the voiatilization

ol'iodinr from FrIl solution. This esperimcnt kvas performed toward the end of the projects. It

was performed to conîirm the observation that L,,, \vas produced from a saturated solution of

Fe[:. The analysis Lvas based on the reaction of I? with 1- producing 1;'. The concentration of

1;- \vas rsamined usine its absorption at 350 nm. This method had been demonstrated by

Palmer et.al. [XI and was later used by Tsukaue et-al. [8.9]. Palmer et.al. reported a value of

3.750 L~rmol.crn for the molar absorptivity of 1;- at 350 nm. This esprrimrnt along u-ith the

results are discussed in more detail in section 5.4.3.

5. RESULTS AND DISCUSSION

[n this chapter. the results of the esperiment are discussed'. In section 5.1 the variables

&tining the rspenmental conditions srudied are classitird. In section 5.2. the main deposition

parameters usrd to drscribed the results. the deposition wAocitu (kd) and maximum surface

Ioading (C,"). are discussrd. In section 5.3. the impact of the operating conditions on thesr

deposition parameters is drscribed. Various esperiments prrformed to evaluate desorption are

presrnted in section 5.4.

In section 5.5. a mechanistic interpretation of the o\.erall adsorption and dssorptinn trends

is presented and evaluatrd in tsrms o l the rsperimrntal resulrs. It is proposrd that 1, is drpositrd

through three distinct mechanisms. Physical adsorption alone occurs ~vhrn gas phase

concentration is belou. 1 O-' rnol!L. A second mechanism occurs for pas conçrntrntion abo1.r 1 O-''

mol L whrn little watrr is present on the steel surface. for esample at low relati\.e hurniditl- or

Iiigh tcmperature. Uhrn adequatc mater is on the surface. a corrosion type chemisorption

niechanism prrdoniinates. Hsre. 1, is brlirvrd to rract at defect sites on the surface to hm1

hygrosçupic Fe12 n-hich attracts more uatrr propagating the corrosion. Cndrr somc conditions.

the propagation is inhibited while for other conditions. no such inhibition oçcurs resulting in

extensive and \-en. rapid iodinr adsorption. In section 5.6. the rele\.cince of the results of this

study tu losses in sample lines is evaluated.

' 0 n l y Sclected esperiments and compilations of the results are presented in this chapter. The deiailed results of e v e p esperiment are presented in appendis A and B.

5.1 Variables Studied

In studying the effects of 1-arious operating conditions on iodine deposition. si-eq-

possible combination of the variables chosrn \vas atirmpted. The range of thesc w-iables m

dtlscribsd in table 5. l below. During the esperiments. the values u-ere set at certain ranges as

cillowd by the esperimental apparatus.

Table 5.1 Rance of Variables hvestioated

The range of values described in table 5.1 \vas designed so that the set points could be

rittriined despitc üny fluctuations that might occur during the esperimrnt. Furthermore. the

conditions in\-sstipated were sslrctsd in ordcr to provide an adrquatr rcprrscntation of the

operating conditions that might ariscr in actual gas sarnpling systerns. both diiring testings and a

rraçtor accident. The b a i s for the classification used in table 5.1 is discussed in appendis D

alon- with somr of the limitations in controlling the variables and the overall experimental

reproducibility .

I

Steel Types

SS-3 16L and SS-304L

Sizs : 0.47 cm ID. and

0.64 cni OD. 1

Relative

Humidity (*)

Low : 096 to 25%

High : 75% to 100%

Gas Phase

Concentration

( MoVL)

Low : 1 O- ' ' to 1 O-'

Medium : 1 O-' to 51 I O-'

High : 5~ 1 0-\O 5x1 O-'

( * ) S e ~ e r a l esperiments were also pertormed at medium relati\,e humidity ranging bet~~een

45"" to 55O.o

Tube Surface

Temperature

( O C )

ZjoC. 40°C.

60°C. 9OCC

5.2 Deposition Parameters

Bcfore discussing the sffects of the operating conditions on iodine drposition brhavior. i t

is important to tirst describe the two parameters selected to represent the trends. Sprcitically.

most of the results c m be describrd in terms of the initial drposition velocity (k,) and the

maximum surface loading (cic'). Lnder man? conditions. dcsorption did not contribure

signiticantl!. to the obsencrd hrhavior. Dcsorption is descnbed in section 5.4.

5.2.1 The Deposition Velocit'; (kd)

The most common paramersr used to describe the rate of I I retention is the deposition

\-t.locit!- (k,) whiçh reprrisents the t 2 deposition rate normalized by the gas phasr concentration.

This parameter is regarded as conwnirnt to use since it can be applied to an! systsm nithout

considering the amount of 1: contained in the pas strearn. In practice. this is especially useful if

L,,, concentration is the variable bcing msasured. Furthsrmore. k, is also practical in the

identitication of the rate detcmiining stcp in the drposition procrss as the value can be directl!.

compared with the theoretical @as phasr mass transfsr coefficient. k,.. This is of significant

iniportançti since once the iimiting step is identi tkd. an!. effort to lirnit iodine lossss can be

t0cusc.d on nianipulating the factors that affect this rate drttirmining stcp. The follon.iny tigure

shows the variation of kd as a function of I I sas phase concentration for SS-3 16L.

Initial Deposition Velocity Vs [U]g SS-316L

Figure 5.1 Variation of k, with gas phase concentration for SS-3 16L

--\s can be seen in t i g r e 5.1 the k, values apprar to be dividcd into two groups in temw of the

rrlati\-t. hurnidit).. At liigh relative humidities. at gas concentration above 1 O-" rnol..,L. kd is

rslati\+sl! constant nith a value on the ordrr of 0.1 cmis. This lack of dependence of k, on

concrntration yivrs support to the assumption undrrlying the use of k,. that drposition rate is a

tirst order tùnction of gas concentration. .At Iow rrlatiw humidities. k, drcreases nith incrtiasing

conccntratiun indicating that the deposition rate is only a u-eak function ol'tlie gas concrntntion.

This trend is not consistent with the tirst ordrr assumption implicit in the definition of k,. This

type of behavior has never before bern idsntif r d in relation to the deposition of 1: on steel. .A

niore drtüiled discussion concerninp the variation of L, under differrnt operatin- conditions çan

bç found in section 5.3 whilr speculation rrgarding the underlying mechanisms is presrntcd in

section 5.6.

From the results of this research. there is evidence that the use of k, alone ma) not

adequate1'- represcnt deposition trends. A second important assumption in the use of k, alone is

that I 2 loading increases linearly with time. This assumption. in most cases. can only be applied

during the carly stage of deposition. Bryond this initiai period. the deposition rate will ohen

dscline until evrntually a m a ~ i r n u m loading is achieved. Thus. b q o n d the initial stngr of

drposition. the assumption of linear accumulation no longer holds. .An esample of such rssults

c m br seen in figure 5.2 belon.. A drclining drposition rate \vas observed undrr most

expsrimrntal conditions

SS-316 123 C HlGH RH 1 HlGH [12]g

7.OE-07

3.OE-07 N - . lodine (mollcmA2) 2-0E-07 -

z 1 .SE-08 [12]g(mollL) 1 .OE-07

-1 .O€-08 - O .O E+OO 0.0 5i.0 100.0 150.0 200.0 250.0 300.0

TlME (min)

Figure 5.2 Temporal variation of iodinr surface loading

Basrd on tlicse considsrations. caution should be taken in using k, as the oniy parametsr.

.-\lthot~gli kd contains infom~ation un how fast I 2 will initiail!. deposit on the tube surfrice. i t dors

not üIn a!.s dsscribs how deposition u il1 brhave o w r rstendr.d psriods oE timr. Because k, does

not contain information on the estent of iodinr dcposition. it dors not predict the occurrence of

maximum loading on the steel surface. Thus. the use of k, has to be accompanied by rhr

knowledge on how iodine deposition is changing with timr.

This problem has tppically not bern identiiied as crucial mainl'. due to the methodology

used prr\iousIy in studying L,,, deposition. In the literature. the reponed values of kd are

commonly bassd solely on the amount of 1, accumulated on the tube surface at the end of an

rsperiment. or on the fraction of the iodine transmitted through a length of tubing i.è. the

transmission fraction. Hencs. the actual accumulation of 1: on the surface as a funçtion of tims is

abstint. For such sspcriments. a constant deposition rate is implicitiy assumcd. The values of

deposition \docit)- used in this report are only relevant to the initial stage of deposition during

~i.hich a constant deposition rats uas o b s s n d .

5.2.2 Pseudo-saturation Surface Concentration (c:)

-4s mrntioned pre~iously. the use of kd alone which impliss iodine accumulation at a

constant rate. does not predict the existence of any masimum surface loading. Therefors another

parameter must be used to represent the maxin~um loading. The combination of the t\vo

parameters bcttcr rrpresents the phenomena of iodine deposition. Figure 5.3 shows the variation

ot'niaximum iodins loading as a function of gas phase concentration. The \dues include data h r

SS-3 16L and SS-3OlL at ail the temperatures and humiditiss esarnined.

Maximum lodine Surface Loading Vs [IJg

Gas Phase Concentration (mollL)

Figure 5.3 'ulasimum iodine surface loading as a function of @as phase concentration

.-1s s h o w above. the point where the system reached maximum loading varied depending

on the rrle\*ant operating conditions. As there was no single value of saturation concentration. the

term pseudo-saturation concentration. dcnoted b! c,". is usrd to represent this apparent

masimum loading. I t is important to note that figure 5.3 only retlccts some of the espcrimental

rssults. For esamples. esperiments at high and medium concentration. under high relative

tiurnidit!- xith tube temperatures of 40°C and 60°C. did not show any maximum loading : I 2

Iciading continued to climb drspite the higli surface concentration already achirvrd ( > I O '

7

mol.'cm-1. Furthemore. some esprrimrnts. particularly thosr undrr low @as phase concentrations

that resulted in very low iodinr loading rates are not included in figure 5.3. It ~vould have taitrn a

long time for the system to reach masimum loading. hence. man? of thesr csperiments w r e

terminatsd before the plateau region was achisved. Furthemore. in sonic othrr csprriments. the

\.alus u-as belou. the detection limit. thcrefore the!. are not retlected in figure 5.3.

5.2.3 S teady s tate and Surface Pseudo-saturation

hlasimum surfice loading may occiir due to t~vo possible phenornena. The tirst one is the

stead!. state condition. Steridy stats occurs whrn the rate of adsorption is equal to the rate of

desorption prewnting an). further 1, accuniulation on the steel surface. This phrnomena c m he

identitied bj- the hi& rate of drsorption. Furthsrmore. it also suggrsts that physical adsorption is

rcsp«nsiblc for the deposition procesS. ~4atheniaticall~-. the balance b r twen the two rates.

descrihed in chapter 7. is given by rquation 2.3.

Ano thcr possibi 1 i ty is that surface saturation uas ac hieved during the rsperimen t. 1 f suc h

saturation occurred. kd would be rffectively zero. Consequently. equation 2.5 above would no

longer bs valid. A different basic rquation to drscribe iodine deposition would have to be used.

This squation must take into account the possibility of maximum surface loading due to

saturation. An rquation which cnn be usçd to drriw the Langmuir isotherm is usrd in this report.

The squation requires the use of an additional parameters that represtints surface saturation.

(eqn. 5.1)

The ratio of C,, c," can he esplainrd as the t m n that describes the distance betwen iodinc

surface concentration at an! point of time to the surface concentration at saturation. In sffcct. this

terni \\-il1 bring the cun-e of iodinr: surface concentration to asymptotically approach the

saturation valus. .As equation 3.1 also includes a desorption t em . it also rrprrsents the role of

dcsorption in determining the value of maximum surface loading.

In the discussion to follo\v (section 5.4). it a-il1 be shown thnt. particularly under high

relative humidity conditions. the maximum loadings under some conditions are more consistent

\\ i t t i surface saturation than a wady state.

5.3 The Effects of Opcrating Conditions on Deposition Parameters

Throughout the project. the rate and estent of drposition \vas obseneed to \-an. with the

operating conditions. In this section. the effects of the operating conditions on the two main

drposition parameters. k, and c,". are discussed. A nuinber of relationships bctwen tliesc

paranieters and the operating conditions are identified. The discussion begins by considering the

effect of gas phase concentration. Based on this division. the effect of tube surface temperature

and relative humidity are also considcred. The data from SS-316L and SS-5O4L are presented

togrthrr so that cornparison c m br readily made.

5.3.1 High Iodine Concentration

Cornparrd to the two other concentration ranges. the results for high concentration

shoued the clearest trend. The obsen-cd values of k, for high gas phase concentration are shown

in figures 5 -4.

High Gas Concentration z HlGH %RH High G a s Concentration 0 High %RH SS-316L LOW %RH SS-304L 2 Low %RH

1 .O€+OO 1 .OE+00

O 20 4 0 60 80 100 Temperature (C)

O 20 40 60 80 100 Temperature (C)

Figure 5.4a and 5.4b Deposition Vriocity ai High [12 ]?

Both figures illustrate the substantial sffect of humidity on k,. The inipact u s

particulad>. pronounced at 40°C and 60°C but lrss signitïcant at Zj°C and 90°C. .At 90°C. the

ratc of deposition appsared to bs supprsssed. In hct. at 90°C. for both SS-3 1 6 L and SS-3O-IL.

thsre was littlc or no difference in the rate of deposition at high and low relative hurnidiiy.

The highrst iodine loadings were obsen-ed with high relative humidity and tube

tenipcratures of 40°C or 60°C. Under thcse conditions. iodine deposition continucd to climb n-ith

a Iiigh deposition i.elocit>- on the order of !O-' cm/s and no widencr of psrudo-saturation of the

surface was observcd (figure 5 . 5 ) . It was observrd that undrr these operating conditions. thrre

\vas an initial delay period afier which iodine accumulated at a constant and very rapid rate.

Lhen this r>.pr of behavior was obsen~rd. for a more accurate represrntation of the deposition

rate. k, associatrd uith the linear part during which the highrst deposition rats occurred was

reportsd.

lodlne (mallcrn*2) 2.OE-07 4- [121g moUL

O 5 0 too 150 200

TlME (min)

Figure 5.5 Iodine surface loading at 40°C. high a s phase concentration and high rrlati\.r

humidi ty

.A high drposition rate \vas also obsen-sd for SS-3 16L at 23°C and Ion- rclatiw humidit!.

(tigure 5.6). This was somewhat unsspectrd. The surface concentration actually surpassed the

masirnuni loading at high relative hurnidity n-hich \vas on the order of IO-' moli.cm2. Whcn

relative hurnidity was increasrd to near 100%. rapid iodinr desorption occurred. The surface

-l

mol!cni- and \vas still decrrasing slowly ivlirn the rsperiment kvas trrminated. It is not clrar \vh-

such behnior would occur. Similar deposition behavior \vas not obsen-rd for SS-304L. ljnder

the same operating conditions. the value of k, \vas much lower. I t is not clear as to what caused

the di fferences2.

- Ii sliould noted rhat this rsperiment \vas perfomed using tubing obtained from the same supplier as tliat used by Deir [7.17]. In al1 the othor esperirnents performed in the present study. the tubing was purchased from an alternate source.

S S - 3 1 6 L 1 2 3 C R H = L O W to HtGH I H I G H [I2]g

4.OE-06

fodine (mol lcmA2)

- 3 .O€-O6 O.OE+OO

O 50 100 150 ZOO 250 300 350 300

TlME (min )

Figure 5.6 Iodine surface concentration high [Ljg and Ion. relative humidity

Jifticult to justib- an!. detïnite dependence of c," on temperature or relative humidity. In spite of

this. tigiire 5.3 suggests that a combination of liigh tube temperature and l o ~ rrelati\-t. humidit!-

ciecrsasçs the maximum loading. For esample. at 73°C and high huniidity. C," \vas 10- niol,'cni2.

Whsn the temperature \\as increasrd to 90°C. c," \vas reduced to 1 O-' molicni2. At tube

tcnlperaturc of 90°C but lon- relatii-e humidity. c," was funhrr suppressed to 10"' molicm'. .A

similar trcnd \vas obsenxd at -10°C. Cndcr high relatiw humidity. ai the end of the esperinient.

-l

the surface concentration \vas highrr than 7x10- molkm- and \vas still increasing. .At lou

re1atii.r Iiurnidit>-. ho\vr\.rr. c," \vas reduced to 5s 10'' rnol!cm2. The above observation. again.

contirnicd the role of water in snhancing the rats and estent of iodine dcposition on stainless

steel. .-\ closer analpis of tipure 5.5 shows that the impact of gas phase concrntration on C," \vas

more signikicant than the impact of both relative humidity and tube tempenture. Linder high gas

phase concentration. the impact of relative humidity is still clearly visible. When gas phase

concentration was reducrd to belon. 1 O-' mol/L. howrver. c," seenied to be unaffrcted b>- the

diffcrenccs in gas relative humidity.

