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

September 2009 2

Contents

• Background• Conventional Technologies• New Method of Co Release Reduction• New Method of Cr Release Reduction• Experimental Procedure• Results• Application• Conclusion

September 2009 3

Background

• 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%

Influence of metallic ion release from stainless steels on the dose rate

September 2009 4

Background

pH

Co

Ab

sorb

ed

ra

tio

, %Increasing Cr

・ Cr release

・ Corrosion of the stainless steels release Cr into coolant

・ Decrease pH in the coolant with increasing Cr content inCoolant

・ Absorbed Co ion on the surface resolved into the coolant

September 2009 5

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 6

New Method of Co Release Reduction

1. Reduction of Co releasePure raw material selection;

・ Hot metal; Pure Fe

・ A little selected scraps

・ Less than 0.02%

・ Low cost

September 2009 7

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

September 2009 8

New Method of Cr Release Reduction

0 1000 2000

Temperature, degree C

Free

ene

rgie

s of

for

mat

ion

of o

xide

s (

Δ－

G0 =

RT ln

Po 2

), k

J/m

ol O

2

100

500

1000

Po 2

, atm

10-10

1

10-20

10-30 10-50 10-100

4/3Cr + O2 = 2/3Cr2O3

2Ni + O2 = 2NiO2

2Fe + O2 = 2FeO2

2. Reduction of Cr release

・ Control of oxygen content during heat treatment in manufacturing process

・ Selective oxidation of Cr in stainless steel

September 2009 9

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 BWRCold draw

15.9 mm diaPre-filming; Solution treatment >1000degCH2 environment adding H2OSelective oxidation of Cr

Melting

September 2009 10

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

September 2009 11

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.

September 2009 12

Result -Co content-

New method Conventional method

Melting ・ Small amount of selected scraps

・ Hot metal, pure Fe from blast furnace

・ Selected pure scraps

Large cost impact

Facility ・ Combination of blast furnace and electric furnace

・ Suitable mixing, small cost impact

・ Electric furnace

Co Less than 0.02% 0.05%

September 2009 13

Result - Pre-filming oxide in the lab. test

DP, O2 ; Low High⇒

SEM imagesSurface pre-filmed at -25deg. C

• Thin oxide formed by heat treatment under controlled dew point in H2

September 2009 14

Result -Depth profile of the pre-filming oxide

September 2009 15

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 16

Result - Application of pre-filming

September 2009 17

Result - Cr release from the tube

・ Pre-filming reduced 25% of Cr release

Autoclave test; 215deg.C

0.0000.0020.0040.0060.0080.0100.0120.014

0 200 400 600Time, hr.

Cr

rele

ase,

g/m

2

Pre-filmingTube

Bare Tube - 25%

September 2009 18

Result - Experience of Japanese BWR

Radioactivity, Bq/cm3100

Onagawa

Higashidori; Pre-filming Tube applied

0 10,000 20,000EFPH (Equivalent Full Power Hours)

Jun-ichi Satoh, Proceedings of Thermal and nuclear power engineering society, p72-p73 October 23 2008, Sendai Japan

10

1

0.1

0.01

0.001

September 2009 19

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 20

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 21

Co release into coolant

September 2009 22

Depth profile of the pre-filming oxide

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

0 10 20 30 40 50 60 70 80 90 100Depth,nm

Com

positio

n, a

t%

C1sO1sSi2sCr-OMet.CrMn2p1Fe2p3Ni2p3

DP= - 30C

Surface

September 2009 23

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

Fe

DP=-20

C

DP=-30

C

Intensity, Arb. Unit

DP=-25

C

Fe2O3

Binding Energy,

eV Figure 4 Fe2O3 formed on the surface,

showing effectiveness of dew point

Fe2O3

Cr-Mn oxide(mainly)

（FeO and Fe, Ni）

TP304 matrix

Figure 5 Multi-oxide layers in the

pre-filming on TP304

September 2009 24

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

0

10

20

30

40

50

60

70

- 35 - 30 - 25 - 20 - 15

Dew point, degree C

Thi

ckne

ss, n

m

September 2009 25

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.

Cr in Cr2O3

Co in Feo.8Cr0.2O3

Fe in Fe0.8Cr0.2O3

Fe in Fe2O3

Fe2O3

Cr-Mn oxide(mainly)

（FeO and Fe, Ni）

TP304 matrix

Figure 5 Multi-oxide layers in the

pre-filming on TP304