isoe ･ atc 2006 alara symposium 1 approach of hitachi for dose rate reduction october 12, 2006...

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ISOE ･ ATC 2006 ALARA 　Symposium

Approach of Hitachi for Dose Rate Reduction

October 12, 2006Hitachi, Ltd.

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Trend of Occupational Exposure

☆Dose rate reduction

•Apply water chemistry control, low Co content material, decontamination and shielding

•In recent year, occupational exposure in Japan is in the highest level in the world.

☆Suppressing of Number of worker and working time•Apply remote or automatic machine•Review frequency and item of inspection•Apply on line maintenance etc.

Man-Sv ＝ Σ ｗ (Dose rate ×Number of workers ×Working time) ｗ

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Dose Rate Reduction　　　　　　　　　　　　 dΓ/dt = δ× Ｃ－ λΓ

Deposition Deposition ∝ × 60Co Conc.（Ｃ）Amount （ Γ ） Rate Coeff. （ δ ）

How should we reduce the deposition Γ ？

1) To reduce the concentration of radio activities in reactor water as low as possible2) To adopt the methods to suppress the incorporation of radio-activities in the surface film of pipings.3) Best combination of the methods should be selected. It may be different plant by plant depending on the specific plant condition.

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Dose Rate Reduction Methods　　　　　　　　　　　　 dΓ/dt = δ× Ｃ－ λΓ

Deposition Deposition ∝ × 60Co Conc.（Ｃ）Amount （ Γ ） Rate Coeff. （ δ ）

Decontamination（ Chemical Decon. et

c. ）HOP(*1) method

Decontamination（ Chemical Decon. et

c. ）HOP(*1) method

-Zn Injection-Low Fe/High Ni Control-Reduction of Surface Roughness (Electric Polishing or Mechanical Polishing)-Alkaline Prefilming-HiF-Coat(Hitachi Ferrite Coating)-Air Oxidation Treatment-H2O2 Preconditioning-RHR Low Temp.Operation

-Zn Injection-Low Fe/High Ni Control-Reduction of Surface Roughness (Electric Polishing or Mechanical Polishing)-Alkaline Prefilming-HiF-Coat(Hitachi Ferrite Coating)-Air Oxidation Treatment-H2O2 Preconditioning-RHR Low Temp.Operation

Hitachi Tech.

１） Reduction of Parent 　 Nuclei of RI 　　 -Low Cobalt Mater.　 -Wear Resistive Mater. （ Hitachi Hyper Valve ）２） Stabilization of RI on Fuel Surface　　 -Improved Fe/Ni Ratio Control　　 -Zn Injection３） Increase of RWCU Capacity

１） Reduction of Parent 　 Nuclei of RI 　　 -Low Cobalt Mater.　 -Wear Resistive Mater. （ Hitachi Hyper Valve ）２） Stabilization of RI on Fuel Surface　　 -Improved Fe/Ni Ratio Control　　 -Zn Injection３） Increase of RWCU Capacity

*1)Hydrazine Oxalic Acid, Potassium Permanganate

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Dose Rate Reduction Methods HitachiRecommends

Construction PhaseConstruction Phase Operating PhaseOperating Phase

/Adopt low cobalt mater.

/Adopt wear resistive mater. (Hitachi Hyper Valve)

/Apply Alkaline Prefilming (RWCU)

/Replace to wear resistive mater.

/Apply Fe/Ni ratio control

/Apply H2O2 precond. after decon. of PLR

/Apply Zn injection

/Apply chemical decontamination(HOP)

/Apply RHR low temperature operation

NWC*NWC* HWC**HWC**

/Apply surface polishing (for S/S pipings)

Reduction of conc.