The above observation on the variation of both k, and c," with trmpcrature and relative

humidit! suggests that any chrmical interactions on the surface that c m rnhance iodine

deposition. require the presence of water. The amount of water on the surface ma). be affectrd by

a. combination of tube surtàce temperature and relative hurnidity. Higher temperature should

dscrease the amount of surface watrr but ma! increase the rate of chernical rcactions. Higlisr

hurniciit?. should increass the arnount of water on the steel surfacr. Deir [7] suggested that at high

temperature such as 90°C. the nmount of w t e r available on the surface is signiticant1'- reduced

so that an). surface reactions which require the presence of nater are inhibitsd. Deir also

suggested that a mechanism involving corrosion attack of the steel by iodine which requires

surface watsr is l iksl~. .

Furthermore. i t can also be ssen that at 40°C and 60°C bot11 SS-3 16L and SS-5O4L l\.ere

cqually susceptible to high iodine deposiiion rates sspecially at high relative humidit?.. Deir 171

rcponsd lhat similar behavior \vas also ohsrn-ed for medium of relatiw humidity ( - j O O / o ) . .At

3OC. SS-2O4L seemed to perform bctter than SS-3 16L. This was truc for botii ranges of relati~er

humidity. This is consistent uith the obsenation by Lee et-al. [76] but is contrar). to \\-ha[ lias

c.spected. As discussrd in çhapter 3. the main differencr: bctween SS-3 16L and SS-3041. is the

addition of molybdsnum in SS-3 l 6 L . Molybdsnum is known to increase srsrl's resistance to

pitting corrosion attack by chlorine.

5.3.2 Medium Cas Phase Concentration

The plot of kd as a function of temperature for high and Ion- r e l a t i~e hurnidity c m be seen

in the following figures.

Medium Gas Concemon High %RH Medium Gas Concentration High %RH SS-316L : LOW %RH SS-304L 1 LOW %RH

1 .OE+Ol 1 .OE+O1

1 .OE+OO i .OE+OO - -

1 .O€-04 1 .OE-04 O 20 40 60 80 100 O 20 40 60 80 100

Temperature (C) Temperature (C)

Figure i.7a and 5.7b Deposition velocity at medium concentration

.-1s cün bt' srrn above. for SS-3 161. k, \vas consistrntl>- highrr at high rdativt. humidit?.

Scattering occurrcd at 23°C and high relative humidity. It is not clrar \i.h~- this scattering

occurred. Similar to the trend obsrn-ed at high concentration. a high drposition rate \vas also

obsen-cd at high humidit>- and temperatures of 40°C or 60°C. The values of k, at leu. relative

huniidity w r e consistrntly lowr. It should be notcd that the estent of drposition on SS-3 161. ai

90°C \\-as belou. detection indicating that the deposition i-elocity was lrss thnn IO-' cmis. The

~.alut.s of the detrction limit w r e used to calculate the kd values in figure 5.7a for SS-3 16L at

SS-3O-K sliowd diffrrent trends than SS-3 16L. .At 23°C. k, uas Ion. and there appeared

tu bc no signifiçant efkct of relative humidity. Tlic highest value of k, was obsen-ed at 40°C and

loir. relative humidity. This is somewhat perplesing as previousfy. for high gas phase

concentration. the highest kd a-as obsen-ed at 40°C or 60°C and high relative humidity. At loir.

huniidit>.. howtver. the accumulation stopped at loadinys between 3s 10'"o 6s 1 O-" n101:crn'. .At

high humidity the loadings \vas 2x10-%ol/crn2 and \vas still increasing ivhen the esperirnent

\vas tmninated (table 5 .2 ) .

Table 5.2 SS-304L Medium_l12J,

I I

High relative humidity

I I I

Low relative humidity

Masirnum Loading

The data from the two types of steel suygest that undsr high relative humidit) ivith

remperarure lower than 90°C. at medium -as phase concentration SS-304L performs better than

SS-316L. .At 90°C. for SS-?ML humidity appeared to have no sffect on kd. Furthemore. the

value u-as found to br lowcr than k, at temperature between 23°C to 60°C. SS-3O-IL. hou-ewr.

showd high k, at 90°C and high humidity ( - I O - ' crn:s). .At low relatiw humidity. kd was found

to br: approsimatrly 1 O-' c m k

l-nder low humidity condition. SS-3 16L cippears to perfomi brtter tlian SS-3O4L

t.spt.çiall!- at 40°C. .-ln intsrcsting obsen-ation is that for SS-3 16L at lou- huniidit!.. tenipenture

sccms to have no sffecr on k,. The oniy differencr is in the values of maximum loading (table

5.3 1. .At 60°C. the mnimum loading achieved was higher than 10-Io mol/cm2 while at 23°C and

-10°C. the loading \vas between 2s10-'% 8s10-"' rnolhn' and w s still increasing when the

espt'rirnent u s trrminated.

kd Gas Phase Temp

Table 5.3 SS-3 16L Medium [IzJz - - Low %RH

Cas Phase Temp kd maximum Loading

From tigure 5.3. i t can also bs seen that m a ~ i m u m loading w s highcr at high than at low

rcilliti\.t: humidity. This brhavior \\-as sirnilar to the one obssn.rd undsr high gas phase

concentration. The rffect of trmprrature is much lcss clear. tlou.evsr. highrr temperature

apperired ro lou-er maximum iodine loading. This can bc ssen in both tigurr 5.3 and table 5.3

above.

5.3.3 Low lodine Gas Phase Concentration

Figures 5% and 5.8b belon- show the plot of deposition vrlocity as a function of

temperature for both SS-3 16L and SS-jO4L. It should be noted that the SS-3 16L data are mostly

hassd on the drrtrction limit. I t can be scrn that scattering occurred for SS-3 16L rspeciall> undcr

lot\ rslati~x humidity.

Deposirion Velocity Vs Temperature . H~~~ RH Deposition Velocity Vs Temperature ~ ~ g n RH S S 4 1 6 L - Law Gas Phase Concentration - L;w R H SS-3O4L - Low Gas Phase concentration , ~ 0 . ~ R H

' ,jE - C C ' GE *cc

:O 4 50 J C 'GO

Temperature (Cl

2 O 4 0 6 O

Temperature ( C )

Figure 5.8a and j.8b Deposition Vclocity for SS-3 16L and SS-3O4L as a function of

Temperature

Thsss results suggest tliat at low gas phase concentration. tubs surface temperature and

relative hurniditl- had very little rtTsct on k,. .~ lmos t al1 values tluctuate between IO-' to IO- '

cmis. .At high humiditu. SS-3 16L appeared to psrforrn better than SS-304L. Under low humiditv

condition. howevrr. the picture is less clear as there is scattering in the data.

Tube surface temperature and reiative humidity appeared to haw little or no sffrct on the

values of maximum surface loading. .-\II the values are on the order of 1 O-'( ' mo1:cm2. The only

txception \\.as that at 60°C and 90°C. with humidity in the lower range. surface luading

clt.crt.iis<td to 1 O-' ' mol cm'. .-\ summary of results for SS-3O4L çan be Iound in table 5.4 and 5.5

Table 5.4 SS-304L Low [II& - - Hieh %RH

Cas Phase Temp kd Deposition rate .Masimum Loading

(mol/L) ( O C ) (cm/s) 1

(moVcm'.s)

l X I O - ' ~ ' 23 1.9 s l 0 - ' 1-87 s 1 O"' 3 s 1 O-'" molicm-

1 23 I S I O - 1 1 I .09 S IO-" 1.7 x I O-'" ~ o I / c ~ -

Maximum Loading Deposition rate

(mo~cm'.s)

Z . S S I O - ' ' '

2.8 s l0 - " '

6.6 s 1 O*"'

6.6 s 1 O-"'

kd

(crn/s)

Cas Phase

(rnoVL)

Temp

( O C )

60

60

90

90

1.5 s I O-' ' ~ o I / c ~ -

< 2 s 1 O-' ' mol/cm-

4 s I O- ' ' niol/cm'

4 s 1 O-' ' m o k m '

1 ~ X I O - - I S I O - ~ "

6 si O"

7 s 1 O*'

1 . 6 ~ 1 0 - '

2

4.4 SI O+

1.1 s l0 - "

5.3.4 The overall effects of Gas Phase Concentration

In sections 5.3.1 to 5.3.;. the eft'ect of relatil-e humidit>- and tube surhce temperature are

considered for the three ranges of pas phase concentration. This section is focused on presenting

the o\erall picture of the rffects of gas phase concentration on the drposition parmeters. k, and

C,". In the beginning of the projrct. it was thought that the mechanism of iodine deposition might

bc different at different reiati~x humidity. Figure 5.1 sho\vs the plot of k, as a hnction of gas

phase concentration. The t i g r e contirrns the previous observation of highrr k, at high relative

hiimidit!.. The slfect of temprraturr. however. is mucli lrss clear with the exception of 90°C. At

90°C. relative humidity does not have sisnitkant inipact on k, with the value of k, hllowing the

trend ol'ssperiments performed under dry condition. .At tiiis temperature. the amount of u ater on

the surtrice \vas Iiksl! suppressed. .An intrresting obsen-ation is that at gas concentration less

than IO-" mol. L. temperature and relative humidit~. did not seem to have a signiticant effsct on

k,. It is speculated that the rntchanisrn for iodine deposition at concentration less than 1 O-' mol 'L

is predominantly ph>.sical adsorption. in which case relative humidity ~ o u l d not Iiaw niuch

impact. .-Il concentration above 10"' rnol;L. it is speculated that a corrosion type chemisorption

nicçhaiiisni prcdominated. but only il' adequate Luter \vas nvailable on the surface. This

nirchanisnis nia!. have been self perprtuating. Speçiticall>. if the chemiçal interactions hrtn c m

I 2 and the steel resulted in a cornpound that was hygroscopic in nature. the cvmpound \t.ould

Iiave cittracted nater from the gas phase thus increasing the amount of surface u-ater. This

h!.groscopic proprrty ma>- have further sustained the chernicol interaction thus incrrasing the

probability of the reactions to propagate. The consequrncr was the enhancement of rate and

estent of iodine deposition.

Initial Deposition Velocity Vs [12]g SS-304L

lE+Ol , 23C-High RH

Figure 5.9 Variation of deposition velocit). m-ith gas concentration (SS-304L)

The dependence of k, on gas concentration for SS-3O4L is less clcar than for SS-3 16L

(tigurcs 5.1 and 5.9). At gas concentration higher than 10%ndolil. the dependence of k, on

relative humidity is surprising. in that for some gas concentrations the k, values for Je- and

Iiurnid conditions appear to br reverseci. Howrver. at gas concentrations lower than 1 O"' mol ,L.

the trend is similar to that for SS-3 16L and k, did not appear to be affected by relative hurnidity

The obsen-ation basrd on the value of maximum surface loading rewals the same

signi ticance O t' gas phase concentration of 1 O-' rnoliL. In t i y e 5.3- il çan be srrn that above 10.'

mol L. c," is hiyher undrr humid conditions whilr at pas concentration below 10"' mo1;L. the

w.alues o t ' ~ , " do not sccm to bs afkcred by relative hurnidity or temperature. Furthemore. results

undrr high hurnidity. showed a sudden decrease in c," lrom 1 O-' mol!crn2 to 1 O-"' rnol/cm2 when

@as phase çoncrntntion was lowered to below 10-' rnol!L.

.An additional observation can bc made in regard to the values of Cs". .At low relative

hurnidity. the èffect of clianging gas concentration was more dominant than the impact of tube

surhce irmprrature (figure 5.3) . Furthemore. the siope of the plot was approsimately one

suggesting a constant ratio between the surface loading and the pas concrntration. Calculation of

the distribution coefficient (K,). the ratio of surface loading to gas concentration. demonstrated

that this ratio iras in t'act indrpendrnt ot'gas concrntration undrr these conditions (figure 5.10).

Distribution Coefficient (K,) Vs Gas Concentration Low Relative Hum id ity

Figure 5.10 Variation of distribution coeficient with gas ccncrntration at IOM. relative humidit)

From figure 5.10. it can br secn that under lou- relative. the values of K,, is approsimatrl>.

1000 cm l i s long as the pas concentration rernriins brlow 10-%d/~ . It is possible tliat undsr this

rnngc of yas phase concrntration. iodine concentration on the surtàcr was drtrrmincd hy an

rquilibrium with the gas phase concentration. Furthsrmore. this suggests that physical adsorption

\vas the predorninant type of adsorption. .As a point of comparison. tigure 5.1 1 below

suniiiicirizes the \.alurs of R,, h r rrsults ~indrr high relative humidity. It can br sren that the

trend is quitc different. It is intrresting that when yas concentration is below 10" mol,L. the

values of K, for high gas phase concentration at high relative humidity is sirnilar to that obsemed

for low relative humidity. This further supports the previous conclusion that belou. 1 0 ' ~ mol!L.

the mcchanism of deposition is similar regardless of the relative hurnidity with ph>.sical

adsorption likely bcing the most dominant form of interaction.

Distribution Coefficient (K,) Vs Gas Concentration High Relative Humidity

100000 0 23C

1 O000 40C

E 1000 O

O A 60C u" 100

Ci ; 90C .,

10

Figure 5.1 1 Variation of distribution cocftïcient with gas concrntration at high relative humidit:.

5.3.5 Summrq of the Effects of Operating Conditions on li, and Cso

Gcncrally. both k, and C," are higher at high relative humidity. The hiphest rate and

estent ot'depositivn can bs sspected at 40°C or 60°C under high humidity. This is {rue for both

SS-3 I6L and SS-3O-IL. At these conditions. both t!.pes of steel appcar to br equally susceptible

to chernical attacli b ) iodine. Specitically in trms of c,". temperature seemed to have Iess

impact than gas phase concentration.

When yas pliase concrntration \vas highrr than IO-' moliL. the deposition rate dependcd

strongly on relative humidity. Brlow IO-' rnoliL. h o ~ v s ~ e r . a differcnt picture emrrged. .At rhis

range of concrntration. the impact of relative humidity and temperature on C," [vas srnail as

comparrd to the rffect of gas concrntration. I t is likely that at concrntration bclow IO-'' rnol,'L

single mechanism is behind the dsposition. One possible interpretation. which is elaborated in

section 5.6. is thar. at low iodine loadings. insufficient amounts of hygroscopic FeIz is present on

the surface to enhance the accumulation of water by the surface.

In mm? instances SS-304L appsars to perform bctter than SS-3 16L. This is somen-ha1

çontrap to what is predictrd. SS-3 16L is espectrd to perform better due to its high content of the

elemsnt mol?.bdenum ~vhich is gensrally known to protrct the steel tiorn pitting corrosion attack

b). chlorine. The resuits at low gas phase concentration are not conclusive as practicall) al1

rrsults Ml below detection limit. During these esperiments. hou.e\w. similar radioiodinr

specifiç activity was ussd which implies that at low gas concentration. SS-3 16L ma). pcrtorm

bettcr than SS-3O-L

5.4 Desorption

Dcspitc the îàct that the data obtained on the rate of drsorption \vas lirnitsd. it did pro\ ide

soms usetid insight. espccially on addrrssing the question as to n-hrther the masin~urn surface

loading \\-as dur to surface saturation or steady state. Specifically at strady stats. the rares of

iodinc deposition and drsorption should be cqual. Henct: it \vas possible to çalculats desorptivn

rate constants from the obsrned mnsimum loadings and deposition vslocities using eqiiation 7.3.

Lt: h r a g iwn rsperiment. the rnasimum loading was actually a stead) statc. the calculated

desorption rate constant would have b e n sirnilar to the value obsenxd espcrirnrntally . The data

obtained on drsorption is surnmarized in appendis C.

-4s an esample. data from an esperiment with SS-3OJL under dry conditions. at 90°C and

high gas concentration \vas esaminrd. The value of k, associated with the esperiment was 0.026

cm.*s and a masimum surface loading of 1 . 8 ~ 1 O-' rnol/cm2 \vas achiewd. based on the

assumption of steady state. the calculatsd value of the desorption constant \vas 1 .h 1 o - ~ s-' .

Hou ever. the esprrimrntal data indicated a much lower desorption rate constant of 4.6s 1 O-' s-' .

This large discrepancy bstween the drsorption constant predicted by squation 2.3 and the

rsprrimental value. indicated that the rna~imum loadiny was not dur to a steady state having

hern açhieved. Figure 5.12 \vas obtaincd by calculating iodine surface concentration bassd on

squation 3.3 and squation 5.1. Houe\w. for equation 5.1. the ssprrimsntal valut: of the

drisorption ratr constant was used in the calculation.

, Eqn 5 1

1 , Surface Concentration Vs Time . Eqn 2 3 Experimental

2 SE-09

Desorption

- 5 OE-10 O 50 100 150 200 250 300

Time (min)

Figure 5.1 2 Surface Ioading as a function of timc : t.spsrimrnta1 and calculatsd ( [LI,. =

8 s 1 O-' mol/L. T=90°C. high relative hurniditl- ).

Both cquations w r e capable of predictiny iodine loading as a function of timc u-ithin

reasunahlrt psrtorrnrtncc.. Ho\ve\w. rquation 2.3 which is basrd on the assumption of srcady stcitr

ot-cr predictrd the ratr of dcsorption. On the other hand. rquation 5.1 \vas capable of predicting

the trend of iodine drsorption n-ith good accuracy. In grnsral. the desorption rate obsened

cspsrimentally were much too slow to esplain steady state situation. Hence. it c m be drducrd

tliat. for most ssperimeni. surface pseudo-saturation brttrr esplained the obsened maximum

loadings.

