1. FAST NEUTRON FLUX EFFECT ON VVER RPV’s LIFETIME ASSESSMENT
Influence of atomic displacement rate, neutron spectrum and irradiation temperature on radiation-induced ageing of power reactor components
FAST NEUTRON FLUX EFFECT ON VVER RPV’s LIFETIME ASSESSMENT
D. Еrak
Ya. Shtrombakh, P. Platonov, А. Аmaev,
Yu. Kevorkian, А. Chernobaeva
Research Institute of Atomic Reactors
Dimitrovgrad - Ulianovsk, Russia
October 2005
RUSSIAN RESEARCH CENTER
“KURCHATOV INSTITUTE”

2. CONTENT VVER-440
Why we should investigate Neutron Flux Effect in VVER RPV’s materials radiation embrittlement
The Investigation Program carried out
Conclusions, further works
Some words about VVER-1000

3. RPVs VVER-440

4. Two generation of VVER-440 type units are in operation: VVER-440/230 and VVER-440/213
VVER RPV EOL basically depends on weld seam Radiation Embrittlement (RE)
High P and Cu contents:
Up to % P
Up to % P
Up to 0.20 % Cu
< 0.3 % Ni
No Surveillance Specimens
Surveillance Specimens Program
Without cladding
With cladding
All units were annealed –
EOL depends on re-irradiation
EOL depends on the primary RE

5. RPVs VVER-440/230 End of Life: NVNPP-3 - 2001 NVNPP-4 - 2002
Unit
Start
Annealing
Templet cutting
Weld № 4
P, %
Cu, %
Ni, %
NVNPP-3
1971
1987, 1991
1991, 1995, 2003
0.033
0.135
<0.2
NVNPP-4
1972
1991
1991, 1995
0.029
0.17
KolaNPP-1
1973
1989
2001
0.034
0.14
KolaNPP-2
1974
1999
0.039
0.18
End of Life:
NVNPP
NVNPP
KolaNPP
KolaNPP

6. RPVs VVER-440/213 End of Life: RovnoNPP-1 - 2011 RovnoNPP-2 - 2012
Unit
Start
Weld № 4
Base metal
P, %
Cu, %
Ni, %
KolaNPP-3
1982
0.010
0.03
<0.3
0.011
0.09
KolaNPP-4
1984
0.018
0.04
0.013
0.11
RovnoNPP-1
1981
0.028
0.18
0.014
0.14
RovnoNPP-2
0.023
0.012
End of Life:
RovnoNPP
RovnoNPP
KolaNPP
KolaNPP

7. VVER-440/213 surveillance programs
Unit
Start
Weld № 4
Tested (unloaded) sets
P, %
Cu, %
Ni, % 1 2 3 4 5 6
KolaNPP-3 82
0.010
0.03
<0.2 +
KolaNPP-4 84
0.018
0.04
RovnoNPP-1 81
0.028
0.18
RovnoNPP-2
0.023
(+)

8. The problems of VVER-440/230 RPV
All VVER-440/230 RPVs were annealed because of extremely high rates of radiation embrittlement of the core welds
Re-irradiation embrittlement kinetics determines RPV steels lifetime
Re-irradiation embrittlement Data Base is highly restricted

9. CUTTING OF TEMPLETS ALLOWED TO OBTAIN FIRST ACTUAL RESULTS OF THE 1ST GENERATION RPV MATERIALS PROPERTIES

10. First templets results showed effectiveness of annealing procedure
The only way to predict RPV material behavior – accelerated irradiation in VVER-440 SS channels

11. REIRRADIATION OF TEMPLETS WAS PERFORMED IN VVER-440/213 SURVEILLANCE CHANNELS
Location scheme of the surveillance chains in VVER-440/213 pressure vessel
Full core – “high flux” irradiation
Reduced core – “low flux” irradiation

12. Is it correct to use the results of irradiated specimens
for RPV lifetime assessment?
Are the irradiation conditions of specimens equal to RPV wall conditions? Temperature Neutron Flux

13. Input water temperature
Direct measurements of irradiation temperature carried out with thermocouples in surveillance channels of Kola NPP-3 showed that overheat of surveillance specimens as compared to RPV inner surface in the core region does not exceed 5°C.
Tracing scheme of thermocouple
COBRA project results
SS temperature
Input water temperature

14. Neutron flux on SS 10-20 times higher than on inner surface of RPV wall
Core center
Flux on SS
SS capsules
Core center
Reactor pressure vessel
Core barrel

15. Flux effect study is very important
for VVER-440 RPV lifetime assessment.
In 1987 special program was started

16. Materials Materials C Mn Si Ni Cr Mo V Cu P S BM 109868 0.17 0.42 0.20
0.15
2.60
0.59
0.10
0.022
0.013
Weld 12
0.06
0.77
0.29
0.14
1.51
0.53
0.12
0.08
0.010
Weld 28
0.05
1.21
0.45
0.13
1.31
0.44
0.18
0.028
0.017
Weld А2
0.07
1.30
0.56
0.16
1.63
0.50
0.22
Weld 37
1.32
1.11
0.38
0.036
0.011 BM
0.49
0.28
2.72
0.62
0.33
0.014

17. High flux irradiation –Аrmenia-2 (full core) Low flux irradiation – Rovno-1 (reduced core) Irradiation temperature - 270оС

18. Experimental data matrix consists of 52 points

19. Comparison of the data obtained after irradiation by high and low dose rates
Function dependence DTК=AFF0.33 was chosen as in Russian Guide
95% upper and lower boundaries:

20. The statistical test was used for the evaluation of difference between high and low fluxes data
It tests the hypothesis that corresponding coefficients (AF) of two models are equal.
Small (< 0.05) Р-values means that hypothesis should be rejected (on 95% significance level), and each data group should be described by its own model.
Otherwise two data groups should be described by one model.

