reduction of radiation exposure at aged bwr plants by water chemistry

1

Copyright 2008, Toshiba Corporation.CDCC-2008-100442PSN-2008-1296

2008 ISOE International ALARA Symposium, Tsuruga Japan,13-14 Nov., 2008

Reduction of Radiation Exposure at Aged BWR Plants by Water Chemistry

Nov.13 2008

Hidehiro Urata and Kenji YamazakiToshiba Corporation

1/23

2/23Copyright 2008, Toshiba Corporation.CDCC-2008-100442, PSN-2008-1296


Content

1. Background and Introduction2. Water Chemistry Approach and

Results3. Conclusion



Roles of Water Chemistry at Operating BWRs

Increase of radiation level due to ageing and up-rating of plant and higher burn-up fuel implementation is a concern regarding roles of water chemistry at operating nuclear power plants.

Main purposes of water chemistry

– Radiation level reduction

• Improvement of work environment

• Reduction of radwaste

– Improvement of safe and reliable operation

• Structural material integrity

• Fuel integrity

Background and Introduction



At aged BWR plants, radiation dose reduction for maintenance repair works, and mitigation of SCC are most important roles of water chemistry

Optimum water chemistry

Radiation reduction

Secure and Reliable Operation

Fuel material integrity

Structural material integrity

Radwaste reduction

Radiation dose reduction




<0.5

0.5-

1.0

1.0-

1.5

1.5-

2.0

2.0-

2.5

2.5-

3.0

3.0-

3.5

3.5-

4.0

>4.0

J apan

02468

10

12

J apanUSA

Japan:2000.12-2003USA :2005.1

Median:0.89 mSv/h

mSv/h at PLR Piping

Num

ber o

f pla

nts

Median:1.55 mSv/h

From Y. Hayashida 2007 ISOE Asian Symposium, Seoul Korea Sep 12-14 2007

Ra

dia

tion

Exp

osu

re (

Pe

rson

S

v)

Outage (days)

Oversea BWR（ Good case)

Japanese BWR

A: Reduction of works and Improvement of works

B: Improvement of radiation environment

BA

Rationale Exposure Reduction by A+B

C: Possible effects of up-rating and higher burn-up

Importance of radiation level reduction




Water chemistry for control of recontamination under low ECPWater chemistry for control of recontamination under low ECP

Reduction of dose rate

Hydrogen Water Chemistry

Dos

e ra

te

Dose rate increase

Water chemistry modification

Operation

Objective

ChemistryRemedy

Mitigation of SCC

Modification of oxideIncrease of

Co-60 Conc.

Chemical Decontamination

Influence on Dose

Rate

Factors

Recontamination

Decon.

Operation

Dos

e ra

te

Factors affecting radiation level buildup at aged BWR plants




Low DF Medium DF High DF

DF ( Decontamination Factor) Dose before decon/after decon)

Re-

con

tam

inat

ion

Rat

e

(arb

itra

ry u

nit

)

Re-contamination after chemical decontamination depends on “ Decontamination Factor”.


Medium Decontamination factor is preferable but not always obtained. Mechanism of the dependence should be evaluated.



Water Chemistry Approach and Result

1. Issues to be evaluated(1) Dependence of Recontamination on Decon.

Factor(2) Water Chemistry for suppression of radiatio

n level build up after water chemistry modification and chemical decontamination

2. Approach(1)Experimental approach by using BWR simul

ation loop(2)Parameters

– Low ECP (ex. HWC)– TiO2, Zn (countermeasures)



Water Chemistry Approach and Results

Experimental (1) Procedure

Specimen preparation

Pre-filming

Chemical decontamination

Co-60 deposition test

Measurement/Analysis

(TiO2 treatment)

Reduction (Oxalic acid)

Oxidation (ozone)

Reduction (Oxalic acid)

HClRepetition

Oxalic acid： 2000ppm， 95COzone： 3ppm,80CHydrochloric acid ： 2mol/l,80C



40mm

10mm

0.3mmt

SUS316L

High Pressure Pump

Pump

Heat Ex.

PreHeater

Cooler

H2

N2

Air

DH

H2O2 Co-60

ZnDO

Con

Ion Ex.Resin

ChemistryMonitor

Filter

downstreamtest section.

upstreamtest section.

Zn Injection Line



High Pressure Pump

Pump

Heat Ex.

PreHeater

Cooler

H2

N2

Air

DH

H2O2 Co-60

ZnDO

Con

Ion Ex.Resin

ChemistryMonitor

Filter



Zn Injection Line




Experimental (2) Co-60 deposition test loop



HWC

m-HWC

• The V-shape dependency was reproduced.• Dependency is much strong under HWC.• The V-shape dependency was reproduced.• Dependency is much strong under HWC.