Reduction of 　 δ

/Apply HiF-Coat. after decon. of PLR

* NWC:Normal Water Chemistry

**HWC:Hydrogen Water Chemistry

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Formation & Deposition Processes of RI

Inflow of 58Co ， 58Ni, and Fe from FW①

Dissolution of 59Co ， 58Ni（ spacer spring etc. ）

Depo. on fuel surface（ by boiling condensation and dry-out ）

Activation by neutron（５９ Co （ｎ、 γ ）６０ Co 、５８ Ni （ｎ、ｐ）５８ Co ）

Redissolution of radioactivity②

RW radioactivity conc.③

Deposition on piping surface④

Removal at RWCW

Different in process between Fe-control and low Fe control

Fuel area

Fe control : By controlling inflow amount of Fe , suppress the radioactivity concentration .① ③Low Fe control: By suppressing inflow of Fe , increase Ni conc. to reduce incorporation of radioact.① ④

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Deposition of Radioactivity in Oxide Filmof C/S under NWC

(2)Ni(Co)xFe3-xO4

Base metal

(1)α-Fe2O3（ Crud ）

Co2+

Dissol.Recrystl. Recrystl.

CrudCrud

RWＦｅ，Ｎｉ ions

Ｆｅ

Co2+ Co

2+

・ Oxide film is mainly composed of Fe based oxides. Cr contained film is not formed

in this case.

・ In this case, as supply of Fe component is abundant compared with S/S, even high Ni

concentration cannot prevent 60Co deposition in oxide film.

Crystal structures in oxide films

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Concept of Prefilming

酸化皮膜厚さプレフィル　ミング(放射能

)放射能付着開始

酸化皮膜厚さ

放射能付着量実運転

時間時間

放射能付着量

酸化皮膜厚さ

放射能付着量

図　3　プレフィルミングの概念従来プレフィルミング

Oxi

d e F

ilm

Thi

ckne

ss a

n d/o

r R

adi o

Act

ivi t

y D

epos

i ti o

n A

mo u

nt

Oxide Film Thickness

Radio ActivityDeposition Amount

Time

Pre-filmOxide Film Thickness

Radio ActivityDeposition Amount

Time

Start of Plant Operation

Ordinary Operation Effect of Prefilming

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60Co Deposition Rate Coefficient of RWCU System Piping

dt

δ=

60 C

o de

posi

tion

rate

coe

ff.(

cm/h

)

EFPH

N.Suzuki ‘An ABWR Water Chemistry Control Design Concept for Low Radiation Exposure and the Operating Experience at the Fist ABWR’9 ｔｈ International Conference on Water Chemistry in Nuclear Reactor Systems (Avignon,Apr.22-26,2002) 　

•The deposition rate of RWCU carbon steel piping with alkaline prefilming is smaller than that without alkaline prefilming.

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Plant 1999 2000 2001 2002 2003 2004 2005

AA 　　　　　　 ▼

EB 　　　 ▼

CA 　　　　　 ▼ 　　　 ▼

FA 　　　　　　 ▼ ▼

ID 　　　　　　 ▼ 　 ▼

IG 　　　 ▼

GD ▼ 　　　　 ▼ 　　　　　　 ▼

BD 　 ▼

IE 　　 ▼

CB 　　　 ▼ ▼

GB 　　　 ▼

J A 　　　　 ▼ 　　　　　 ▼

BA 　　　　　 ▼

• HOP method has been applied 20 times for total 13 plants.

▼ 　 Application

HWC

Application Experiences of HOP Method

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・ After　chemical　decontamination,　dose　rate　increased　under　HWC

・Dose　rate　was　decreased　by　Zn　injectionR.L.Cowan ‘Modern BWR Chemistry Operating Strategies’10 ｔｈ International Conference on Water Chemistry in Nuclear Reactor Systems (San Francisco,Oct.11-14,2004) 　

2500

BR

AC

　D

ose

　R

ate(

mR

/hr) 　

2000

1500

1000

500

0

Mar-83 Mar-85 Mar-87 Mar-89 Mar-91 Mar-93 Mar-95 Mar-97 Mar-99 Mar-01 Mar-03

DZO

Pleated Condensate Filters

HWC

Recirc SystemDecons

10 12-15scfm 20 scfm 35 scfm 39.6scfm

Dose Rate Behavior of a US BWR

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化学除染後リバウンド率

（｜

）

Year after Chemical Decom.

0

1.0

2.0

-1 1 2

Rebound Rate

Dose Rate before Decon.