Similar bcha~ior \vas obssned for iodine deposition and desorption at high relative

humidit!.. The rate of drsorption was much lowcr than the value required for steady state to

- occur. For ssarnple. in an csperimrnt with SS-3 16L at high iodinr concentration ( - 10' mol! L i .

high relati\-r humidit!.. and a tube temperature of 23°C. the obsrneed kd [vas 0.19 cm. S . The

calçulated k basrd on the strady state assumption \vas 1.6s 10" s-' while the trspçrimental valus

\ras 7 . 3 ~ 1 0 ~ s" (Figure 5-13). .As brtfore. using a dcsorption rate constant bascd on the

assumption of steady statr over predicts the rate of drsorption and surface pseudo-saturation is a

n-iore plausible intsrpretation.

lodine Surface Concentration Experimental, Eqn 2.3 and Eqn 5.1

1.5E-07

, . ' n

8

S b - - - Equation 2.3 . l + I -Eqn. 5.1

I

\ . Experimental

Time (min)

Figure 5. I 3 Surface Ioading as a function of time at high huniidity : espsrinisntal

and eq~iation 3.3 ([I71g = 10-7 molPL. T = 23°C. high relative humidit~.)

.An esampis of desorption results that shows rapid iodine loss frorn the surface is shown

in figure 5.6. This result suggests the possibility of iodine physicai adsorption on the surface. The

7

initial iodinr loading was pcrformed under high gas concentration ( - I O - ' mol/L). low relative

humidity and 23°C. Adsorption \vas found to be very rapid. quickly reaching a surface

concentration hipher than 10" mol/cm2. This value is approsimately 1 O00 timrs the reporteci

due Ior monolayer coverage. Rapid drsorption occurred when the relative humidity was

drli beratsl!. increased. men t hough the iodine concentration remained constant. An accurats

estimate of the desorption rate constant is ditticult to obtain as thrre u-as a suddcn jump in iodine

retention on tube surface. The rate of drsorption. howvcr. appcared to match that of adsorption.

.A rough estimate of the iodine desorption rate from the surface gave a value on the order of 10-'"

rnoLcm2.s. The associated rate of adsorption was approsirnately 10"" nwlm-n2.s (ai k, = 0.28

cm. S I .

I t shouid be noted that the behavior obsrnred in this rsperirnrnt \vas quitc unusual. The

adsorption rats was much higher than that obsen-ed in other esperirnents pttrformed at high

iodine concentration w-ith low relative humidity. In addition. desorption is usually more rapid

undrr dn. conditions than humid conditions. This is the opposite of the trend ssen in figure 5.6.

The esprriments a-rre repcated four timcs and the samr behavior \vas a l w i ~ s obserwd. It is

spcculatcd that the iodine concentration usrd in this txperiment may have bsrn high cnough to

produce niultilaysred physical adsorption. It should aiso be notrd that these four rsperimrnts

\vert. perlornied using steel provided by a difkrent supplier than that uscd to obtain the tubing

L I S ~ ~ in al1 other tests.

5.4.1 Desorption Under Different Range of Humidity

Esperirntrnts were conducted to evaluate the tffect of relative humidity on desorption

rate. L o w r hurnidity kvas found to cause a highcr rate of dcsorption. Figure 5-15 shows

desorption results for high relative humidity (>75%) and low relative humidity ( ~ 2 5 % ) ai 40°C.

During the latcr stage of desorption. temperature was decreased to 3OC to see the effects of

luwering the temperature. Desorption occurred faster at lorv relative humidity. This \vas truc for

lodine Adsorption and Desorption L = Low %RH

Under different Relative H umidity 140 C 1 Medium [l,], H = High %RH

Adsorption

500 1000

H 23C

Desorption

1500 2000 2500 3000 3500 4000 4500

TlME (min)

Fig 5.1 5 Desorption under di fferent relative humiditirs and tube temperatures ( loading occurred

for [ 1 7 - l., - = 1 O-' rno1;L. T = -10°C. relative hurnidit'. as indicated in tipiire)

.A similar trend was not obsenred at a tube temperature of 90°C. in that. the rats of

dssorption ivas unaffected by changes in relative hurnidity (figure 5.16). This fiinlier supports the

prt.\.ious assertions that at surface temperature of 90°C humidity played a less siyniticant rok in

dctem~ining the interaction brtwern iodine and the steel surface. .An important obsen-aticin is that

the rats ot'cirsorption for this esprrimrnt was similar to that espcctrd based on the strady stats

assurnption. Givrn the observcd deposition velocity of 0.1 cm/s and assurning a distribuiion

çoefticieni of 1000 cm ( figure 5. I O ) the dssorption constant basrd on the steady state assumption

would b s IO-' s-'. The observed value was approsirnately 3 times higher. This provides further

supports that at 10 w concentration iodine retention occurred predominantl y b y phy sical

adsorption.

OCT 24 . 95 - 1 SS-316L 190 C LOW %RH 1 LOW (121

1 .SE-10

DESORPTION : 90C 1

'.pLowR"

DESORPTION : SOC 1 . HJGH RH

. / -1- : 8

. lodine (mollcmA2)

- a - (121g moUL

O 500 1000 1500 2000

TlME (min)

Figure 3.16 lodine drsorption undrr diffrrcnt relative humidity at 90°C. Iodins loading involwd I I [12]g = 3 . 5 ~ 1 0 - rnoI/L. Io\\. relati\x humidity and 90°C.

5.4.2 Desorption and Iodine Reloading

Most of the data for iodine deposition genrntrd in this study only addresses shon trrm

adsorption. Thrre is littlr data relevant to the long term impact of the interaction brt~vern the

dcposited iodine and the steel surface. Howrw-. a fe\v rsprrimsnts w r e carricd out to

inwstigate as to whrther long trrm interactions would cnhancr funhcr iodinr deposition or

inhi hi t the proces-

SS-316 123 C HIGH RH 1 HlGH [12]

- . . - . [12] gas phase Conc. moUL

O 1000 2000 3000 4000 5000 6000 7000

TlME (min)

" Figure 5.17 Iodine Desorption and Reloading ( [ I& > 1 O- ' mol/L. high relative humidity. 3 ° C ) .

- .An esperimrnt was prrformed with a high iodine concentration ( 10-' mol/L) under high

relative humidity with a tube temperature of 23°C. Initial iodine deposition was rapid and a

maximum loading approaching 1 2s 1 O- ' molicm' \vas quickly achir\-rd. Dssorption. l i ou r~ t r .

occurred at a much slou.er rate. Nrarly thrrt: days wtre rcquired For most of the iodinr deposited

to desorb from the surface. Fo l lo~ inp drsorption. iodine reloading undsr similar pas phase

conditions \vas attempted. Only negligible amounts of iodine w r r deposited during this

rcloading esperimrnt. tt appears that during desorption. somc interactions betwcn steel and

iodine çauscd the steel surface to bccome much less susceptible to iodine drposition. Possibly.

the change in the surfacr conditions of the steel \vas brought b'. soms chemical interactions

br twen iodine and the steel tube.

5.4.3 Volatilization of lodine from Feil Solution

Iodine ma). be ph>-sical1'- adso:kd ont0 the steel or i t ma). undergo chemical

trmsfomiations through some chemical reactions with rnrtal constituents of the steel. It is also

possihlr that hoth phenomcna occur on the surface. in othttr words. boih physically and

cht.rnicril1~ adsorbed iodine may esist on the same steel surface. Hou.txc.r. it is common sense

that prior to an!. surface chemical transformation. the iodine must tirst be physicnlly adsorbcd by

the surface.

If an>- chrmical reactions occur on the surface. one likely product of the reactions w u l d

be 1- which is not volatile. Therefore. desorption of 1' directly from the surface is not possible.

The desorption procrss ma? takr place if the 1- is osidized back into a volatile î'orm Le. I?. To test

whethrr such an osidation procrss could take place additional esperiments were performed.

Esperiments wsre carrisd out by passing an air strearn above IO0 mL o h saturated FeI,

solutions ( figure 5.1 8 ). Any 1, formed by the osidation of 1- in the tirst tlask would bs carricd by

the pas stream into the second tlask. The second tlask which contained 100 mL of 0.1 41 Na1

tvould trap the inçoming 1: by its reaction u-ith 1' to form 1:'. h a l y s i s w s perfonned hy

spsctrophotometric detection of 1;- xhich has strong absorptivity at 350 nrn.

Fe12 ( a d 1 M

Figure 5.18 -4pparat~is to stud!. iodinci volatilization from FcIZ ,:,,,

'The result \\.hich shot\.s the amount of iodine detscted in the second tlask can bs sesn in

[12] (molfL) Trapped in 0.1 M lodide

Volatilized From Saturated Fe12 solution 6.OE-07 . Unfiltered

-& Fiitered - , - Saturated -> diluted 4x

4-O€-07 - Fiitered - N2 A *-/ =

E - - 4 - -

0.00 0.50 1.00 1-50 2.00 2.50 3.00 3.50 Tirne (hr)

Figure 5.19 Volatilization of iodine from Fr& solution

Based un these results. iodine volatilization from Fe[: solution is possible. The procrss

in\-ol\.ss the production of 1: which is volatils and thersfore c m br releasrd into the gas phase.

The process that is responsible for the production of 1,. however. is not known. Initially it \vas

thought that a reaction was occumng invol\-ing the conversion of ~ e ' - to ~ e ' - dur tu osidation

by O2 ~vith subsrquent osidation of I'by F?. Ho\vever. when the air strrarn \vas replaced with

3,. n similar rats of I 2 production \vas observsd ~vliich suggests that the a\.ailability o f 0 2 is nut

necessac. for the procrss. The 1, production kvas found to be reduced by lowsring the

concentration uf Fe& in the original solution. In this esperimrnt a saturatrd solution [vas dilutrd

h: (i factor of four. The results however only showed a factor of two reduction in the production

of 1,.

Bascd on the resulrs on figure 5.18. it can bc sstimatsd that the rats of loss of 1: tiom the

100 niL saturatcd Fe12 solution was approsirnately 3s 1 O-'' rnol/s. This rats is actually îàstctr than

1

the drsorption rate obsemd for an! of the 16 cm- tube samples used in the studv. Howt\-sr. it is

not y t known if or how the 1: loss depends on the solution volume. In addition. it is difficult to

estininte the quanrity of water on the tube samplrs. klence direct cornparison of the rate obsen.cd

tor tliese solutions and the drsorption rats h m the tube is not )-et possible.

5.5 Iodine Retention on Compression Fittings

.-in additional espcriment was carried out to esamine the possible contribution of

stainless steel titrings to the overall 1: retention on stainless steel sampling lines. The esperinient

\vas performed by including a steel fitting at the rniddle of the steel tubing samplr. The trend of

1, deposition a-ith timr as well as its distribution along the tubing are shown in figures 5.20 and

5.2 1.

lodine Deposition : MC-1 Steel Tubing with Stainless Steel Fitting MC -2

1 JO€-08

Time (min)

Figure 3.20 Iodins deposition w-ith time : SS-3 16L tube with stainless steel t'itting at the

middle ( [I& = 3.5E-8 mul/'L. T = 23 C. relative humidity = 8596).

The \-alues of k, associated w-ith the above duplicatr esperimrnts are 1 . 1 3 1 O-' cm, s Sor

SIC4 and 4 . 9 ~ 1 O-' cm's for MC-'>. An intrresting observation made is that in the above figure. a

dela! period ivliich is usuall>- absent at 23°C was obsçirved. Funhrrniorr: based on the plot of 1,

distribution dong tubing. most of the iuading \vas due to 1: accumulation on the niiddlc pan

nhers the stainless steel titting \{+as attachrd (figure 5-21 ). The body of the titting itself onl!

retained sniall a arnount of 1'. The highrist rrtcntion apprarrd ro bç on the pan of the tubing tIiat

\vas attachttd to the steel fitting. Possibly the high retention of iodinr on this part \\as dur to the

damage to the steel surface causrd by cutting and smoothing. Additionally. the increased

turbulence in the region may have funhctr increased the retention. Another observation based on

the data is that the estent of the damage on the t u b i n may be different g k e n that the amount of

retention ar al1 the tube ends are different. For esample. the part attachcid to the teflon fitting (2.5

cm and 3 . 5 cm) seemed to retain substantially less I 2 compared to the part attached to the steel

tubing.

Distribution o f lodine Activity Along Tubing

6 O€-O4 - 5OE+O4

O

0 3 4 OE+04 .- w Fitting at 14 c .- - 3 OE+04 * 2 A

2 OE+04 A .- O 0 1 OE-04 -

A

P O O

0 oE+OO -m 8 - 6

O 5 10 15 2 O 2 5 cm from lnlet

Figure 5.1 1 Distribution of iodine actkity along the tubing

Based on this result. care should bs taksn in relation to the sections of sarnple linss nhrrr

damnge to the surface ma' rsist. such as the damage made by cutting. One possible solution is to

chsniicall!- treat the surface by imnirrsing the strel in HNO1 solution to recovrr the damagr

oside tilm and repassivate the steei surface. The high retention at tubs ends also suppons the

tlieory of incrrnsing iodine retention duc to chernical interaction at locations wherr: siIrface

Throughout the esperiments. higher I 2 retention at the inlet and outlet w r r obsenrd. In

al1 of thcse esperimsnts. tenon fittings tvere used to connect the steel sarnple to the teilon tuhing

that carrird the gas Stream. Othrr than the inlet and the outlet parts. the rest of the tubing showed

rrasonably et-en 1: distribution dong the length. The plots of iodine distribution alony the tubing

are includrd in appendix A.

5.6 Passible Mechanisms

Cpon r e v i e u i n the obsen-cd trends of deposition under various oprrating conditions. it

is now possible to speculate in regards to the possible rnechanisms behind the retention of iodine

on stainless steel. The mcchanisrns for iodine deposition can be divided into thrre different

catepories. The tirst one involves physical adsorption of 1, which in general predominates at lou

iodine concentrations. The second is a slow chemisorption process that occurs undsr dry

conditions uith highrr iodine concentration. The third is a rapid corrosion basrd chrmisorption

of II. .As this process requires the presence of uater to occur. it only occurs undcr medium to

tiig1-i relative humidity conditions. with higher iodine concentrations.

5.6.1 Physical Adsorption

Some theoretical consideration of physical adsorption is discussed on section 2.4.1.

hg w h (R 5.1)

Pti>sical adsorption involves w a k or indirect bonding betwern the iodine niolecules and thc

srccl surface. This attractke force is opposrd by the concentration gradient gcnsratsd b> the

accumulation of iodine on the surface. Hrnce. aftrr a certain prriod of loading. the surface ma!.

açhicw a stsady statr. Thcrefore. it should be esprçted that the o b s e n d deposition rate ~ o u l d

be initially hi&. The ratc will decrease as steady state is approached. To predict the variation of

iodine surface concentration as a tùnction of time. rquation 7.3 is appropriate. Anothrr

characteristic of physical adsorption is the relatively high desorption rate compared to chemical

adsorption. Finally. multilayered physical adsorption is only rspected for partial pressures of the

samr order of magnitude as the vapor pressure of the compound.

Basrd on the data obtainrd. physical adsorption predominatcd for iodinr concentrations

bslow approsirnately IO-' mol:L. This upper h i t corresponds to a surface loading o i 10-"

7

moli'crn-. giwn the obsrn.rd distribution coefficient of 1000 cm (figure 5.10). Hrncc. it apprars

that phyical adsorption alonr occurred for loadings bctlow that esprcted for rnonolayrr cowragc.

5.6.3 Slow Chemisorption

-1 different beha\+ior of iodinr deposition \vas obsen-rd at &as concentration higher than

10'" rnoliL and lou- relative hurnidity. The sarne anomal! kvas also obseneed for high rc1atii.e

hurnidit!. and a tube surface temperature of 90°C. The deposition \vas characterizrd b!. a sloivrr

k, comparrd to othrr esperimental conditions (figure 5.1 ). In fact if deposition rate is plottrd as

oppose to kd. i t can bs seen that the ratr of iodine deposition appearrd <O be relativel>- constant

rrgardlrss of the pas phase concentration or temperature (figure 5.22)- This trend could on[> br

seen for SS-5 I6L. For SS-304L. no clrar trend \vas widrnt (figure 5-23 ).

Figure 5-22 Iodins deposition ratr (N,) as a function of gas concentration (SS-3 16L)

lodine Deposition Rate Vs [IJg SS-304L

1 0 0 E - 0 9 , 23C-Hign R H

1 OOE-1o d POC-Hiqn R H

Figure 5.23 Iodincl deposition rate (N,) as a function ofgas concentration (SS-3OJL)

Furtliçr analysis of the data sugprsts that physical adsorption is not the phenornena

behind the Jeposition. Lnlike physical adsorption which predominantly occurs at gas

concentration helow 10"' mo1:L. the value of K, \vas found to be btitwen 10 cm and IO0 cm

( figure 3-10). Bclow 10" mol/L. for physical adsorption. KD \vas found to be approsimatrly 1000

cm. This suggests tliat the maximum loading is not dctcrmined by the balance beiwesn 1: on the

surfàse and 1: in the gas phase. Furthtirmore. dtisorption data shows that the desorption rate was

rnucli s l o w r tlian the rate of adsorption (ti.g. figure 4.13. This indicatrs that the 1, ma! ha - e

k e n cht.niiçall>. adsorbed on to the steel. It should be notrd. howsver. that desorption data n n s

\.en. liniited. Further investigation is requirt-d.

