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Surveillance test specimens : flux effects and 1st - 2nd generation RPV steels / New positron annihilation methods and their application to Fe-Cu model alloys
Prof. Masayuki Hasegawa et Dr. Y.Yasuyoshi Nagai (DEN/DANS/DPC/LECA, Bât.458, p.201)
Institute for Materials Research, Tohoku University, JAPAN
Thu, Sep. 11th 2008, 09:45
, CEA-Saclay

Part 1. Surveillance test specimens : flux effects and 1st - 2nd generation RPV steels

Masayuki Hasegawa and Yasuyoshi Nagai* The nanostructural evolution of irradiation-induced nanoprecipitates and vacancy clusters in the surveillance test specimens of reactor pressure vessels (RPVs) of a Calder Hall (Magnox) Reactor (CHR) and pressurized water reactors (PWRs, Belgium Doel-1, -2 and -4) has been studied. For the CHR test specimen, using positron annihilation spectroscopy, we here reveal a novel kinetics of irradiation-enhanced precipitation. Very low dose rate irradiation in the CHR for 20 years can significantly enhance Cu nanoprecipitation. This phenomenon causes the RPV embrittlement at a much earlier stage than that found from accelerated test in a material testing reactor. For the test specimens of the first generation RPV steels (A508B) for Doel-1 and -2, combining the three dimensional local electrode atom probe (LEAP) and the positron annihilation techniques, we have found that in both medium (Doel-1, 0.13 wt.%) and high (Doel-2, 0.30 wt.%) Cu welds, the Cu-rich nanoprecipitates (CRNPs) are found to form readily at the very beginning of the reactor operation time. Thereafter, during the subsequent 30 years of operation, the residual Cu concentration in the matrix shows slight decrease while the CNPs coarsen. On the other hand, small vacancy clusters of V3~V4 start appearing after the initial Cu precipitation and are accumulated steadily with neutron dose. Grain boundary segregation/precipitation is found to proceed with irradiation. For the test specimens of the second generation RPV steels (A508B), in which Cu, P and S impurity contents were much reduced, for Doel-4 (0.05 wt%Cu), the formation of nanoprecipitates, such as Si-Mn-Ni nanoprecipitates and CRNPs are much suppressed. Results of MTR irradiation experiments of the first and second generation RPVs (A533B) are also presented.

Part 2. New positron annihilation methods and their application to Fe-Cu model alloys

We will present positron annihilation techniques recently applied to the study of nanoprecipitates and irradiation-induced defects in metals, including coincidence Doppler broadening (CDB), age-momentum correlation (AMOC) and two-dimensional angular correlation of annihilation radiation (2D-ACAR). CDB/ Chemical analysis and nature of positron trapping sites: nanoprecipitates and vacancy-type defects AMOC / Can positron be used as a qualitative tool to estimate the number density of the positron trapping sites? 2D-ACAR / Electronic structure of nanoprecipitates and vacancy-type defects. Prior to the introduction of the above methods, we will briefly review positron trapping at vacancy-type defects and affinity-induced trapping to embedded nanoprecipitates. Finally we present experimental results obtained in combining the three mutually complementary methods: the coincidence Doppler broadening (CDB) of positron annihilation, 3D atom probe (3DAP) and electrical resistivity (ER) measurements. It is demonstrated how the use of this complementary methods leads to clarify the very early stage of Cu nanoprecipitates (CNPs) in Fe-Cu model alloys. The experimental results are well reproduced by calculations of the early stage evolution of the CNPs from a classical nucleation and growth theory.
Contact : Luc BARBIER


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