0

＝

US HWC Plants

Japanese NWC Plants

・ Rebound rate of Japanese NWC plants ： 20 ～100 ％

・ Rebound rate of US HWC plants ： 200 ％ and over

Chemical Decon.

Dose rate behavior of NWC is different from that of HWC

Dose Rate Behavior after Chemical Decontamination is applied

Reb

ound

Rat

e (-

)

Dose Rate after Decon.

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Oxide film is formed

Oxide　film　is　removed

Chemical Decon.

Oxide film is restored

Outer　LayerInner　Layer Base metal

Crud DepositionRadioactibity ion

HWC、NWC

NWC： normal　water　chemistry

HWC:hydrogen　water　chemistry

Deposition Behavior on SUS after Chemical Decontamination

Base metal Base metal

•After the chemical decontamination, the surface of the pipings is restored to the original condition as time elapses.

•Under these circumstances, dose rate behaviors of the pipings are expected to be strongly affected by the water chemistry.

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IndexH2O2/ppb0 5 10 200

Prefilming

NO ○ △ □ ◆

Yes ○ □ △

0

20

40

60

80

100

120

140

0 200 400 600 800 1000 1200Exposure time （ｈ）Am

ount

of 60

C o d

e po s

itio n

(Bq/

cm2 ) HWC

( 0.5ppm H2 injection into feed water)

HWC(1.0ppm H2 injection int

o feed water)

NWC NWC Prefilming*

Time Dependency of Co-60 Deposition on Specimens

N. Usui, M. Fuse, H. Hosokawa, S. Uchida., “Effects of Hydrogen Peroxide on Radioactive Cobalt Deposition on Stainless Steel Surface in High Temperature Water”, Nucl. Sci. Technol., . 42, 75 (2005)

*:200 hours pre exposure

in NWC before HWC)

•The 60Co deposition on stainless steel under HWC condition is more than that under NWC condtion

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• Fine magnetite film is formed by Hi-F Coat.

• RI deposition on stainless steel is mitigated by this film.

Hi-F Coat ： Hitachi Ferrite Coating

Hi-F Coat Actual Plant

Oxide FilmOuter Fe3O4

Fe3O4 、 Fe2O3 、Ni （ Co)Fe2O4

Inner － CoCr2O4 、 Cr2O3

Particle Size ＜ 0.2μ ｍ 1 ～ 10μ ｍ　Thickness ＜ 0.5μ ｍ 3 ～ 10μ ｍFilm Formation Temperature　　

90℃ 280℃

Countermeasure to reduce the Dose Rateー　 Hi-F Coat ー

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Fine film (Magnetite) is formed by Hi-F Coat

Deposited Carbon

Base Metal（ SUS304）

After exposed under NWC condition for 200 h(DO: 300 ppb)

After Hi-F Coat

Outer Layer(Magnetite)

Inner Layer(Chromate）Coating Layer(Magnetite）

Countermeasure to reduce the Dose Rateー　 Hi-F Coat ー（ SEM 　 photographs)

Base Metal（ SUS304）

Deposited Carbon

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Co-60 deposition in HWC could be suppressed by Hi-F Coat

0

50

100

150

200

250

0 500 1000 1500 2000 2500 3000 3500

(h)浸漬時間

Co-60

(Bq/cm

2)付

着量

Hi-F Coat

NWC Prefilming*

Reference(No prefilming)

About 1/5

Exposure　time(h)

Countermeasure to reduce the Dose Rateー　 Hi-F Coat ー（ Effect on RI Deposition)Amount 　of 　60Co　

deposition(Bq/cm2)

*:200 hours pre exposure in NWC before HWC

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Summary•Dose rate reduction methods are reviewed stressing the role of oxide films formed on the surface of the structural components. The control of the oxide film is considered to be an essential factor for a reduction of dose rate of piping. From this point of view, we should further understand the nature of oxide films for developing an effective method of dose rate reduction.

• It was found that alkaline prefilming for carbon steel and ferrite coating(Hi-F coat) for recirculation piping were promising methods for dose rate reduction .

isoe ･ atc 2006 alara symposium 1 approach of hitachi for dose rate reduction october 12, 2006...

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