Anothrr observation made u-as that. for the case of slow chrrnisorption. k, ma). not be

appropriate. In the use of k,. it is implicitly assumrd deposition rate is a first order function of

gas phase concentration. As can be seen in figure 5.72 . iodine deposition rate appeared to b r

constant dsspite the change in gas concentration. Possibly the rate of reactions on the surface was

covemed by the amount of physically adsorbed iodine which was limited to one monolayer -

( 10" mol!crn2). Hence. within the region whrre slow chsmisorption prrdominatrs. deposition

rate (N,) is suggestrd to be used to replace kd. .AS to the nature of the reactions that occur on the

surface. the information obtained is not adcquate to allow an>- speculation. Furthrr study in this

matter is needed.

5.6.3 lodine Chcmisorption with Corrosion

The rhird process is iodins chrmisorption with corrosion of the steel surface. Throiighout

the presentation of rrsults. it has bern obsrncd that humidit). play a signiticant rolc in aî-feçting

the rats and estent of iodinr deposition on stainless steel. This suggests that cheniical reactions

on the surface are grratly accelrratrd by the presrnce of \vater. Several authors such as Deir and

Tsukaue et-al [7.8.91 have proposed interactions which involw pitting corrosion attack of strrl

ty iodinr. For such corrosion reaction. the presence of n-ater on the surface is nrcrssan. to

hçilitate the elecrrochsniical reaction.

The proçrss of chemisorption is usiially precsded by ph~.sical adsorption. Initiall!.. I 2 mil!.

be physicall>. adsorbed ont0 the mrtal surface or it ma! also bc adsorbed into water micro-

tiroplets that nia>- esist on the surface. The adsorption of 1' into mt r r droplttts on stainlcss steel

hns bsen obscnsd b!. Tsukauc et.al. [8]. In this investigation. siniilar condensation of watrr

which adsorbed 1, was also obsrwed. .-in). I 2 drpositrd near surface defects will react with the

steel to form 1-. Most probably. II ni11 first react with the most reactive component of the steel

i.e. Fe. Cornparison of reactivity with iodine among the thrett major metal constituents of

stainlcss steel namely Fe. Ni and Cr shows that Fe is the most prone to the chernical attück.

followed by Ni and then Cr (table 3.5). The same data shows that Cr is largely unreactivr with 1,. -

This coincides a-ith the fact that the oside tilm of Cr is mainly responsible for the stainlrss

quality of the steel. For stainless steel to remain passi\-r. the level of Cr as drtined by the bulk

concentration must bs maintained throughout the entire mrtal matris. If local regions rsist tt herr

the çhromium concentration is drplcted. thesc regions \vil1 brcome susceptible to loçalizrd

corrosion [37]. The surface. however. may become repassivatsd rhrough rhs dissolution of the

niost reaciive component. i.r. Fe. \!-ith the consequencr of Cr enrichment of the steel surface

tvhich may lsad to a renewed stainless qualitp [JI.

The following reactions have been suggested as the possible meçhanisms of interactions

betwrn F r and I2 [ I 1. As the reactions require the prrsence of rvater. it is implicitly assumsd that

some nater already rsists within the surface defects thus making it possible for the reaction to br

1 -1dsorption of 12rgi on to the surface or water droplets :

6 i y " h i ( R 5.1)

13g [2 i ïq i ( R 5.2)

2 ) Electroçhemical reactions between 1: and Fr :

'1 *

Fe- ,,,, - 2s- tt Fe,,, (-0.44 V )

Li:, + 32- H 21- ( -0 .53 V )

Conibining the two reactions will giw :

*

F , , 1 tt Fc- ,,,, + 21- ,Jq, (-0.975 V ) ( R 2.3)

3 ) The resulting ~ e ' * ions rnay react with 0: diffusing f?om the gas phase giving the final

corrosion products FqO, :

- I F + 0: - ( i + l s ) H 2 0 + 2Fe20,.sH20,,, + 8H- ( R 2.4)

Another observation made during the investigation is that no Fr" \vas found in the wash solution

iiom the inside surface of the tubing. This is possible as the brniation of ~ r ' + ma!. be prwsnted

b?. 1- through the follo\ving reaction :

7 -

FrJ-uq, - [-,aq) - Fe- ,3qj + k,;q (R 5.3)

7 -. Reaction 5.3. hoivever. ma) occur during desorption in that Fe- ma) be osidizrd to ~ e ' - \\-hich

subscquriitl>. reacts with 1- to producr 1, which drsorbcd frorn the surface. Once the niajorit~. of 1-

has besn rernovrd from the surface. ~ s ' - ma- csist and tom1 a protectivc oside Iayer.

The abo\.r reactions suggest a deposition mechanism involving the osidation of Fr to

1 -

Fe- by Il. Bascid on this hypothesis. it can be drduced that the amount of iodine adsorbcd by the

steel surface. ~vhich is rrtlected b ~ . the magnitude of k, and C,". inay depend on sc~ernl factors

such ris the number of available sites to initiate the reactions and how \\.el1 thess sites will

propayate under the relevant esperimsntal conditions. Additionaliy. the rate may also br affectcd

hy the propenirs of the corrosion products formrd on the surface of the steel.

.As indicated above. the number of initiation sites ma- affect iodine mention on the steel

surtace. 'These sites are locations whrre defects esist in the protectiw oside l a y r . The number of

sites a\.ailabls ma>- be a property spt'cilic to an' tubing particulnrly with respect to its

nianufacturing history. For esample. Lei: c u l [26] obsenxd diffrrences in iodine drposition

hehavior in stainless steels originated from two different rnanufacturers. Corrosion reactions in

thrss initiation sites may lead to localized reactions which. in the end. leads to pitting corrosion.

Thrse sites. howevcr. ma- not al1 propagatr. Some would cease to grow if the nrccssar!

conditions for growth are not satisfied. Tsukaue et-al. [9] have obsewed a period of dela>- brfore

pitting corrosion occurs. Moreover. based on their observation. not et-en- initiation site can

propagats. According to their observation. only pits with depth > 0.1 mm would srow stably

under environment containing 1:. hmong the factors that can affect the probabilitiss of pir

propagation are gas relati~e hurnidity. tube surface temperature and. possibly. iodine

concentration.

Gas phase hurnidity will afkct the corrosion reactions by supplying thti surface with the

required u-atrr. It can therefore btt espectrd rhat. at high humidity. iodinr retsntion would be

high. This phrnomcnon has bccn contirmrd by the esperimental results. Furthermore. the Fel,

salt producrd is hj*grroscopic in nature. Hencr. i t will cause furthrr condensation on the steel

surface. Due to its hygroscopiç nature the salt \vil1 also improve the stability of the watcr already

csisting on the surtace. The rnhancrd presencs of w t e r on thti surface will furthrr increasi: tlic

corrosion attack by iodine on the steel surface.

Tube surface temperature wi I l affect the reactions in sr\.rral w y s . B y increasing the

temperature. the rate of an>- chernical reactions ma' br: increased. Furthsrmore. pitting

probability will also increastt at higher temperature. If the temperature is increased sufticisntl>,.

h o w w r . the aniount of surlàce w te r ni11 be reduced as more ivater niIl be driwn off the

surface due to evaporation. Morcover. incrrased trmprrnturr \ d l decrease I 2 physical adsorption.

l'lius. i r sliould br espctcted thrit increasing temperature ma>- initial 1)- increasr 1, retrntion ( <60°C

as found in the current study). When the temperature is sufficiently high as to substantially

reduce the arnount of surface water. the iodine retention should also be reduced.

The effect of both temperature and relative hurnidity wcre relatively weak at loa. iodine

concentration. This can be rsplained in terms of the small loading of I I on the surface. It is

sprculatrd that at l o ~ iodinr loadings. the amount of iodinr present on the surface was

ina&quate to allow the rapid propagation of the corrosion reaction. .As a resuit. the anount of

Fe[: fomed u.as 1 0 ~ - m d insufficient to substantially incrrüsc the presence of water on the

surface-

The situation is diftierent under higher gas concentration. Highrr amounts of iodine are

d~posited n-ith the consequence of higher arnounts of hygroscopic Frl, on the surface. The

amount of çondrnsing rvater wili also be greater as to commence the corrosion cycle: producing

more Fe[: and condensing more water frorn the pas phase. This interpretations implics the

existence of a critical relative humidity ( Rh,,,,). which is likely quite low given the hygroscopic

nature of Fd,. if the gas phase relative humidity is reduced to a value below the critical \.alur.

the arnoiint of wnter condensing u-ould be suppressed such that tiirther production of Fei: salt

u-ould also be inhibited. In this in~sstigation. the specific value of Rh,,,, \vas not identified. The

tdue ma'.. in fxt. drprnd on several factors such as tube surface temperature. Rtrsults obtainrd

point out that Rh,,,, ma)- be hiphrr than 25%. Deir [7] also found that iodinc deposition beha\-ior

nt medium relative humidit). (-50% ) is sirnilar to the behavior at high relati\.r Iiiiniidit>. ( > 7 5 " * a ).

Hençc. i t is possible that Rh ,,,, ma). lie brtwern 2joh to 50%.

The parameter used to dcscribe the estent of iodinr deposition is the pssudo-saturation

surt'act. concentration ( c,"). The magnitude of C,* appears to be dstemiined by the capability of

the corrosion sites to propagate. As in the abo1.e discussion. the extrnt of propagation will be

dictnted by the provision of the conditions that would suppon corrosion reactions. Ir is for these

reasons thai the d u e of minimum loading differs depending on the relevant operating

conditions. I t is therefore likely that Cso is not surface saturation but rather the point where the

surface reactions cease to propagate. This is consistent with the obsenxtion o r drclining

drposition rate as C," is approached. Conditions such as warm temperature (40°C to 60°C). high

relative humidity and high iodine -as phase concentration would cause high iodine deposition on

the steel surtàce. While low re1atik.e humiditp. low iodine -as phase concentration and high tube

surîàce temperature d l yisld a lower value of C,".

The corrosion reactions that occur on the surface will s\.rntually producr some f o m of

mstal osides such as iron osides. The properties of the surhcr \vil1 to some estent be deteminrd

by the physical propenies of this film of corrosion products. Metal osides are genrral1~- not

soluble in Nater. Therefore unlike Fei'. i t \vil1 not cause further condensation of water. If the film

is porous in nature. dcspite the formation of this tilm. 1: from the =as phase ma!. still ditfùse

throuyh the matris of corrosion products at a reasonable rate and tinally react with Fe atoms. On

the other hand. the film may have low porosit).. increasing the resistance tor difhsion of12 on the

surface. This ma)- ei~entually lead to a decrease in the numbsr of sites availabls for chemisorption

on the surface limiting the propagation of the corrosion.

This intcrpretation is consistent with the observed trends. The highrst iodinr drposiiion

w s obsrrwd at high humidity and tube surfacr trrnpcraturt. b r t w x n 40°C and 60°C.

Presumably cit this temperature the presence of watrr on the surface tvas sri11 adequate whilr the

wmn temperature sen-ed to increasr I 2 corrosion of the surface as kvell as incrrasing the

\,ulnrrability of the steel to pittiny corrosion attack. When the trmperature \vas increased to

90°C. the rate as w l l as the estent of iodinr deposition w r e reducrd. In fact. at this temperature.

hun~idity seems to have no impact on iodine deposition. Watrr condensation was suppressed and

the chernical reactions on the steel surface were also inhibited.

c,' \ a s not obsen-ed when the rsperiment was conducted at high -as concentration

(>j\- [O-"&L). high relative humidity and tube temperatures of 40°C or 60°C- lodine

accumulation continued to climb at a high rate evrn though the surface concentration was a1read~-

higher than 10-' mol/L. Hençe. it appears that under these conditions. the process of pitting

corrosion \vas able to propagatr stabl).. The conditions sermed to be sufficittnt to commence the

cycle betwecn the production of h>-proscopic Frl, and the condensation of nater. The a-arm

temperature l ike l~ increased the rate of reacrions as wrll as increasing the stsel susceptibility to

pitting corrosion attack bu iodinr. Additionally. Cr dissolution from steel surfaces in saturatrd

solutions of 1, at trmperaturrs brtwren 30°C and 60°C has also bcen obsen-rd [8]. The oside

tilm of Cr is maini!. responsible for the passkity of stainless steel surhce. I t ma!. br chat at 40°C

or 60°C. this protective layer was brokcn down by a solution containing I 2 thrn the steel \\.ouid

have lost its inert property.

From the cspsriment xith iodine reloading h hi ch uas carried oui afier a substantial

aniount of iodine dssorbed from the surface. the failure of reloading ma!. have bcrn causrd b>- ri

protectiw l a y r of oside scalr torming on the surface. The scale ma! have creatrd hiphsr

resistançe to iodine diffusion to reach any reactive sites underncath. Evsn if the scale \vas

initinlly porous in nature. it is possible that during the desorpiion proccss the surface was yiven a

pcriod of tinie to precipitate more iron osides which eventuall!. sealed the pores. Furthemore.

any csposed reactive sites on the surface would have besn consumed and subsrqurntly covcred

b!. either the inert la>.er of corrosion products or passivatcd through Cr enrichmcnt of the siirface.

In othcr words. the drsorption process provided the steel surface with the nreded time to de\.elop

its protective oside tilm. Similar xaling of the pores may also occur during the period of

deposition hence lowsring the value of k,.

I t should be noted. hotvek-er. the current investigation did not include an!. visual

esamination of the steel surface. This hypothesis was simpl>- developed basrd on the obsen-ation

of k, and C," as wsll as the information a~eailable in the litsrature. It is recommendrd that the

steel surface be studied visuall!. under magnification to obsrmed the changes in the propenies of

the steel dur to iodinr deposition.

Therefore. in surnmap. the rate and estent of the reactions between I 2 and Fe will be

cistermined by sewral factors u-liich includs :

1 . The numbsr of reacti1-e sites original ly present on the steel surface: This number u il1 be

dictated by the manuîàcturing history of the steel sample.

2. The wmbined efkcts of tube surîàcs tsmpsrature and relative hurnidity thess two factors

will determine whether the corrosion sites will grow in a stable miinnrr tlius causing mors

iodine to deposit and react on the surfacc

i. The iodine @as phase concentration: Gas concentration will affect the retrntion through the

deposition of iodine and the subsequrnt torniation of hygroscopic Fe1 ,. This hwroscopic r C

property reduces the necessac- rillatiw humidity to cause \vater condensation which in st'fect

will assist the formation of more FeI,. The hygroscopic effect. howxer. may onl). be

eI'fc t i w if the surîàce concentration surpasses a certain critical valrie

4. The cycle of reactions may be stopped if the nrcessary conditions required for them to

propagate are not satisfied. Furthemore. the formation of corrosion product scale on the

surface may increase the resistance for iodine diffusion to reactive sites beneath the scale.

5.7 .-\pplications to Gas Sarnpling Lines

The following table summarizes the values the deposition parmeters obserwd under

larious oprrating conditions. Thesc values give a general indication of the bchavior undrr the

specified condition. However. as c m be seen frorn figures 5.1. 5.3. 5.9 and 5-10. somr exceptions

ciid oçcur.

Tabk 5.6 Espsctsd iodine deposition parameter values for stainless steel linss

1 Ill, moVL

< 10“

1 0 - ~ - I O - '

&position rate (rnol:'cm

Reia t ive

Humidin;

Lo~b/High

Lon

> 1 0-

> IO-"

> 10 - ' ( (316~)

> 1 ( j 0 - 1 ~ )

High

High

H ish

\vas not appropriate (ses section 5.2). Therefore.

'.SI is useci instead.

1 Tube Temp. I ! (OC)

23 - 90

23 - 90

23

40 - 60

( a ) kJ iindsr these conditioi

The tollowinp assessrnent is carricd out using the case of tlow rates o l 1.5 L. miri. 4.5

L min and II L min 11-ith a linr length of I O m. If the atmospheric conditions during the use of

the snmple Iine is humid and u-arm (40°C - 60°C). the hiplirst losscs of iodins should be

espsctsd. If iodine retention associated with pitting corrosion continues to propagate. with the

relevant values of k, on the order of 1 O-' cm/s. practically no iodinr will makr it through the

sampling lines. .4 more credible accident situation would yield iodine concentration of IO-'

ni01 L or lowsr. Values in table 5.6 abo1.e are used to generate figure 5.24. Gas concentration is

ki (c m/s)

IO-- - I O - l

- 1 O - ' ?

