Cr and Co release reduction from
stainless steels in PWR and BWR
2009 ISOE Asia ALARA Symposium Aomori
EPRI Radiation Protection Conference
September 9, 2009
Sumitomo Metal Industries, Ltd.
Hiroyuki ANADA
Kiyoko TAKEDA
Tetsuo YOKOYAMA
Contents
•
•
•
•
•
•
•
•
Background
Conventional Technologies
New Method of Co Release Reduction
New Method of Cr Release Reduction
Experimental Procedure
Results
Application
Conclusion
September 2009
2
Background
Influence of metallic ion release from
stainless steels on the dose rate
• Co release
・ Co content in stainless steels from contamination in
raw material
・ Co release to coolant
・ Co raise the dose rate in coolant
・ 60Co has long half-life, 5.7 years
EPRI; Restricted less than 0.05%
September 2009
3
Background
・Corrosion of the stainless
steels release Cr into coolant
・ Decrease pH in the coolant
with increasing Cr content in
Coolant
・ Absorbed Co ion on the
surface resolved into the
coolant
Co Absorbed ratio, %
・ Cr release
Increasing Cr
pH
September 2009
4
Conventional Technologies
1. Reduction of Co release
・Pure raw material selection; Scraps
・Min. 0.05 – 0.20%
・High cost
2. Reduction of Cr release
・No commercial productions for stainless
steels
September 2009
5
New Method of Co Release Reduction
1. Reduction of Co release
Pure raw material selection;
・Hot metal; Pure Fe
・A little selected scraps
・Less than 0.02%
・Low cost
September 2009
6
New Method of Cr Release Reduction
2. Reduction of Cr release
Pre-filming technique
Cr oxide film by control oxygen content
during a heat treatment
Protective Cr oxide film for Cr release into
coolant
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7
New Method of Cr Release Reduction
・Selective oxidation of
Cr in stainless steel
1
2Ni + O2 = 2NiO2
100
2Fe + O2 = 2FeO2
500
10-10
4/3Cr + O2 = 2/3Cr2O3
1000
10-20
0
1000
2000
Temperature, degree C
10-100
September 2009
10-50
10-30
8
Po2, atm
・Control of oxygen
content during heat
treatment in
manufacturing process
Free energies of formation of oxides
(－ΔG0=RT ln Po2), kJ/mol O2
2. Reduction of Cr release
Experimental Procedure -Material• Material
TP304L; Raw material selection, hot metal in addition
to scraps
• Pre-filming on inner surface of the tube,
15.9 mm dia.
1. Laboratory test
• Heat treatment in H2 with slight amount of O2 content
controlled by dew point; -10 to -50 deg. C in H2
2. Application to feed water heater tube for BWR
Melting
September 2009
Cold draw
15.9 mm dia
Pre-filming; Solution treatment >1000degC
H2 environment adding H2O
Selective oxidation of Cr
9
Experimental Procedure
• Characterization of pre-filming oxide
1. Color and Oxide morphology
• Naked eyes and SEM
2. Oxide structure identified by XPS
• Depth profile of the chemistries by Ar sputtering
• Chemical state analysis
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10
Experimental Procedure
• Cr and Co release from the pre-filmed
tube to coolant
Corrosion test in pure water
– Refreshed type autoclave at 215 deg. C for 450 hr.
– Cr and Co content in the test water was analyzed.
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11
Result -Co contentNew method
Melting ・Small amount of selected
scraps
・Hot metal, pure Fe from
blast furnace
Facility ・Combination of blast
furnace and electric furnace
・Suitable mixing, small cost
impact
Co
September 2009
Less than 0.02%
Conventional method
・Selected pure
scraps
Large cost impact
・Electric furnace
0.05%
12
Result - Pre-filming oxide in the lab. test
• Thin oxide formed by heat treatment under
controlled dew point in H2
DP, O2 ;
Low
⇒
High
SEM images
Surface pre-filmed at 25deg. C
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13
Result -Depth profile of the pre-filming oxide
80
70
Cr rich oxide (60nm)
Composition, at%
60
50
Fe
Dew Point=
-20 degree C
40
30
20
O
Cr
Mn
10
Ni
0
0
Surface
September 2009
50
100
150
200
250
Depth, nm
300
350
400
14
Result - Application of pre-filming
Application of pre-filming for feed
water heater tube
・Specification; TP304L
・15.9mm dia.