Co

-60

　W/O Zn

　W/O TiO2


Results (1) Dependence on decon. factor

0.0

0.5

1.0

1.5

2.0


On

Sta

inle

ss s

teel

spe

cim

en HWC

FW DH>1.0ppm

m-HWC

FW DH<0.5ppm




After decon

Oxide removed unevenly

No residual oxide

No residual oxide

But rough

After Co-60 test Large oxide

evolved infrequently

Smallest Oxide and uniform

Large oxide evolved

No decon

Oxide deposited uniformly

Not changed






After decon

After Co-60 test Residual oxide causes the

irregular oxide growth and enhances Co-60 deposition and migration

Oxide film uniformly formed suppress further corrosion and Co-60 deposition

Rough surfaces causes the increase of surface area and oxide growth and enhance the Co-60 deposition

： Cr2O3： 60Co： Fe2O3： NiFe2O4

Chemical decontamination with adequate control is important to suppress re-contamination.

Chemical decontamination with adequate control is important to suppress re-contamination.





• Zinc is effective for suppression of recontamination under both HWC and m-HWC, even after inadequate decon.

• Zinc is effective for suppression of recontamination under both HWC and m-HWC, even after inadequate decon.


Co-60

0.0

0.5

1.0

1.5

2.0


On stainless steel specimen)

HWC- W/O ZnHWC- With Znm-HWC-W/O Znm-HWC- With Zn

Results (2) Control by water chemistry - Zn -




After decon

After Co-60 test

W/O Zn

With Zn

Oxides are formed uniformly and closely packed by zinc injection.


Results (2) Control by water chemistry - Zn -



• TiO2 can suppress Co-60 deposition to very low level and has synergy effect with Zn.

• TiO2 can suppress Co-60 deposition to very low level and has synergy effect with Zn.


Co-60

＊

0. 0

0. 5

1. 0

1. 5

2. 0

DF低 DF最適 DF過剰

Ref . ( )抑制対策無し

酸化チタン付着

酸化チタン付着後亜鉛注入

0.0

0.5

1.0

1.5

2.0


On

Sta

inle

ss s

teel

spe

cim

enRef No treatment

TiO2 deposition

TiO2 deposition and Zn

Results (3) Control by water chemistry - TiO2 -



Low DF Medium DF High DF Remarks

Ref.No water chemistry

counter measure

TiO2

TiO2 covers surfaces and

smaller corrosion oxides are produced.

TiO2 + Zn





HWC + Zn M-HWC and TiO2

Surface morphology

Suppression effect on Co-60

deposition

Zn prevents oxide film growth.

ZnCr2O4 is formed in the inner oxide layer and prevent Co-60 migration.

(1)TiO2 layer may block the migration of Co-60 and prevent CoFe2O4 formation

(2)Radicals produced by TiO2 may oxidize Fe2+ to Fe3+ and prevent ferrite formation.

： 60Co： Fe2O3： NiFe2O4 ： Cr2O3： TiO2 ：Ｚ nCr2O4





• Before ferrite (CoFe2O4) formation, Fe2+ can be oxidized to Fe3+ and Fe2O3 is formed.

• Further discussion is needed to establish the mechanism for Co-60 deposition prevention.

(Cathodic reaction)2O2 + 8H+ +8e- 　 -> 4H2O

TiO2

Heat

Excitation

h+

e-

SS

H2O + h+-> OH ・ + H+

OH- + h+ -> OH ・

O2 + e- -> O2-

(Anodic reaction)Fe ->Fe2+ + 2e-

3Fe2+ + 4H2O ->Fe3O4 + 8H+ + 8e-

SS

Possible explanation on role of TiO2




-0.4

-0.2

0

0.2

0 50 100 150 200 250TiO2 amount / g/cm2

ECP

/ V v

s. SH

E

○：<50W/cm2

□：50~200W/cm2

△ ：>200W/cm2

Co

rro

sio

n P

ote

nti

al (

EC

P)

Current

Oxidation ofSteel

Reduction of Oxygen

Reduction of ECP

Phto-electric Current

TiO2 for SCC Mitigation

K. Takamori, International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, Aug.14-18 2005 Snowbird Utah

TiO2 technology for BWR SCC mitigation is developed by Toshiba and TEPCO.

ECP of stainless steel is decreased by TiO2 with photon, or without photon under low-HWC chemistry.



Conclusion - 1

Effects of Decontamination factor on re-contamination

1. The V-shape relationship between Co-60 deposition and DF could be clearly seen, especially in case of HWC condition.

2. In case of low DF, poorly protective residual oxides accelerate the re-contamination.

3. Under HWC condition, recontamination with inadequate DF is accelerated and thus control of decontamination is particularly important.



Water chemistry control

1. Zn injection can effectively suppress Co-60 deposition regardless of DF.

2. TiO2 can effectively suppress Co-60 deposition regardless of DF.

3. TiO2 technology is a promising candidate for simultaneous accomplishment of SCC mitigation and radiation exposure reduction.

4. Synergy effect of TiO2 and Zn can be seen.

Conclusion - 2

reduction of radiation exposure at aged bwr plants by water chemistry

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