- (il1 rnoI/cm-.s

> I O - '

> I O - '

C,"

( rnollcm')

1 O+" 1 O-'

1 O-'( - l

-10-

-z

KD

(cm)

- 1 O 0 0

assumed to he IO-' mol/L while kd is assurned to be MO-' cmis. Frorn KD rquals to 1000 cm. a

value o t- Ci" near 10" rnol/crn' c m be assumed. Furthemore. the assumption of steady statr:

lodine Transmission Fraction Vs Tirne L = 10 rn, OD=l14" (0.032" wall)

Time (hour)

Figure 5.24 Variation of iodine transmission fraction with rime

.-1s crin be sesn above. transmission fraction is initially low but increases as rht. surface is

uradually loaded. E\-entuaIl>.. the transmission fraction \vil1 asymptotically approach 100%. It Ci

can also be stxn that incrtsasing the Ilou rate can substantially increass the transmission fraction.

For cornparison. table 5.8 summarizes the values of transmission fraction as calculated by

cquation 1.16. Equation 1.16 \\.il1 only predict a single valus of iodine transmission hction.

Similady. rquation 2.16 ais0 prrdicts highrr iodinr transmission fraction at higher t l o ~ rate.

Tahlr 5.7 Estimatsd iodine transmission fraction based on equation 2.16

k, (cm/s)

5s i O--

5s 10--

js1 O-'

Volumetric Flow rate

1.5 Lllmin

4.5 L'min

12 L/min

Transmission Fraction

5 '/O

3 7%

69%

96

.\ sirnilar behavior of increasing iodine transmission fraction with time has been obsenxd

b > Edson rt.a1.[6]. Thrir report is surnmarizsd in table 3.4. The effrct of increasing tlow rats has

to soms estent been confirmed by a test perforrnrd by Ontario Hydro. The test \vas performed on

Novsrnbrrr 30. 1992 (table 3.1 1 ). By increasing the tlow rate. the transmission fraction r u s also

increased. Based on the same set of test results. the trend of iodine loss with tims. ho~vtxsr.

srrmsd to çontradict the results sho~vn on tigure 5.24. [odins loss wiihin subsequent tests uas

increasing. The trend of incrsasing iodinc loss over timr map haw been a rrsult of the conditions

usrd. The gris concentration \vas on the ordsr of 10 'holol i~ \\-hile the humidit). u-as liksl?

ambisnt (>50°/d Cnder this conditions k, for SS-304L ma'. have been initially lou (figure 5.9).

Clow\+cr. the iodine depositrd during the tirst few tests ma? have bern adequate to form Fe[.

thereby initiating the corrosion cycle. and increasing kd.

6. Conclusions and Recommendations

6.1 Conclusions

1 . It is believrd that 1, is deposited through both physical and chemical adsorption. Physical

adsorption predominated at gas concentrations below i O-' mol/L. Under this condition k, was

found to be betu-een 1 O-' to 1 O-' cmis. A distribution corfticient ( KD of approsirnatsi). 1000

cm u-as obsen-rd. The highrst maximum loading obsen-ed in this condition \\-as on the ordcr

of 1 O-" mol/cm2.

2 . .At gas concentration higher than IO-' rnol/L at low relatiw humidity. slow chrmisorption

ripprcirrd to bc responsible for iodine drposition. The trend was characterized by a slon but

relati\-ely constant iodine deposition rate ( 1 O-' ' - 1 O-'' mol!cm2.s ). Drposition i.sloci t!. ( k, ) is

not appropriate to be uscd under this condition as it appears to vary with gas concentration.

The masirnurn loading was between 1 0 - ~ to IO-' mol/cm2.

7. .At gas concentration higher than 1 0 ' b o l . ' ~ and high humidity. a pitting corrosion t).pr

çtiernisorption mechanisni ma!. have snhancrd deposition. The deposition \.el«cit) \\-as on

1 the order of 10' cmis. Possibl?. the rate close to being limited by &as side mass transfer. 1. is

hrlirvrd to have reactrd with the steel at dekct sites to forrn hygroscopic FrL. FsI. attracted

niors water facilitatitig the propagation of the corrosion reaction. Under some conditions. the

propagation was inhibited resulting in an apparent ma~imum surface loading (C,"). Ho~vrvcr.

at temperature of -10°C or 60°C. no such inhibition occurred resulting in vrp rapid iodine

adsorption. .A slow initial deposition rate that might be associated with pitting incubation

prriod was obsrn~ed.

4. Despitr the espectation that SS-5 16L should perform better. particularly undrr medium and

high gas concentration. there appearcd to be no difkrence bstwesn the tkvo t>.pçs of steel. In

tàct. soms results suggest that SS-304 performed better than SS-3 16L. Cndsr loi\ gas phase

concentration. however. most of the results for SS-3lGL were below detection limit.

suggrsting that SS-3l6L would perform better under low iodine concentration. The most

liksl'. concentration in an accident condition ri-ould bs within the Io\\ rangs (c: 1 O" mol. L 1.

Theretors the use of SS-3 16L ma! be justifieci. This trend. howewr. should still be

contirmed. A different method of yaseous iodinr generation ma!. bs required as to cnablr the

use of higher iodins specitic activity.

5. .Attention shou!d be given to minimizing any damage to the steel surface prior to irs

instnliation in radioiodine sampling lines. todine retention may bci accelerated in regions

wliers darnagc to the protectiw oside layer esists. This is especially true for regions such as

the inls t and outlrt where damagr ma! occur during the cutting of the steel tubing.

.-kiditionally. the initiai steel surface conditions ma) also affect iodins retention. This ma!.

depend on the rnanutàcturing histop* of the steel tubing.

6.2 Recommendations

1. To obtain more svidrncc on the occurrence of corrosion of the steel by II. visual

rnagnitiçation of the conditions of the surface after its rsposurr to iodine should be

performrd. This will provide more evidcnce on the occurrence of corrosion reactions dur to

attack by iodine. Specitically. attention should be placed on whethsr pitting actually occurs

on the surface. The physical appearance as well as chernical composition of the corrosion

products. if an).. should also be esamined. .An oside Iayer that is porous in nature would

provide pathu-ays for the iodinc to diffuse and reach an esposed Fe atom. Identification of the

reaction products accumulated on the surface should also bci performrd. The recognition of

the chemical cornpound formed ti-ould furthrr corroborate rnrchanisrns that are proposed for

th< corrosion rnhanced deposition. Funhrrmore. such study n-ould also providr somr in sight

into the possible rnrchanisms responsibls for the slow chemisorption phrnomrnon.

1. i t is sprculated rhat one of the factors that causes the pitting corrosion to cease to propagatr is

the enrichment of the steel surface with Cr dter the dissolution of the mure rsacti~x

componenr of the metal matris such as Fe. Confirmation of this hypothesis can br ubtained

b! studying the meral composition of the surface after the corrosion product is removed.

3. This rcsearch effort placrd more emphasis on studying the phenomena of iodine drposition.

Ver!. iimited data \vas obtained on the desorption process. More data is requircd to coniirm

the difirent niechanisrns proposed in this report. For physical adsorption. for esample.

desorption rate would be suftkiently fast to match the drposition rats obsrntd in the

heginning of the loading. For chemisorption. desorption ratc would bt. much sloii-sr

compared to the rate of adsorption.

7. References :

I.,4.E.,A.. Off Gas and Air Cleaninrz Svstems for Accident Conditions in Xuclear Power

Plants. Technical report Senes No. 3 8 . IA.E.A. Vienna. 1993

Fluks. R.J. and Jmisson. T.J.. Pickering NGS B : Long term Containment Analysis md

Radiolooical - Conseoliences of Loss-of-Coolant Accidents. Nuclrar Studiss and Sakty

Department Report 82005. 1982.

Incropera. F.P. and Dewit. D.P.. Fundamentals of Heat and Mass Transfer. third edition. John

W ' i l q - & Sons. Toronto. Oh' 1990.

Fos. R.W. and McDonald. AT.. [ntroductions to Fluid Mechanics. third sdition. John U'ils?-

& Sons. Toronto. ON 1 985.

Perry's Chrmical Enginrrrs handbook. sisth cdition. Pem. R.H. and Green. D.W. Editor.

McGraw Hill. New York 1 984.

Funkc F. and Hsllmann. S., '-Reaction of Iodins with Steel Surfaces. Part 1 : Litcrature

Ctud~ " EVR 15668; 1 EN. L.

Deir. C.A.. ".A Study of Interactions of Gas Phase Molecular Iodine nith Stainlsss Steel

. . Tubino,. hl..-\.Sc. Thesis. L'nivcrsity of Toronto. 1995.

Tsukaue. Y.. Kudo- .A.. Nakao. G.. Yamasaki. H. and Kimura. S.. "jiccumulation of-rrihalide

Ions Causeci b>. Corrosion.-' Corrosion vol. 49. no. 3. hlarch 1993.

Tsukaue. A.. Nakao. Takirnoto. Y. and Yoshida. K.. Ynitiation Behaviour of Pittino, in

Stainless Steels bu Accumulation of Triiodide Ions in Water Droplets ." Corrosion vol. 50.

no. 10, October 1994.

10. .A ~vorkine - Pam Reoort on Practical Corrosion Principles. European Fsdrration of Corrosion

Publications. Number 2. Published for the European Federation of Corrosion Publications

Instituts of Metals- London 1989.

1 1 . Sedrik. J..A.. Corrosion of Stainless Steels. second sdition. John U'ilcy & Sons. Toronto

1996.

12. E\.ans. G. J.. .A Study to Idrnti-. Options in Response to PARbIS Samplinc Problems at

Ontario Hydro Nuclear Stations. Universit>. of Toronto. 1996.

. . 1 3. GIisme>-sr. J.A. and Sehmsl. G A . "Line-loss Determination for Air Sarn~ler S ystçrns.

Pacitic 'Iorthwest Laboratory. NL'REGICR-4757. Februq. 199 1

14. Cnrein. P.J.. Pelletier. C.A.. Cline. J.E. and Voilleque. P.G.. "Transmission of Radioiodinr

Throusl h Sanipl ing Lines." Proc. DOE 'iuclear Airbomc Waste blanagsrnsnt and Air

C lsaning Cunkrtincs. i 984.

15. Dick R.S.. '-Bruce NGS-B Stack Monitorinrr Sarnple Inte~ritv Tests. June to Septembrr

1993." Ontario Hydro NESD-OHL Nowrnber 1993

16. Knutson. M.. "PNGS Post Accident Radiation \Ionitorin~ System t P.4RMS ) Sarnple

Intcerit? Tcsts." Ontario H? dro N-03480. .\pril 15 1 993

17. Deir. C.A.. "Retention of iodins upon Stainless Steel Tubin.:." B.;\.Sc. Thesis. Cni\.ersity of

Toronto. April 1991.

18. Edson. J.L.. Duce. S.W. and Tkachyk. J.W.. "Transport Behavior of Iodinr in Effluent

. . Radioactivit?. Monitorinc S!*strms. Idaho National engineering Laboratop. NUREGICR-

4786. Ssptsmber 1987.

19. Grnco. J.M. B t m . . W.E.. Rosenberg H.S. and Momson. D.L.. '-Fission Product deposition

and Its Enhancement under Reactor Accident Conditions: Deposition on PrimaeV Sytcm

Surfaces.' Battelle ~Memorial Instituts. march 1969.

20. Morris. J.B. and Nichols, B.. "The Deposition of Iodine Vapour on Surfaces-.' AERE

E-Ianvell, AERE RS502. 196%

1 1 . T~ler . J. W.. "Surface ..\nalvsis Usine ?(-Ra\. Photoclsctron S~ectrosconv of lodinr Deoosits

. . on 17% Cr ' 129~0 Ni and blild Steel Surfaces Osidized in CO+CH;I Gas blistures, Journal

of 'iuclear Materials 16 1. 1989.

Tyler. J.W.. "Chernical State and Distribution of Iodine Deposits on 17% Cr 1 Ni Steel

Osidized in C02/CH;I - Gas Mixtures." Journal of Nuclear Materials 1 70. 1990.

.\brefah. J. De -4brt.u. H. F. G.. Tekanian. F.. Kim. YS.. Olander. D.R.. "tntsraction ot'

todine with Preosidized Stainless Steel." University of California. Berkeley. Nuclcar

Trchnology. vol. 105. February 1994.

34. Frrnke. F. and Hellmann. S.. "Reaction of Iodine n-ith Steel Surfaces. Part 2 : .\sscssn~tlnt c i t '

Iodint. Steel t nieractions and F.sprrimenta1 St~idy." ECR 1 5668. 2 ES.

25. Fukuda. T. and Omori. S.. "Corrosion of AISI 3 16 and AISI 304 Stainless Steel with todinc

Vapor." Japan Atomic Energ Research Institute. ORNL-tr-4691. January 1975.

26. Lee. B.S. and Jesrer. W.A.. '-Gaseous Radioiodine Deposition Losses in Yuclear Rractor

Saniple Lines." Nuclear Tschnology vol. 1 13. February 19%.

77. Armstrong. G.S.. "BNGS-A PARMS Iodine Line Loss Tests on the SS-j 16 Sampling Lins."

Ontario Hydro. CPSD EPD O H 8 3 .?. June 29 1989.

28. Pecber. D and Bernstein. 1.M.. Handbook of Stainless Steels. McGraw Hill Book Company.

Xsw York 1977..

19. C:jiro. T.. Yoshioka. K. and Starhle R.W.. "Differencss in Corrosion Behaviour of Ferritic

and Austenitic Stainlsss Steels.-' Corrosion 1-01. 50. no. 12. December 1994.

30. Hermas. ;\.A.. Opun. K.. Takapi. S. and Adachi. T.. "Effects of .lllo>.ing Conditions on

Corrosion and Passi\.ation Behaviors of Type 304 Stainless Steel." Corrosion vol. 5 I . no. 1.

January 1995.

3 1 . Nerdharn. N.G.. Freeman P.F.. Wilkinson. J. and Chaprnan. J.. "Atmospheriç Corrosion

Resistance of Stainless Steels:'

32. Froats. J.P.. "Sarnnle Re~resentativeness Testinrr on the PXGS Unit 4 Stack.'. Ontario Hydro.

June 2 1 1990.

33. Barathwal. D.. '-PNGS Stack Samolinc Reoresentativeness Tests - Lnit 1 Eshaust Stack.

Juns 1 7-2 1 1901 .'- Ontario Hydro. CPSD 03480. August 29. 199 1 .

34. Chart of the Niidides. 14th Edition. Gencral EIectric Company. 1989.

35. t l a n q . A.E. Ir.. Srnart. J.A. and Amis E.S.. Simultanrous Spectrophotomctric determination

of Iront I I i and Total Iron with I . I O-~henantroline. J. of r\nalyticai C'hemistry. C'ol. 27. So. 1.

p. 23. Jariuary 1955.

36. Palnier. D.A. Ramette. R.W. and Mesmer. R.E.. Triiodide Ion Formation Eauilibrium and

.-lctivity Coefficient in Aaueous Solution. J . of Solution Chemistn;. Vol. 13. No. 9. pp. 673.

1984.

7 S h i L. L.. Jarman R A . and Burstein. G.T. (eds.). Corrosion : Volume 1 .

bletai/Environmsnt Reactions. third edition. Buttenvsrth Heinemann. Oxford. 1995.

8. Nomenclature

iodine gas phase concentration

iodine gas phase concentration

iodine gas phase concentration

iodine surface concentration ( mol!cm-)

tube inside diamster ( c m )

hinaq- difiusion coefticient (cm' s )

volunietric tlow rate ( cm?s )

iodine transniission fraction

desorption rate constant (s" )

dsposition velocit>. (crnls)

distribution coeftïcisnt (cm )

cas phase niass trans fer cor ftic irnt ( cm/ s )

surface adsorption rate coefticient (crnh)

tube length (cm) 1

iodine deposition rats ( mol,'crn-.s )

iodine dssorption rats (niol'crn-.s)

Rqnulds number for t70u. through the tube :

relative humidity ( O/O)

critical rsl;iti\-s humidity (O'O)

Schmidt nuniber:

Shsnvood nuniber

temperature (OC o r Kelvin)

gas linear velocity (cmis)

p = dcnsity (@cm')

v = kinernatics viscosity (cm2k)

= viscosity of the gas (kg/m.s)

p, = viscosity of the gas near the wall (kg!m.s)

APPENDIX A - DATA PLOTS

StainlessSteel316L page 106-144 StainlessSteel30JL page 145-172

23 C - HlGH [IZ]g - HlGH RH 23 C - HlGH [12]g - HlGH RH

MAR 21,95 - 1 SS-316L 1 23 C High RH 1 High [12]g

iodine (rnol/cW2)

- - . . . gas phase (ml1L) -1 .OE-07 O.OE+OO

O 500 1000 1500

IODINE DISTRIBUTION ALONG TUBING

- 5.OE+05 E 0. u \

O 5 I O 15 20 25

cm FROM INLET

APR 27,95 - 1 SS-316 1 23 C HlGH RH 1 HGH [R]g

1.3E-07 4.5E-07

i hi-

U

0 . O.OE+OO 0

. lodine (mol/cmn2) 5.0E-08

. . . . - 112) gas phase Conc. -2.5E-08 -5.OE-08

O 500 1 O00 1 500

IODINE DISTRIBUTION ALONG TUBING - 1.OE+05 E P a

g O.OE+OO

O 5 10 75 20 25 cm FROM INLET

23 C - HlGH [IZ]g - HlGH RH 23 C - HlGH (1219 - HlGH RH

MAY 4,95- 1 SS-316 1 23 C HIGH RH / HIGH @2]

MAY 4,95 - 2 SS-316 1 23C HlGH RH I HlGH [12]

1.2E47 7.0E-07 DESORPTION

1 .OE-07 6,OE-07

8.OE-08 i 5.0E-07 E . . . @2] gas phase Conc. O c : 6.0E-08 '. 4.0E07 E .

in Y

Z 67 2.OE48 RELOADING 2.0E-07 =

G

O.OE+OO 1 -0E-07 RELOADING 2.6E-07 7 ?- . '. ..