・Pre-filming condition
Atmosphere ; In H2, DP= - 25deg.C
Temperature; 1060deg.C
September 2009
15
Result - Application of pre-filming
80
Atomic Concentration, %
Fe
Cr rich oxide
(1.5 nm)
70
60
50
40
Dew Point=
-25 degree C
30
O
20
Cr
10
Ni
0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Sputter Depth, nm
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Result - Cr release from the tube
Cr release, g/m2
・ Pre-filming reduced 25% of Cr release
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
Pre-filming
Tube
0
September 2009
- 25%
Bare Tube
400
200
Time, hr.
Autoclave test;
215deg.C
600
17
Result - Experience of Japanese BWR
Radioactivity, Bq/cm3
100
10 Onagawa
1
0.1
0.01
0.001
Higashidori; Pre-filming Tube applied
0
10,000
20,000
EFPH (Equivalent Full Power Hours)
Jun-ichi Satoh, Proceedings of Thermal and nuclear power
engineering society, p72-p73 October 23 2008, Sendai Japan
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Conclusion
• New Method of Co & Cr release
reduction from stainless steel tubes to
coolant :
(1) reduces Co content in stainless steels less than 0.02%
without large cost impact.
(2) reduces 25% of Cr release from stainless steels tubes
(3) is applied for Higashi-dori BWR plant, and contributed
to reduce the dose rate and to be No.1 plant in whole
BWR
September 2009
19
Future work
• Challenge to reduction of Ni release from steam
generator tube for PWR.
– Pre-filming technique using by oxygen potential control
Thank you for your attention!
September 2009
20
Co release into coolant
September 2009
21
Depth profile of the pre-filming oxide
80.00
DP= -30C
70.00
Composition, at%
60.00
C1s
O1s
Si2s
Cr-O
Met.Cr
Mn2p1
Fe2p3
Ni2p3
50.00
40.00
30.00
20.00
10.00
0.00
0
Surface
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10
20
30
40
50
Depth,nm
60
70
80
90
100
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Structure of the pre-filming oxide
• Cr-Mn mixed oxide layer formed adjacent to the matrix.
• Fe2O3 or Fe3O4 layer formed at the surface of the oxide
Intensity, Arb. Unit
DP=-25
Fe2O3
Fe
Fe2O3
Cr-Mn oxide(mainly)
C
3 to40n
（FeO and Fe, Ni）
DP=-30
C
TP304 matrix
DP=-20
C
Binding Energy,
eVFe2O3 formed on the surface,
Figure 4
September 2009
showing effectiveness of dew point
Figure 5 Multi-oxide layers in the
pre-filming on TP304
23
Thickness of pre-filming
• Thickness of the pre-filming increase with increasing DP.
• Suitable pre-film thickness for will be selected easily.
• This might contribute to the effectiveness of the barrier layer
70
Thickness, nm
60
50
40
30
20
10
0
-35
September 2009
-30
-20
-25
Dew point, degree C
-15
24
Diffusion of Co in oxide
• Diffusion coefficient of Co decrease with increasing Cr content in
oxide.
• This suggests that Cr rich layer adjacent to the matrix acts as a
protective film.
Fe2O3
Cr-Mn oxide(mainly)
3 to40nm
Fe in Fe0.8Cr0.2O3
Fe in Fe2O3
（FeO and Fe, Ni）
Cr in Cr2O3
TP304 matrix
Co in Feo.8Cr0.2O3
Figure 5 Multi-oxide layers in the
September 2009
pre-filming on TP304
25