21. The results of experimental data evaluation by statistical test
Material
Р-value BM
0.11  0.05
WМ 12
0.24  0.05
WМ 28
0.02  0.05
WМ А2
0.04  0.05
WМ 37
0.61  0.05 BM
0.01  0.05

22. Р-value=0.610.05

23. Р-value=0.04  0.05

24. Р-value=0.01  0.05

25. All experimental data

26. Experimental data for materials with high Cu-content show significant flux effect

27. For flux effect evaluation the difference in ΔTK was used: d= Δ TK(low flux)- Δ TK(high flux) for the fluence 4x1019 сm-2.

28. Correlation parameters (R) between d and P and Cu contents
Р-value
d(41019cм-2) и СР
0.28
0.30  0.05
d(41019cм-2) и СCu
0.75
0.04  0.05
Dependence of flux effect on Cu content seems to be

29. Flux effect dependence on Cu content
Flux effect dependence on Cu content. Preliminary estimation: effect is significant if Cu content more than ~0.13 %

30. The results of mechanical tests of the VVER-440 pressure vessel steels show that copper influence on ТК shift is insignificant for re-irradiation

31. It agrees with the data of microstructural studies of the steels in irradiated and annealed conditions, and after re-irradiation

32. The microstructural studies of VVER-440 materials carried out by
P. Pareige, O. Zabusov and others,
B. Gurovich, Е. Кuleshova and others
show the following:
At primary irradiation of the RPV steels the copper-enriched clusters occur. They are of 2-3 nm in diameter with high distribution density and are effective barriers for dislocation movement.
There is depletion of solid solution by copper atoms.

33. Cu distribution in VVER-440 irradiated weld P. Pareige, O. Zabusov, M
Cu distribution in VVER-440 irradiated weld P.Pareige, O.Zabusov, M.Miller etc.

34. Copper content does not change in solid solution during annealing, there is coagulation of copper-enriched clusters and formation of copper precipitates of diameter ~ 5 nm with much smaller distribution density.
Large copper precipitates formed during annealing are of low density
They are not effective barriers for dislocation movement
There is no more intensive formation of copper clusters at re-irradiation.

35. Cu in irradiated, annealed and re-irradiated VVER-440 weld

36. Copper precipitate in irradiated, annealed and re-irradiated VVER-440 weld

37. The dependence of transition temperature shift for VVER-440 pressure vessel materials on neutron flux is not expected under re-irradiation.

38. Results of templets material study

39. CONCLUSIONS (1/2)
1. The work concerning establishment of fast neutron flux influence on radiation embrittlement for VVER-440 pressure vessel materials has been carried out using standard VVER-440 RPV materials irradiated in the surveillance channels at high of ~31012сm-2s-1 and at low ~41011сm-2s-1 fast neutron fluxes.
2. Flux effect occurs in VVER-440 pressure vessel materials with the level of copper content higher than ~ 0.13 %.

40. CONCLUSIONS (2/2)
3. Radiation damage under re-irradiation does not depend on copper significantly.
4. It is confirmed by the data of microstructure studies.
5. There is no dependence of transition temperature shift for the VVER-440 pressure vessel materials on neutron flux under re-irradiation after annealing.

41. The studies made within the last 10 years enables NPPs extend the life time of annealed units for 15 years with licensing for each 5 years

42. Line of the further works
The quantitative assessment of flux effect for different values of fluence is necessary for the solution of practical problems of VVER-440/213 lifetime assessment.
It requires development of models for radiation embrittlement under irradiation at high and at low fluxes in wide range of fluence , based on modern understanding of mechanisms of microstructural changes under irradiation.

43. Representative data on Kola NPP Unit 3 and 4 RPV steels radiation embrittlement
Neutron flux corresponds
to RPV wall
Application of the
reconstitution
technique
provides
representativenes
of test results

44. RPVs VVER-1000 EOF depends on the primary Radiation Embrittlement
Low P and Cu contents
High Ni content in weld metal (Up to 1.9 %)
EOF depends on the primary
Radiation Embrittlement
Elaborating RE depends are based on SS and research results

45. SS assembly VVER-1000 Research assembly

46. Melting monitors from VVER-1000 irradiation programs Surveillance program
Research program
314°C 308°C 302 °C 292 °C 288°C
304°C 300°C 293°C

47. The capsules with surveillance specimens are located above the core baffle in a place with high neutron flux gradient

48. It is very important to use results with high accuracy fluence data

49. New radiation embrittlement dependence of VVER-1000 RPV steels based on SS and research results
TF = ,4Ni1,5F1/3
The standard reference dependence specified in the Russian Guide for weld seams:
TF = 20 F1/3