-5.OE-08 -.*

.-*.a-....*......... . . . .* ., O.OE+OO O 1000 2000 3000 4000 5000 6000 7000

1 IME (min) TIME (min)

K)DINE DlSTRlBUTlON ALONG TUBING IODINE DISTRIBUTION ALONG TUBtNG 800

E

O 5 40 15 20 25 cm FROM INLET

5 1 O 1 5 20 cm FROM INLET

23 C - MED (1219 - HlGH RH 23 C - MED [12]g - HlGH RH

JULY 7,95 - 1

1.0E-08

SS316L 1 23 C HKàH RH / MED [IAg

JULY 10,95 - 1 SS-316L 1 23 C RH LOW to HlGH 1 ME0 0219

C'

HGH FW k .

3.OE-08 O a

4 ..' , . 2.OE-08 \ 1 . '. -

RH LOW \ . 1 . + \

1,OE-08 \ !. ?

TllllL (mn) TlME (min)

IODINE DISTRIBUTION ALONG TUBING IODINE DISTRIBUTON ALONG TUBING

1 O 15 cm FROM INLET

O 5 cm FROM INLET

23 C - HlGH [I2]g - LOW RH 23 C - HlGH [12]g - LOW RH JUNE 20,95 - 1 SS316L 1 23 C JUNE 20,95 - 2 SS-316L 1 23 C

LOW RH to HlGH RH I HlGH [IZ] LOW RH / 23 C 1 HIGH [U]g 7.5E-06

6.0E-06 7.5E-07

8.0E-07 , Lodine (mllcrrP2) * . Mine Retention . . . .[iqgmoln 5 . 0 ~ 4 6 ( I T W V C ~ ~ Z )

- r - Gas phase Conc. ( m w

T M (mn)

1 O0 200 300

TlME (min)

lodine Distribution Along Tubing

5 1 O 15 cm from lnlet

23 C - MED [12Jg - LOW RH 23 C - MED [12]g - LOW RH

JULY 7,95 - 1

1 .OEm

JULY 10,95 - 1 SS-316L 123 C RH LOW to HlGH I MED [12]g

5.OE-08 5.OE-08

. * -.- RH LOW \ . ' O

---+ \ ' I n r nn l e

LOW RH

0.oHXl . .&@ O.OE+OO -1 .OE-08 O.OE+OO

O 200 400 60d 800 d o 0 0 lm 1400 1600 O 500 1 O00 1500

TniE (mn) TIME (min)

IODINE DISTRIBUTION ALONG TUBING h

KIDINE DISTRIf3UTK)N ALONG TUBING

10 15

cm FUOM INLET 10 15

cm FROM INLET

r d V > V ) b 0 0 0 0 + + + + W W W W ? ? ? Y C I F l r r

23 C - LOW [12]9 - LOW RH 23 C - LOW [12]g - LOW RH O C T 1 2 , 9 5 - 1 SS-316L 123 C

L O W R H 1 L O W [IZ] 3.OE-10 1 .O€-09 . lodlne Retention (mollcrnA2) 2 .5E- t 0 - a- Gas phase Conc. (mollL)

O.OE+OO ...mm..a.a.w*~. l

RUN S T A R T E D

TIME (min)

IODINE DISTRIBUTION ALONG TUBING

O 4 8 12 16 20 24 28 cm FROM INLET

O C T 2 1 , 9 5 - 2 S S - 3 1 6 L 1 2 3 C L O W R H I L O W [12]g

r : + O.OE+OO +' . lodino (mol /cmA2)

2.OE-1 1

I -- [12]g mollL -5.OE-11 O.OE+OO

O 1 0 0 200 300 4 0 0 600 6 0 0

T IME (m in )

ODINE DISTRIBLJTDN ALONG TUBING 300

O 5 10 15 20 25 cm FROM INLET

23 C - LOW [12)g - LOW RH 23 C - LOW [12]9 - LOW RH OCT 23, 95 - 2 SS-316L 123 C

RH LOW to HIGH 1 LOW (1219 5.OE-11 8.OE-11

lodine(mollcmA2) O

4 , 0 ~ - 1 1 -3- 1121 gas phase Conc. moVL m * 7.OE-11

*. 6.OE-11 ( 3.OE-11 cm.' I y - . & O 5.OE-11 \ - * / * - * - *: **-. _ -

O g 2.OE-11 - * . 0 *:'* 4 .0E- i l E

' / fl 8 rn

W n

z # 4 **%* 2' 0 1.OE-I l *r e.4

3.OE-11 0

# *ye :a. an- 2.OE-11

#

-1 .OE-11 8

O.OE+OO O 100 200 300 400 500 600

TlME (min)

IODINE OISTRtBUTION ALONG TUBlNG

5 10 15 20 25

cm FROM INLET

OCT 24 1995 - 2 SS-316L 1 2 3 C LOW R H 1 LOW [IZ]g

-5,OE-11 0.00€+00 O 200 400 600 800

TlME (min)

IODINE DISTRIBUTION ALONG TUBlNG 30000

10 15 20 cm FROM IN LET

23 C - LOW [I2]g - LOW RH 23 C - LOW [12]g - LOW RH JULY 21,1995.1 SS-316L 1 23 C

LOWRH 1 L W @Ag 1 .OE99

JULY 21,1995 - 2 SS-316L 1 LOW RH 23 C 1 LOW [IZlg

2.5E-09 1.50E-09 2.5E-09 , lodine Retention (mollcmA2)

- - - Gas phase Conc. (mollL) 2.OE-09

TlME (min)

(JULY 21-1 and JULY 21-2 ; SEE PAGE A4 FOR IODINE DlSTRlBUTlON ALONG THE TUBING)

2 " .- A.. . .-

O

40 C - HlGH OhRH - MED [12]g

FE6 13 1996 - 1 SS-316L 140 C RH : LOW to HlGH 1 M E D (1219

' HlGH R l l il = lodine (mollcrn42)

- a - [12]g molll.

1000 2000 3000 4000 5000

TlME (rnln)

IODINE DISTRIBUTK)N ALONG TUBING

5 10 15 20 25 cm FROM INLET

40 C - HlGH %RH - LOW [12]g M A R 2 9 , 9 6 - 1 SSs316L 1 4 0 C

R H : L O W - M E D - HIGH 1 L O W [ I ~ ] Q

1 .OE-10

lodine (rnollcmA2)

1.SE-11 - m- [IZ]g mollL , L O W R H I

M E 0 R H H l G H RH

1.OE-11 ; , I

' ; 7.OE-11

IODINE DISTRIBUTION ALONG TUBING

5 10 15 20 cm FROM INLET

40 C - HlGH %RH - LOW [12]g 40 C - HlGH %RH - LOW [12]g M A R 2 9 , 9 6 - 2 S S - 3 1 6 L 1 4 0 C J u l y 4 , 96 - 2 SS-316L 1 4 0 C

R H : L O W - M E D - H I G H 1 L O W [12]g HtGH %RH I L O W (12]g

1 .OE-11 1 .OE-10 5.OE-11 3.OE-10

4

-6 .OE-12 IL - , I

r t ' - - - 4 . 2.OE-11 t

-8 .OE-12 - - * a 1 .OE-11 m M E 0 R H a H l G H R H

T I M E ( m i n ) l o d l n e ( m o l l c m A 2 )

' 1 - lodine (mollcmA2) A - r - [12]g mollL

O.OE+OO O 1 0 0 200 3 0 0 4 0 0

TlME (min)

IODINE DISTRIBUTION ALONG TUBING IODINE DISTRIBUTION ALONG TUBING

300 A 300 P. y 200

L 0 200

W Ill z IO0 g 700 O cl 0 O 0 0

O 5 1 O 15 O

20 5

25 1 O 15 2 0 25

cm FROM tNLET cm FROM INLET

40 C - LOW %RH - HIGH [12]g 40 C - LOW %RH - HIGH [12]g D O C 2 3 , 9 6 - 2 S S - 3 1 6 L 140 C

L O W R H I HIGH [I2]g

û c 2 3 , 9 6 - 1 SS-316L 140 C LOW RH IH1GH (1219

5 . lodine (mollcmA2) O.OE+OO 0 . 4 ~

a 2.OE-08 - e - [12]g moll t

-5.OE-10 O.OE+OO O 50 100 150 200 250 300

TIME (min)

-1 .O€-1 O O.OE+OO O 5 0 100 160 200 2 5 0 300

TIME (mln)

IODINE DISTRIBUTION ALONG TUBlNG I O D I N E D I S T R I B U T I O N A L O N G T U B I N G - 4 , 0 0 0 -

O 5 10 15 2 0 2 S

cm FROM INLET c m F R O M I N L E T

40 C - LOW %RH - MED 11219 40 C - LOW %RH - MED [IZlg S E P T 10 , 96 - 1 SS-316L 140 C

R H : L O W - M E 0 1 M E D [IZ]g 5.OE-07 6.OE-08

FEB 13 1996 - 1 SS-316L 140 C RH : LOW to HIGH 1 MED (1219

OESORPTION

; \ HlGH %RH

LOW R H 2.OE-O8

todlne (mollcmA2)

a - [IZ]g mollL

.. ~ o â i n e ( r n o 1 1 c m ~ 2 ) 1.OE-08

- r - (I2)g moilL

O.OE+OO O 1 5 0 0 2000 2 5 0 0 3000

TIME (min)

1000 2000 3000 4000 5000

TIME (min)

KIDINE DlSTRlBlJTlON ALONG TUBING IODINE DISTRIBUTION ALONG TUBING

cm F ROM INLET

40 C - LOW %RH - MED [12]g

SEPT 10 ,96 - 2 SS-316L 1 4 0 C R H : L O W - MED I M E 0 il219

IODINE DISTRIBUTION ALONG TUBING

O 5 10 15 2 0 2 5 cm FROM INLET

40 C - LOW %RH - LOW [12]g 40 C - LOW %RH - LOW [12]g M A R 2 9 , 9 6 - 1 S S - 3 1 6 L 1 4 0 C

R H : L O W - MED - HfGH I L O W [lz]g M A R 2 9 , 9 6 - 2 S S - 3 1 6 L / 4 0 C

R H : L O W - M E D - H I G H I L O W (12jg 1 .OE-10

lodine Imol /cmA2)

1 .5E-11 - m- [IZ]g molli. , L O W RH l

M E 0 RH HlGH RI1

1 .DE-1 1 ; , , a e 7.OE-1 1

. , - t *

- 6 .OE-12 b - , 1; 2.OE-11 l C

1 - - - - i , 1 ) ' - 8 . O E - 1 2 - - * e 1 .O€-1 1 1 H l G H R H * M E 0 R H ,

-1 . O E - I l * O.OE+OO O 1 0 0 0 2 0 0 0 3 0 0 0

T I M E ( m i n ) lod lne ( m o l l c m A 2 )

- a - (1219 mol lL

DDlNE DISTRIBUTION ALONG TUBING IODINE DISTRIBUTION ALONG TUBING

5 10 15 20 cm FROM INLET 1 O 15 20 25

cm FROM INLET

60 C - HlGH %RH - HlGH [ l i ]g 60 C - HlGH %RH - HlGH [h]g M A Y 8 , 9 6 - 1 SS-316L 160 C

HIGH %RH 1 HIGH [12]g

lodine (rnollcrnA2) 1.2E-07 O

M A Y 8 , 9 6 - 2 SS*316L 160C HIGH %RH / HIGH [ 1 2 ] ~

3.5E-07 lodine (mol lcmA2) O

-2.OE-08 O 100 200 300

TIME (min)

IODINE DISTRIBUTION ALONG T U B I N G IODINE DISTRIBUTION ALONG TUBING - 30,000 1 .OE+06 P

0 20,000 W - 2 E 1.0€+04 W

10,000 0 0 O 1 .OE+00

O 5 10 1 5 20 25

cm F R O M INLET O 5 1 O 15 20 25

cm FROM INLET

60 C - HIGH %RH - LOW [h]g J U L Y 2 , 9 6 - 1 SS-316L 160 C

HIGH %RH 1 L O W 11239

-1.OE-11 (* . O

O

O O

. 1 . lodine (moIlcmA2) 5 . O E - i l -2.OE-11

- a - (1219 rnollL -3 .0E-11 O.O€+OO

O 100 200 300 4 0 0

TtME (min)

KIDINE DISTRIBUTION ALONG TUBlNG = 80

O 5 10 15 20 25 cm FROM INLET

60 C - LOW %RH - HlGH [lz]g APR 26,96 - 1 SS-316L 160 C

LOW %RH to MED %RH / H1GH [12]9

1.2E-07 2.OE-07 . lodine (rnollcrnA2) 1 .O€-07

1.8E-07 4- (1219 mollL

1.6E-07

O 100 200 300 400

TlME (min)

IODINE DISTRIBUTION ALONG TUBING 1 E+O5

O 5 I O 15 2 0 2 5 cm FROM INLET

60 C - LOW %RH - HlGH [12]g APR 26,96 - 2 SS-316L 160C

LOW %RH 1 HlGH (1219

-+- lodine (mol/cmA2) 1.8E-07

2.OE-09 - - - [ I Z ] ~ ~ O I I L 1.6E-07

1.5E-09 a 1.4E-O7 5 - - E l.OE-O9 t 1.2E-07 i - - O

w 1 .OE-07 E 5 5.OE-10

0

n 0

8.OE-O8

O.OE+OO 6.OE-08

4.0E-08 -5.OE-10 I

2.OE-08

TlME (min)

IODINE DISTRIBUTION ALONG TUBING

O 5 1 O 15 20 25 cm FROM INLET

90'C - HlGH %RH - HlGH [ I z ] ~ 90'C - HlGH %RH - MED (1219 APRlL 19, 96 - 2 SS-316L 190C

HlGH %RH 1 HlGH (1219

MAY 2 3 , 9 6

1 .4E-10

1+2E-10

1 .O€-1 0

N 8.OE-11

C

6.OE-11 Z

4.OE-11 - W z 2 .OE- i l O 0 O.OE+OO

-2.OE-11

-4.OE-11

t 1 SS-316L 190 C HIGH R H I MED [I2]g

1.6E-09 8.OE-07

1.4E-09 lodine (mollcmA2) O - - - [I2]g mollL 7.OE-07

1.2E-09 a

. .

O 50 100 150 200 250 300

TlME (min)

O 100 200 300 400

T1ME (min)

IODINE DISTRIBUTION A L O N G TUBING

O O 5 10 15 20 25

cm FROM INLET 5 10 15 2 0 25

cm FROM INLET

90'C - HIGH %RH - MED [I~]cJ M A Y 2 3 , 9 6 - 2 S S - 3 1 6 L 1 9 0 C

H I G H R H I M E D [ U j g

- 1 .SE - 1 O . . l o d l n e (rnol lcrnf i2)

O 1 0 0 2 0 0 3 0 0 4 O 0

TJME ( m i n )

IODINE DISTRIBUTION ALONG TUBING

5 10 15 20 cm FROM INLET

0 0 0 0 0 O O O O r O O O r 0 0 - O r - ( w W

90'C - LOW %RH - HlGH [h]g 90'C - LOW %RH - MED [h]g J A N 1 7 , 9 6 - 1 S S - 3 1 6 L 1 9 0 C

L O W % R H I H I G H [12]g

2 .OE -09 2,OE -07

lod ine ( m o I I c m A 2 )

- - - f I2 )g. mol lL

1 . 5E - 0 9 . 9 1 . 5 E - 0 7 ..

-. a

I

* - 5 . 0 E - 1 0 O.OE+OO

O 1 0 0 2 0 0 3 O O 4 0 0

T I M E ( m i n )

IODINE DISTRIBUTION ALONG TUBING

O 5 10 15 2 0 25 cm FROM INLET

M A Y 2 0 , 9 6 * 1 S S - 3 1 6 L 1 9 0 C

L O W % R H 1 M E D [12]g

, l od ine ( m o l l c m A 2 ) t 4.5E-09

4.OE-10 - a - (1219 mollL 4.OE-O9

T IME ( m i n )

IODINE DISTRIBUTION ALONG TUBING 1 O00

O 5 10 15 20 25 cm FROM INLET

90'C - LOW %RH - LOW [Iz]g O C T 2 3 , 9 5 - 2 S S - 3 1 6 L 1 9 0 C

R H c 1 0 % I L O W [12]g

90'C - LOW %RH - LOW (l2]g O C T 2 4 , 9 5 - 1

S S - 3 1 6 L 1 9 0 C L O W % R H I L O W (121

1 . 5 E - 1 0 4 . 5 E - 1 1

a

: D E S O R P T I O N : BOC 1 4.OE-1 1 L O W R H

1 . 0 E - t 0 3 .5E -1 1

N < ' HIGH R H

O ? ' ' Li

=. .- 2 . 5 E - 1 1 8 UJ g O

F 2.OE-11 E

a a N C

1 1 . 5 E - 1 1 I

4

a . lodine ( m o l l c m A 2 ) a 5 ,OE-12 I - r - (12)g moll i .

