APT measurement of stainless steel welds after long-term aging and irradiation

ENVDEG 2015, 9 - 13 Aug, Ottawa

K. Fujii, K. Fukuya Institute of Nuclear Safety System

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Purpose

• Influence of irradiation on thermal embrittlement of dual phase stainless steel components is a new concern for long-term operation of LWRs.

• This study intends to get better understanding of microstructural evolution in ferritic phase of SS welds after long-time aging & irradiation.

• Atom probe tomography (APT) was applied to two SS welds exposed to similar thermal history with & without irradiation:

- Irradiated FTT (flux thimble tube) weld (304) 324ºC, 111,000h, ~3 dpa - Aged MCP (main coolant pipe) weld (316L) 320ºC, 92,000h

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• TIG weld of top plug (304) and flux thimble tube (316) • 324ºC, 111,100 h, ~3 dpa (7x10-9 dpa/s)

Irradiated FTT weld

20µm 10µm

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APT analysis

• CAMECA LEAP 3000X HR • Laser pulse mode for irradiated FTT weld 35K, 0.3nJ, 200kHz EF=30V/nm, ICF=1.65 •Voltage pulse mode for aged MCP weld 50K EF=33V/nm, ICF=1.1 • Cluster identification Recursive search algorithm >50 Ni+Si+Mn atoms within 0.5nm

EF: Evaporation Field ICF: Image Compression Factor

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Atom maps of irradiated FTT weld

ID: Z01R3-1-uS1-AP1 263×45×45nm

• Ni-Si-Mn clusters containing Cu, depleted with Cr • Cr decomposition

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Fe Cr Ni Si Mn Mo C P Cu

Clusters in irradiated FTT weld

Unit Fe Cr Mn Ni Si Mo Cu Cluster at% 50.39 11.43 5.94 25.15 6.15 0.10 0.53

Matrix at% 66.12 26.55 1.15 4.79 1.06 0.21 0.11 wt% 67.45 25.22 1.15 5.14 0.54 0.37 0.13

• Ni-Si-Mn cluster (Fe-25Ni-6Si-6Mn-0.5Cu) • 5.7 nm in diameter, 1.1x1024/m3 in number density

Diameter (nm)

Frac

tion

(%)

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Clusters in irradiated FTT weld • Larger clusters contained higher Ni & Mn • Ni/Si ~ 3−5, Mn/Si ~ 1 • Clusters were likely to be G phase or its precursor.

Diameter (nm) Diameter (nm) Diameter (nm)

7 N

i con

c. (a

t%)

Si c

onc.

(at%

)

Mn

conc

. (at

%)

Si atom

Ni a

tom

Si atom

Mn

atom

G phase Mn6Ni16Si7

Cr decomposition in irradiated FTT weld • Cr atom distribution in matrix (excluding clusters) • Non-random distribution

Cr concentration (at%) Cr-Cr distance (nm)

Frac

tion

(%)

Frac

tion

(%)

Diff

eren

ce (%

)

Diff

eren

ce (%

)

Data Random

Data Random

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V parameter ~0.98 1NN parameter ~0.133

• SG inlet nozzle safe end / MCP weld • TIG weld (316L) of MCP elbow (SCS14A) & safe end (316) • 320ºC, 92,000 h

Aged MCP weld

MCP elbow Weld Safe end

Weld

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10 Atom maps of aged MCP weld

• Ni-Si-Mn clusters depleted with Cr • Cr decomposition

Fe Cr Ni Si Mn

Clusters in aged MCP weld

• Ni-Si-Mn cluster (Fe-22Ni-5Si-6Mn) • 3.3 nm in diameter, 4.0x1024/m3 in number density

Diameter (nm)

Frac

tion

(%)

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Unit Fe Cr Mn Ni Si Mo Cluster at% 46.94 17.66 6.56 21.79 4.66 2.39

Matrix at% 63.31 26.38 1.24 5.19 1.12 2.76 wt% 63.40 24.59 1.22 5.47 0.56 4.75

Clusters in aged MCP weld • Ni, Si & Mn conc. were independent of size. • Ni/Si ~ 3− 5, Mn/Si ~ 1− 2 • Clusters were likely to be G phase or its precursor.

Diameter (nm) Diameter (nm) Diameter (nm)

12 N

i con

c. (a

t%)

Si c

onc.

(at%

)

Mn

conc

. (at

%)

Si atom

Ni a

tom

Si atom

Mn

atom

G phase Mn6Ni16Si7

Cr decomposition in aged MCP weld • Cr atom distribution in matrix (excluding clusters) • Non-random distribution

Cr concentration (at%) Cr-Cr distance (nm)

Frac

tion

(%)

Frac

tion

(%)

Diff

eren

ce (%

)

Diff

eren

ce (%

)

Data Random

Data Random

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1NN parameter ~0.131 V parameter ~0.65

Comparison of atom maps 14

• FTT irradiation condition (~3 dpa, 7x10-9 dpa/s) caused no significant enhancement of Cr decomposition but enhancement of G phase growth

Cluster size distribution

Comparison of Cr decomposition 15

•1NN parameters are similar in irradiated FTT & aged MCP. • Mo effect is not significant under thermal aging at ~105 h. (after Pareige et al. in Fontevraud 8) • It is likely that no significant enhancement of Cr

decomposition under FTT irradiation (3 dpa, 7x10-9 dpa/s)

Matrix composition (wt%)

■ FTT weld ■ MCP weld

Comparison of G phase formation 16

Cluster composition (at%) Clusters

Diameter (nm)

Density (1024/m3)

Vf (x100)

• Clusters were in larger volume with higher Ni for FTT weld. • Mo enhances G phase formation with higher density & volume

under thermal aging (after Pareige et al. in Fontevraud 8) • Cluster growth is likely to be enhanced under FTT irradiation

■ FTT 304 weld ■ MCP 316L weld

Summary

• Irradiated FTT 304 weld & aged MCP 316L weld were analyzed by APT, which were aged at ~320ºC for ~105 h.

• Common microchemical changes were identified in ferrite phase: Cr decomposition and G phase formation. • FTT irradiation condition (~3 dpa, 7x10-9 dpa/s) caused no significant enhancement of Cr decomposition but enhancement of G phase growth, considering Mo effects on thermal aging after Pareige et al. • Further data are needed to confirm irradiation effects more directly and in other conditions.

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