Approach of Hitachi for Dose Rate Reduction

1
Approach of Hitachi for Dose Rate Reduction
October 12, 2006 Hitachi, Ltd.
2
Trend of Occupational Exposure
Man-SvSw(Dose rate Number of workers Working
time)w

• ?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.

3
Dose Rate Reduction
dG/dt dC-?G Deposition ?
Deposition 60Co Conc.(C) Amount
(G) Rate Coeff. (d)

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

4
Dose Rate Reduction Methods
dG/dt dC-?G Deposition ?
Deposition 60Co Conc.(C) Amount
(G) Rate Coeff. (d)
Decontamination (Chemical Decon. etc.) 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
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 Injection 3)Increase of RWCU
Capacity
1)Hydrazine Oxalic Acid, Potassium
Permanganate
Hitachi Tech.
5
Dose Rate Reduction Methods HitachiRecommends
Construction Phase
Operating Phase
NWC
HWC
/Adopt low cobalt mater.
/Apply Fe/Ni ratio control
/Apply chemical decontamination (HOP)
/Adopt wear resistive mater. (Hitachi Hyper
Valve)
/Apply H2O2 precond. after decon. of PLR
NWCNormal Water Chemistry HWCHydrogen Water
Chemistry
/Apply surface polishing (for S/S pipings)
/Apply HiF-Coat. after decon. of PLR
/Apply Alkaline Prefilming (RWCU)

/Apply Zn injection
Reduction of conc.
/Apply RHR low temperature operation
Reduction of d
/Replace to wear resistive mater.
6
Formation Deposition Processes of RI
Different in process between Fe-control and low
Fe control
Fe control By controlling inflow amount of
Fe?, suppress the radioactivity
concentration?. Low Fe control By suppressing
inflow of Fe?, increase Ni conc. to reduce
incorporation of radioact.?
Inflow of 58Co,58Ni, and Fe from FW?
Dissolution of 59Co,58Ni (spacer spring
etc.) Depo. on fuel surface (by boiling
condensation and dry-out) Activation by
neutron (59Co(n??)60Co?58Ni(n?p)58Co) Redissoluti
on of radioactivity? RW radioactivity
conc.? Deposition on piping surface?
Fuel area
Removal at RWCW
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Deposition of Radioactivity in Oxide Filmof C/S
under NWC
RW
Fe,Ni ions
Crud
2
Crud
2
Co
2
Co
Co
Recrystl.
Dissol.
Recrystl.
Crystal structures in oxide films
(1)a-Fe2O3(Crud)
(2)Ni(Co)xFe3-xO4
Base metal
Fe
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.
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Concept of Prefilming
?
3
???????????
Start of Plant Operation
????????
??
Oxide Film Thickness
Oxide Film Thickness
Pre-film
??????
?????
???
(
???
)
???????
Oxide Film Thickness and/or Radio Activity
Deposition Amount
??????
??????
??????
??????
Radio Activity Deposition Amount
Radio Activity Deposition Amount
??????
???
Time
Time
??
??
Ordinary Operation
Effect of Prefilming
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60Co Deposition Rate Coefficient of RWCU System
Piping
dt
d
60 Co deposition rate coeff.(cm/h)
EFPH

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

N.Suzuki An ABWR Water Chemistry Control Design
Concept for Low Radiation Exposure and the
Operating Experience at the Fist ABWR9th
International Conference on Water Chemistry in
Nuclear Reactor Systems (Avignon,Apr.22-26,2002)
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Application Experiences of HOP Method
? Application
HWC

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

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Dose Rate Behavior of a US BWR
2500
10 12-15scfm 20 scfm 35 scfm
39.6scfm
HWC
2000
Recirc System Decons
Pleated Condensate Filters
1500
DZO
BRAC Dose Rate(mR/hr)
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
After chemical decontamination, dose rate
increased under HWC Dose rate was decreased by
Zn injection
R.L.Cowan Modern BWR Chemistry Operating
Strategies10th International Conference on Water
Chemistry in Nuclear Reactor Systems (San
Francisco,Oct.11-14,2004)
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Dose Rate Behavior after Chemical Decontamination
is applied
US HWC Plants
Rebound Rate
2.0
Dose Rate after Decon.
???????????()

Dose Rate before Decon.
Chemical Decon.
Rebound Rate (-)
1.0
Japanese NWC Plants
0
-1
2
1
0
Year after Chemical Decom.
Rebound rate of Japanese NWC plants
20100 Rebound rate of US HWC plants 200 and
over
Dose rate behavior of NWC is different from that
of HWC
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Deposition Behavior on SUS after Chemical
Decontamination
HWC?NWC
Chemical Decon.
Oxide film is removed
Oxide film is formed
Oxide film is restored
Radioactibity ion
Crud Deposition
Outer Layer
Inner Layer
Base metal
Base metal
Base metal
NWCnormal water chemistry HWChydrogen water
chemistry

• 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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Time Dependency of Co-60 Deposition on Specimens
140
HWC ( 0.5ppm H2 injection into feed water)
120
Index
100
H2O2/ppb H2O2/ppb H2O2/ppb H2O2/ppb
0 5 10 200
Prefilming NO ? ? ? ?
Prefilming Yes ? ? ?
80
HWC (1.0ppm H2 injection into feed water)
Amount of 60Co deposition(Bq/cm2)
60
40
20
NWC
NWC Prefilming
200 hours pre exposure in NWC before HWC)
0
0
200
400
600
800
1000
1200
Exposure time(h)

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

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)
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Countermeasure to reduce the Dose Rate ? Hi-F
Coat ?
Hi-F Coat Actual Plant
Oxide Film Outer Fe3O4 Fe3O4? Fe2O3?Ni(Co)Fe2O4
Oxide Film Inner - CoCr2O4?Cr2O3
Particle Size Particle Size lt0.2µm 110µm
Thickness Thickness lt 0.5µm 310µm
Film Formation Temperature Film Formation Temperature 90? 280?

• Fine magnetite film is formed by Hi-F Coat.
• RI deposition on stainless steel is mitigated by
  this film.

Hi-F CoatHitachi Ferrite Coating
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Countermeasure to reduce the Dose Rate ? Hi-F
Coat ? (SEM photographs)
Fine film (Magnetite) is formed by Hi-F Coat
Outer Layer(Magnetite)
Coating Layer(Magnetite)
Inner Layer(Chromate)
Deposited Carbon
Deposited Carbon
Base Metal(SUS304)
Base Metal(SUS304)
After Hi-F Coat
After exposed under NWC condition for 200 h (DO
300 ppb)
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Countermeasure to reduce the Dose Rate ? Hi-F
Coat ? (Effect on RI Deposition)
Co-60 deposition in HWC could be suppressed by
Hi-F Coat
Reference(No prefilming)
About 1/5
Amount of 60Co deposition(Bq/cm2)
NWC Prefilming
Hi-F Coat
Exposure time(h)
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 .