-5 .OE-1 1 O.OE+OO

O 5 O 0 1 0 0 0 1 5 0 0 2 0 0 0

TIME ( m i n ) TIME ( m i n )

IODINE DISTRIBUTION ALONG TUBlNG

4 8 12 16 20 24 28

cm FROM INLET

lODlNE D(STRiBMDN ALONG TUBlNG 20000

O 5 10 15 20 25 cm FROM INLET

23'C - HIGH %RH - MED [Ii]g APR 3 0 , 9 6 - 1 SS-304L 123 C

HIGH %RH 1 MED [12]g

O O .* 4 O

0 . 0 * . . fi I

I 0 b

0' O

O . , + a , lodlne (mollcmA2)

23'C - HIGH %RH - MED [h]g

APR 3 0 , 9 6 - 2 SS-304L 1 2 3 C HIGH %RH 1 ME0 [I2]g

2.5E-09 4.OE-08

. A . 1 .OE-O8

O.OE+OO 8 f . . . . lodine (mollcmA2) 4- [IZIg. mollL

O.OE+OO -5.OE-10 O 1 O0 200 300 400 O.OE+OO

O 100 200 300 400 TlME (min)

TlME (min)

IODINE D1STRIBUTION ALONG TUBlNG

O 5 10 15 20 25

cm F ROM INLET

IODINE DISTRIBUTION ALONG TUBlNG ., 1500

O 5 I O 15 20 25 cm FROM INLET

23'C - HIGH %RH - LOW [h]g J U N E 2 8 , 9 6 - 1 S S - 3 0 4 L 1 2 3 C

H I G H O/oRH 1 L O W 11219

1 .OE-10 2.OE-10

T I M E ( m i n )

IODINE DISTRIBUTION ALONG TUBlNG

23'C - HIGH %RH - LOW [h]g JUNE 2 8 . 9 6 - 2 S S - 3 0 4 L 1 2 3 C

H I G H % R H 1 L O W [IZ]g

-1 .OE-11 o. a *

H U N SlARlED 4 s . o o E - ~ 1 . . -2 .OE-11

-3 .OE-11 a O,OOE+OO

O 1 0 0 2 0 0 300 400

TIME ( m i n )

IODINE DISTRIBUTION ALONG TUBlNG 300

O 5 1 O 1 5 20 25

cm FROM INLET O 5 10 15 20

cm FROM INLET

' ai-

23'c - LOW %RH - HlGH [12]g 23'C - LOW %RH - HlGH [h]g JAN 2 6 , 9 6 - 1 SS-304L 1 2 3 C

LOW % R H I HIGH (1219 JAN 3 0 , 9 6 - 1 SS-304L 1 2 3 C

LOW lo HlGH RH 1 HiGH I12)g

- lodine (mol/crnA2)

- r - [IZlg mollL

-5.OE-10 O.OE+OO O 50 100 150 200 250 300 350

TlME (min)

IODINE DISTRIBUTION ALONG TUBING 2000

O 5 10 15 20 25 cm FROM INLET

-5.OE-10 O.OE+OO

O 200 400 6 0 0 800 1000 1200

TiME (min)

IODINE DISTRIBUTION ALONG TUBING

O 5 10 15 20 26 cm FROM INLET

23'C - LOW %RH - MED [12]g

JULY 1.96 - MC 1 SS-304L 123 C LOW RH 1 MED [Id9

., lodlne (moUcmA2)

- .- [12] gas phase Conc. moUL LOOE-09

-1.OE-10 0.00E+00 O 50 100 150 200 250 300 350

TlME (min)

23'C - LOW %RH - MED [12]g JULY 1.96 - MC2

ZSE-lu

SS-304L 1 23 C LOW RH

. [IZ] gas phase Conc. mol I l

40'C - HlGH %RH - HIGH [l2]g M A Y 3 ,96 - 1 SS-304L 140 C

HlGH RH

5.0E-07 8 DESORPTION 40 C 1 HlGH RH

CY 5 Q 5 4.OE-07 .DESORPTION \ - - ; 4 0 C I L O W

,+ lodine (moilcmA2)

- i- [12]g rnoVL

500 1 O00 1500 2000

TlME (min)

IODINE DISTRIBUTION ALONG TUBlNG

5 10 15 2 O 2 5 cm FROM INLET

40'C - HlGH %RH - HIGH [I2]g

MAY 3 , 9 6 - 2

7.OE-07

SS-304L 123C HlGH RH

8.OE-07 * 8

c " DESORPTION D E & R P T I O ~ g 4.0E-07 : :; 40 C I HlGH RH 40 C i HiGH RH 5'0E-07 Z - O

O 1 I O

É 3.OE-07 1;; l O 4.OE-07 Ë C O

rn 1 it' I 6'

Z 2.OE-07 , (a 0 . * 3.OE-O7

I '.>

1 t. g 1.OE-07 * O 9 2.OE-07 1 < #

0 ,f .. lodine (mollcmA2) O.OE+OO e>

I - a - (12]g mol/L 1 .O€-07

-1 .O€-07 O.OE+OO O 500 1000 1500 2000

TIME (mln)

IODINE DlSTRlBUTlON ALONG TUBlNG Ê 150,000 n 0 100,000 W z 50,000 O 0 O

O 5 IO 15 2 0 25 cm FROM INLET

40'C - HlGH %RH - MED [h]g

FE5 28 1996 - 1 SS-304L I60C CO 40C HlGH W H I MED [12]g

O 500 1 O00 1500 2000

TlME (min)

DDINE DISTRIBUTION ALONG TUBING 3000

40'C - HlGH %RH - MED [h]g

L

-5.OE-10 O.OE+OO O 500 1 0 0 0 1000 2 0 0 O

T I M E (min)

IODINE D ISTRiBUTiON ALONG TUBING

Ê l E t 0 5 IE+O~

2 1E+O1 O 5 1 O 1 5 2 0 2 6

cm F R O M INLET

40'C - HlGH %RH - LOW [h ]g

JUNE 26,96 - 2 SS-304L 140C HlGH %RH I LOW (1219

4,5E-10

5.OE-11 8 .. . 8 .

*. ' . lodine (mollcmA2) O.OE+OO . *ms.ri - [I2]g rnollL

O 100 200 300 400

TlME (min)

IODINE DISTRIBUTION ALONG TUBING

5 10 15 20 25 cm FROM INLET

40'C - LOW %RH - HIGH [lz]g

A u 9 2 7 , 1996 - 1 S S - 3 0 4 L 1 4 0 C L O W R H 1 HIGH [ IZ]g

3.0E-09

TlME ( m i n )

IODINE D I S T R I B U T I O N A L O N G T U B I N G

c m F R O M INLET

40'C - LOW %RH - MED [12]g

M A Y 7 , 9 6 - 1 SS-304L 140 C LOW RH 1 MED [12]g

I " -+- lodine (mollcmA2) -1 .O€-09 1 .O€-09 - 1 - [12]9 mollL -2.OE-09 - O.OE+OO

O 1 O0 200 300 400

TlME (min)

IODINE DISTRIBUTION ALONG TUBING

5 1 O 15 20 26

cm FROM INLET

40'C - LOW %RH - MED [Ill9 M A Y 7 , 9 6 - 2 SS-304L 140C

LOW RH 1 ME0 [12]g

7.5E-09 8.0OE-O9

+- lodlne (mollcmA2)

TlME (min]

IODINE DlSTRlBUTDN ALONG TUBING

40'C - LOW %RH - MED [12]g NOV 4,1996 - 1 SS-304L / 40 C

LOW RH 1 MED [t2]g

O .

O.OE+OO ..noof. . iodine (mollcmA2) 5.OE-10 1

* - [12]g mollL -1 .OE-09 O.OE+OO

O 5 O 100 150 200 250

TlME (min)

O 5 1 O 15 20 cm FROM INLET

1OOOO

O 6 10 15 20 25

cm FROM I N W

e

'Ji 00

60'C - HlGH %RH - HlGH (12]g APR 22,97 - 1 SS-304L 160 C

HlGH RH I HlGH [i2]g 1.4E-06 ,

I

' 'DESORPTION @

* I I ' 6 0 ~ 1 LOW RI1 ;

23C 1 LOW RI1

TlME (min)

DDINE DlSTRlBUTiûN ALONG TUBING

60'C - HIGH %RH - HlGH [ I z ] ~ A P R 2 2 , 9 6 - 1 SS-304L

LOADING a l 60C I HlGH %RH 1 HlGH [12]g

8.OE-07 1 .O€-06 e ' I

7.OE-07 6OC 1 LOW RI1

6.OE-07 .' .' . QESOHPTION

5.OE-07 :60c 1 HIGH R H < * DESORPTION

60C 1 M E 0 RI1 4.OE-07 O *

I

*

E . m DESORPTION

3 . O E - 0 7 2 23c I LOW R H

2 2.OE-O7 <

. lodine (mollcmA2) O.OE+OO / . [l2] gas phase Conc. mollL

-1.OE-07 a O.OE+OO O 1000 2000 3000 4000

TlME (min)

IODINE DISTRIBUTK)N ALONG TUBING 1 €+O6

5 1 0 15 2 O 2 5 cm FROM INLET

O 5 10 15 20 25 cm FROM INLET

60'C - HlGH %RH - MED [ I z ] ~ 60'C - HlGH %RH - MED [h]g FEB 28 1996 - 1 SS-304L 1 60C to 40C

HIGH W H 1 MED [I2]g 2.5E-09 1 .OE-08

11

8 8

I l

I

8 tD

lodine (mollcm

- 4 - [12]g mollL -5.OE-10 O.OE+OO

O 500 1 O00 1500 2000

TIME (min)

IODtNE DISTRIBUTION ALONG TUBING 3000

F E 6 28 1996 - 2 SS-304L 16OC IO 40C HIGH %RH 1 MED [12]g

7 . lodlne (moI lcmA2) 8.OE-09 2.OE-09 -=- [IZ]g IIIOIIL

IODINE DISTRIBUTION ALONG TUBING 1 E tOs 1EtO4 -

w 1E+03

1E+02 0 1EtO1

O 5 10 15 2 0 2 5 cm FROM INLET

60'C - HlGH %RH - LOW [lz]g JUNE 24,96 - i SS-304L 160 C

HIGH RH I LOW [12]g

1 * - a 5.OE-11 I O lodine (mollcmA2)

4

O 50 100 150 200 250 300

TIME (min)

IODINE DISTRIBUTION ALONG TUBING 4 0 0 0

O O 5 1 O 1 5 2 O 2 5

cm FROM INLET

60'C - HIGH %RH - LOW [lz]g JUNE 24,96 - 2 SS-304L 1 60C

HIGH W H / LOW [123g 5.0E-10 5.OE-1 O

8

-3 .O E-1 O O.OE+OO O 50 IO0 150 200 250 300

TlME (min)

IODINE DISTRIBUTION ALONG TUBING

E 4 0 0 0

O 5 1 O 15 2 0 2 5 cm F R O M I N L E T

60'C - LOW %RH - HlGH [h]g

Au9 23,1996 - 1 SS-304L 160 C LOW RH 1 HlGH [I2]g

3.5E-09 1.4E-07

3.OE-09 . bdine (mollcmA2) ,.ym 1.2E-07 - 4 - [l2]g mollL 07 2.5E-O9 r

a.# ,* . .r œ- ' 1 .OE-07

-5.OE-1 O O.OE+OO O 50 100 150 200 250 300

TIME (min)

IODINE DISTRIBUTION ALONG TUBING 7 5000

0

O 5 1 O 15 20 25 cm FROM INLET

60'C - LOW %RH - HlGH [12]g

SS.304L 160 C LOW RH 1 HlGH I12Jg

O . lodine (molicmAZ) .. . 2.5E-09

-i- [12]g moUL 00 1.2E-07 . .-fl . ' I .

1

-5.OE-10 O.OE+OO O 50 100 150 200 250 300

TlME (min)

IODINE DISTRIBUTION ALONG TUBING 15000

P 0 10000 /'

O 5 1 O 15 20 25 cm FROM INLET

60'C - LOW %RH - MED [12]g J U L Y 18 , '96 - 2 S S - 3 0 4 L 1 6 0 C

L O W %RH / M E 0 11219

3.OE-10

0 . ' . .'

-5 .OE-11 ? . lodlne ( m o l l c m A 2 ) i. -(12]g m o l l L

-1 .O€-1 O

O 1 O0 2 0 0 3 0 0

DDlNE DlSTRlBUTlON ALONG TUBING

5 I O 15 20 cm FROM INLET

60'C - LOW %RH - MED [IZ]CJ

O C T 2 4 , 9 6 . 1 S S - 3 0 4 L 1 6 0 C L O W R H 1 MED (1219

I

@ lod ine (mol lcrnA2) O.OE+OO ~-..,.mo*). . - - [I2]g rnollL

KIDINE DISTRIBWION ALONG TUBlNG

w m * m 0 0 0 0 + + + + W W W W 9 9 9 9

90'C - HlGH %RH - MED [l2]g M A Y 2 4 , 9 6 - 1 SS-304L 1 9 0 C

H l G H %RH 1 MED [12]g

90'C - HlGH %RH - MED [12]g MAY 2 4 , 96 - 2 SS.304L 190C

HIGH RH I M E D [I2]g

2.OE-10

6 - * 1 .O E -09 1 .O€-09

-5.OE-11 lodine (mollcmA2) -1,OE-10 r lodlne (rnollcmA2) 6

. . [ I Z ] g a s phase C o n c . rnollL 5.OE-10 . - . . [12] gas phase Conc. mollL 5'0E-10

-1 .OE-10 6

O.OE+OO -1 .5E-10 O.OE+OO

O 100 200 300 400 500 6 0 0 O 100 200 300 400 500 600

TIME (min) TIME (mi t \ )

IODINE DISTRIBUTION ALONG TUBING IODINE DISTRIBUTION ALONG TUBING 1500 - 500 s

0 1000 !i ul 0 z 500 n

u 260 \ t n ----+---

O 0 . 0 0

O 5 10 15 2 O 2 5 O 5 10 15 2 O 2 5 cm FROM INLET cm FROM INLET

90'C - HlGH %RH - LOW [h]g JUNE 20,96 - 1 SS-304L / 90 C

HlGH %RH I LOW [12]g

. . :5

lodine (mol/cmA2) ~.OE-IO

TlME (min)

IODINE DISTRIBUTION ALONG TUBING 3000

O 5 10 15 20 25 cm FROM INLi3

90'C - HlGH %RH - LOW [h]g JUNE 20 ,96 - 2 SS-304L 190C

HIGH %RH I LOW [12]g

, . ; . lodine (rnol/cmn2) 1 .OE-IO

O.OE+OO .%-os*m ,: v - . . . [12]g mollL

TlME (min)

IODINE DISTRIBUTION ALONG TUBING CL

3000

O 5 10 15 2 0 2 5

cm FROM INLET

90'C - LOW %RH - MED [lz]g JULY 17, 9 6 - 1 SS-304L 1 9 0 C

L O W %RH l M E D [12]g 4.OE-10 . lodine (molrcmA2)

- - (1219 mollL 3.OE-10

7

2.OE-10 N

-1.OE-10 \ '

-2.OE-10 O 5 0 100 150 200 250

TIME (rnln)

ODINE DISTRIBUTION ALONG TUBlNG 300

W \

z 100 . . n . .

O O 6 1 O 15 20 2 5

cm FROM INLET

90'C - LOW %RH - MED [IzJg J U L Y 1 7 , 9 6 - 2 SS.304L 190 C

L O W %RH I M E D [12]g

2 .5E-10

1

-5.OE-11 t , \ , ; j , * . lodine (mol lcmn2) . ! ' 4

. . [12]g mollL

- 1 .OE-10

ODIN€ DISTRIBUTION ALONG TUBlNG 600

0 0 0 . - r c . I

W W W 9 9 9 e w - -

r o r r r r r r r r r r r r r I I I , , ,

w w w w w w 9 9 9 9 9 9

.Appr.ndis B - Tabulated Results : -4dsorption

Table B. 1 SS-3 16L High [LI,, - High O,,6RH

-Pribls B.: SS-3 16L High [I,], - Low %RH

Maximum Loading

7

Deposition Masimum Loading

rd te

Deposition

rate

kd (cm/s)

Lla? 8 96 - 1

Temp

( O C )

Experiment Gas Phase

( m o n )

1.8 s l O " 60 1.4 sl0" 3.6 S I O ' ~ ' > IO" mol/cni-

Table B.3 SS-304L High [LI, - High %H

5

I I 1

> 5s l O- ' mulicrn'

Temp

( O C )

3

Apr 22 96 - 2

Dcposition

rate

( mo~/cm'.s)

(cm/s)

4.0 s l O-'

iMaximum Loading

60

90

90

5 s l O-' mol!cm- 1.4 S I O

Table BA SS-304L High [LI,, - - - Low ?GRH

6.9 SI O-"

3 .0s l0- '

6 sI0-'

5 s 1 O--

1

2

3

I

Cas Phase

(mol/L)

Z.sl0-'

6.5~10-"

9 s 1 O->

9x10- '

910"

8.0 SI O-"

8 . 0~10 - '

Experirnent

Jan 26 96 - I

J a n 3 0 9 6 - 1

Aug 27 96 - 1

1.22slo-"l

1.9 s10-"

1.5 SIO-"

> 4sl 0" rnol/cm-

2s 1 O*" mol/cm-

1 s 1 O-" molicm'

Temp

(OC)

-3 7 3

-3 7 -

40

60

60

90

90

4

5

6

7

Aiig2396-1

. - \ug3 9 6 - 2

Jul 1 1 96 - 1

Jul11 96-2

iq

(crn/s)

5 s 1 O-'

2 s 10-O

1.9 s10"

I .S s l O-'

2.3 s 10''

2.6 s 1 O--

1 .S sl0--

Deposition

rate

( moi/cmL.s)

IO+'

1 O+

1.7~10-"

1.65s 1 O-"

2. I l s l0-"

2 . l s 10"'

1.5 s10-'-

Maximum Loading

> 1.5s 10"' rnol!'cm-

> 2.0s 1 O-" rnol.,cm'

5s IO-' molicm'

> 2.5s 1 O-" niol/cm-

> 2.5s 1 O-" niol/ctn-

2s 1 O*' moL1crn'

2s l O-" rnol/crn'

Table B.5 SS-3 16L Medium [LI, - High %H

Experiment Cas Phase Temp p

1 t

7 Feb 13 9 6 - 1 1 .1 s10-" 10 2.2 s 1 0 - ' r

S Sep 10 96 - 1 2.6 s l o - " -10 3.8 s l O - '

1 I Jul 22 96 - 2 1.3 x 1 O-' 60 S s l O-'

Table B.6 SS-3 1 6 L Medium [I,], - Lmv ?/oRH

Deposition

rate

(mol/crn2.s)

1.8E- 13

8.3 d o - l -

Maximum Loading

1 -6 s l O- ' mol/crn-

> I si 0'" 8 mol/cm'

2 s10-' mol/cm-

1.6 s 1 O-' mul!cm-

> 4.5 si O- ' moi/cm-

> 8 s 1 O-"' molicm'

Espcrimcnt h

1 Gas Phase

(mol1L)

1.2s10- ' 1

Temp Deposition

J u l ? 9 5 - I

;Maximum Loading

(OC)

2 3

(cm/s)

1 s 1 O-'

rate

( r n o ~ c m ' . ~ )

4.9 s l O"" -

> 4 s 1 O-' ' ' rnol!cm-

Table 8.7 SS-3OJL Medium [I,], - High %RH

1 Experiment 1 Cas Phase 1 Tcmp k~

6 FebZ8 9 6 - 2 1 . 0 s l 0 - ~ 60 4 s l O"'

Table B.8 SS-303L Medium [[,je - Lmv '.&RH

Deposition

rate T

(rnoVcm'.s)

6.7 S I O-"

I -5 s I O-"

Maximum Loading

Experiment

8

9

Deposition .Maximum Loading Cas Phase

O c t 2 4 9 6 - 1

Jul 17 9 6 - 1

Temp

I.2s10-'

I.Os10-"

k~

60

90

3 xl O-'

6x10"

3.5 S I O - ~ - '

6 ~ 1 0 " '

6 IO*"' molkm-

> 1 s10-'~ mol/crn'

Table B.9 SS-3 16L Low [LI. - High ?/6Ri-i

Maximum Loading

-

- - -

Deposition

rate 7

( rnollcm'.~)

( 2 SIO- ' " )+

(2 s l O-' ')*

(2 sl0-")*

( 2 s10-'O)*

k~ (cmls)

( 9 SI O=')*

( 7 s 1 O--)*

(7 s10--)*

(9 sl O")*

Temp

fcC)

23

40

40

-10

Gas Phase

( m o n )

2.1 sl0-"

2.9 X I O-' ' 2.9 s l O-' ' 2.1 SIO-"

1

Z

3

4

Experiment

J L ~ I -1 96 - 1

hlar 29 96 - 1

Mar 29 96 - 2

Ju l - l 96 - 2

Table B. 10 SS-3 16L Low [LI,, - - -

3 1 Oct 23 95 - 2 4.0 s l0-" l 4 Oct 21 95 - 2 4.0 s I O- ' '

16 Oct 24 95 - 1 3.6 slo-"

Ternp

tac)

23

23

kd (cm/s)

1 . 6 s l 0 - '

9.8 s l O--

Deposition

rate

(mo~cm'.s)

4.0 SI O-"

9.8 s 1 O-''

.Maximum Loading

1.5 s l O-!" molicm-

> 2.3 s IO"" mo lkm-

Table B. 1 1 SS-304L Low [L], - - - High O6RH

I

2

Deposition

rate

Eaperirnent

4

5

Maximum Loading Gas Phase

( m o W

Temp

(OC)

Jun 28 96 - 1

Jiin 28 96 - 2

6

7

8

lq

(cm/s)

Nat. 8 9 6 - 2

Jun 35 96 - 1

I O

Table B. 12 SS-3OlL L o u - LOX '#;RH

1.5 s 1 O-''!

1.5 X I O-"'

Jun 26 96 - 1

Jun 26 96 - 2

Jun 24 96 - I

1

I I j Jiin 20 9 6 - 2

7.6 s l O-' ' ' L

2.5 s 1

Jun 20 96 - 1

23

23

2.2 s 1 O-"'

2.2 s l O-'"

3.5 s 1 O-'''

1

4.3 sl O-"' 1 9 0

23

JO

4.3 s 1 O-"'

Maximum Loading

1

2

I

I O Jun 12 96 - 2 90 1.6 s10-' 1 . 1 S IO-" 4 s 10'" niol/cm'

( 6 S I O-')*

(6x10- ' )*

1

7x10 ' - / 3 .1 X I O - ' ~

Expriment

4

5

6

7

8

6 ~ 1 0 - -

1.8 s 1 O" I

9 0

3 s l O- ' ' ' mol!cm-

Temp

(IC)

Cas Phase

(mol/L)

Jun 19 96 - I I

Jun 19 9 6 - 2

( 9 X I O-")*

(9x10-" )*

2.5 SIO""

2 . 6 3 ~ 1 0 - "

3.4 s l 0 - "

40

Jun 18 9 6 - 2

No\ 6 96 - 1

N c n . 6 9 6 - 2

Juri 14 96 - 1

J u n 1 4 9 6 - 2

- -

4.2 ~ 1 0 - "

4.4 s l O-"

> 3.5 x 1 O - ' ' ' mol/crn-

> 3.5 s 1 O-' ' mol/cm-

1.5 s10"" mol/cm'

I l 0

9 s l O - -

(crnls)

I sl O- ' ' '

1 IO-'"

2.5 sl0"" rnoh'cm-

> 6 s 1 O-"' rnol/cm-

Deposition

rate

(mo~/cm'.s)

1.2 s l0 -" '

5.5 s 1 O-'"

5 . 5 s 1 0 - ' "

2.8 X I O - ' ~

~ . 8 s 1 0 - " ~

1 0

6 0

3 . 7 s l 0 ~ ' ~

23

7 - -3

1 . 2 ~ 1 0 ~ '

9.7 sIOe'

3 s 1 O-"' -mol,cm-

4 0

4 0

4 0

6 0

6 0

1 . 9 ~ 1 0 - '

1 x 1 0 - l

3.5 d o - ' 1 . 1 0

1.3 x 1

4 s I0- '

6 s 1 O-'

1 . 5 7 ~ 1 0 ~ "

1 .O9 s 1 O-"

3 s 1 O - ' " mol/crn'

1.7 s I O-"' inolicm'

4.56 s10- '~

5.9 xl0-"

7.3 sl O-' '

1 s10- '"

Z s 1 O"?

< 5 s 1 O-'" molfcm'

< 7 x 1 mol/cm-

< 9 s 1 O-"' mol/cm'

1.5 s 1 O-' ' mol/cm'

< 2 H 1 O-' ' mollcm' ,

-4ppendix - C : Tabulated Results : Desorption

Table B. 1 Desorption results

Sep 10.96 - 1

Sep 10 .96 -3

90°C , Higli RH

Loading Conditions Desorption

Conditions

7.1 110-'

4 d o - ' 5 sl O"

1.8 s10-'

1.26 s104

5.8 S I O "

2.7 s 1 O-'

23°C' High RH /High 1, 2 3 T High RH

4O0C,' High RH/ High 1, 40°C ,' High R H

40°C ' Lou RH

Zj°C / High RH

3OC ,' Lon RH

60°C.' High RH/ High I 2 60°C ' H i ~ l i RH

60cC , bled RH

60°C 1 Lou RH

23°C ,' Lon RH

60°C' i-figh RH! High 1: 60°C / Hiph RH

60°C bled RH

60°C Lon RH

Zj°C ' Loi\ RH

40°C.' High RH/ Higli 1, 40°C / High RH

40°C / Lou RH

40°C' Hi211 RH: High 1, 40°C ' Higli RH

40°C ' LON RH

90°C/ Lon RHI/ High 1- 23°C Low RH

90°C; Lon RH/ High II Zj°C ,' LON RH

4O0C/ High RH/ High 1,

4O0C,' High RH/ High 1:

40°C .: High RH

40°C / Low RH

40°C ff igh RH

40°C / LON R H

~ p p e n d i x D - Classifications of Operating Variables and Reproducibilie of Results

D.1 Gas Phase Concentration

.As describcd in table 5.1. iodine concentration was divided into three differsnt

categorics. One problem that was difficult to resolvct \vas control of fluctuations in the gris

concentration during an esperiment. Generallp. the trend was a declining $as concentration

~l-ith time. This drop \vas mainly due to the limitation in the method for gaseous iodine

censration as descri bed in the ssperimental chapter. - -4 substantial drop kvas obserwd especial1~- in espsriments at high and medium gas

concentration. In some cases. the final concentration \vas only 35% of the initial value. Less

se\,rre decrease tus obsenved for rsperiments at low concentration. Between the beginning

and the end of such rsperiments. the concentration declined by 10% to 30%. Rttgardless of

this fluctuation. al1 concentrations xere still kept within the proptir range of iodine

concentration as intrndrd for the corresponding esprriment. For the purpose of calculating

k,. the average value obsrnped during the initial period was uscd.

D.2 Tube Surface Temperature and Relative Humidity

There \vas no significant problem in the control of tube surface temperature.

hiasimum tluctuation for the tube surface temperature was + 3°C from the set point value.

The relati\.e humidity. however. followed different trends at low and digh humidity. At high

liumidit). the uatcr content of the gas strearn decreased with tinie as the amount of watrr in

the bubbler was depleted. The reduction in relative humidity could be up to 10%. As in the

case of iodine gas concentration. despite this declining relative hurnidity. the obtained values

\ver< still nithin the dssired range.

During dn . e:cperimrnts. the humidity was more stable. This \vas mainly due to the

sfkctiveness of the dehumidification systsm. The only limitation was caused by the hurnidit).

brought by the gas strram that carrird the iodine. Based on the downstream RH probe. the

iodine source typically inçrrasrd the humidity to Iess than 1 5 O 6 . In a11 the dry esperiments.

the range ofreIati\-r humidity was baween OOb and 2j0b . Questions arose as to u-hether the

hrhavior of iodine deposition \vould be consistent within this wide range. An rspcriment was

carried out to sre the effect of incrrasing relative humidity within this range.

The espsrimcnt \vas psrformrd at 60°C and medium gas phase concentration. The

relatiw humidity [vas initially set near O0/0. U'hsn the iodine loading kvas at masimum. the

relative humidity was increased incrementally until a value of 30°h \vas reached. The results

showd that at this low surface loading no obsen-able change in the behavior of iodine

deposition uccurred ( tigurt. D. 1 ).

SS-316L / 60 C LOW RH 1 MED [12]g

. lodine (mo~cm"2) LOW RH : 0%

- m - [12]g rnoIlL

TlME (min)

Figure D. 1 Iodins deposition at low iiuniidity ( O O h to 30%)

.-\ similar test \vas also carried out itt 60°C and high iodint: Las phase concentration.

The purposc of thsse esperiment \vas to see the difference in iodine drposition beha\.ior

iindsr diffrrrnt ranges of relative hurnidity. In this test. the hurnidity \vas initiaIl? maintainrd

Iess than 25931. .Appro\iimatrl~- 6 hours into the esperiment. the relative humidit>- \vas

iricrcascd to 5 0 O . 0 and a suddrn substantial incrrase in the rate of iodinr deposition occurred

(ligure D.?). The rate of deposition uas similar to that obsenxd at hi& relative 1iumidit)-.

A P R I L 2 6 , 9 6 - 1 S S - 3 1 6 L 1 6 0 C L O W % R H to M E D % R H / HIGH [IZjg

1.2E-O7 2.OE-07 lod ine ( rno I lcmA2)

1 .BE-07 1 .OE-07 - D- [12] g a s p h a s e C o n c . moi11 *

* 1 .6E-07

8.OE-08 * c -: 1 .4E-07

5 R H = 50% p-:- - - 6.0E-O8 1 * 1.2E-07 4 - E . O

i: 1.OE-O7 E 4.OE-08

UJ

5 . L O W '&RH 1 : n

9 4 - 7 -

8.OE-O8 2 O 1 : - 2 2.OE-08

-. 1 . * '. 1 - 6 .O E -O 8

T I M E ( m i n )

Figure D.2 lodinci deposition at lou- humidity and medium range of relative humidi ty (<25O'0 and 50%)

This esprriment indicatrs that deposition brhavior at medium humidi ty should br

siniilar to that at higli humidity. A similar trend \vas reportcd by sir'. The data also supports

the role ot'watcr in increasing the rate of iodine deposition on stainless steel.

D.3 Reproducibility of Results

The apparatus being uscd in the stud:. allowrd simultaneous testing of two tubing

saiiiplss so that for most esperiments. duplicated results wrre obtaincd. Some problems rvere

rncountrred with one of the 'la1 detectors preventing duplications for a few of esperimrnts.

In such cases. additional espcriment under similar operating conditions were carried out.

Deir. C.A.. ..A S r u d ~ of interactions of Cas Phase Molscular lodine with Stainless Steel Tubing... M.A.Sc. Thesis. Universir? of Toronto. 1995.

Even when simultaneous measurements were obtained. sorne addi tional mns were

still performsd to snsure that the obsrmcd trend of deposition wu reproduciblr. This aas

required especially for esperiments at low gas concentrations in which variations in results

nzre obsrnesd. Iodine dsposition at luw gas concentrations \vas generally l o w r than

&position at higher concentrations. This made it difticult to obtain ri rrnding that \vas tice

tiom tluctuation dus to background interference. One partial solution to the problttrn \ras the

use of higher specific activity by adding a larger amount of " ' 1 tracer. There was. howwr . a

limit to the amount O:' tracer that could bti used in a single esperiment for safety

considerations. Conseqiiently. for some loti- concentration rspcriments. considerable

uncertaint>. was associated with the readings. In the tabulation of results. the uncrrtainty is

retlected in the last signitïcant digit. For cases involving very low iodinr deposition. such

that the rrading w s mostly due to background variation the vaiues reponed are based on the

detcstion limit. Anoiher problem that \vas sncountrred \vas the inhomogcneity of the

matcrial. Some variations in results w r c espectrd dus to this inhonwgsneity.

Drspitr ri11 the variations describrd abovs. the replicates to mrasurçments of

deposition vdocity generall). tluctuated around approximatel y 20°h of the average value (sre

rippcndis B). This variation \vas kss than the effect of changing operating conditions. hence

it did not prevent the recognition of the gcneral trends in the esperirnental parameters under

inwstigation. Moreover. as mrntioned earlier. some esperiments were repeated to ensurc that

the obsen-ed trends could be correctly attributed to the effects of the operating conditions.

Consmative measlires sliould bt: taken when estimating iodine losses in the sanipling linrs

to avoid undrrestimating the loss. Recommcnded values of deposition parameters are lisred

in table 5.6.

concentration but with different relatke humidity. .As çan be sesn. in spits of an' variations

dur to limitation in the mcthod and apparatus. the different trends of deposition under ~arious

operating conditions could still bs recopnized. This is rspscial1~- truc for esperiment under

high gas phase concentration.

lodine Surface Concentration Vs Time (SS-3 1 6 L)

2.5E-07 A 40C - High [12]g - High %RH

2.0E-07 40C - High [12]g - Low OhRH

Time (min)

Figure D.3 Iodine deposition undrr different rrlatiw humidities

IMAGE EVALUATION

APPLIED I M G E . lnc - = 1653 East Main Street - -. - Rochester. NY 14609 USA -- -- - - Phone: 71 614829300 -- = Fa: 7lW288-5989

O 1993. W i e d Image. inc.. All Rqhts Reserved