Nanostructural evolution in surveillance test specimens of a commercial nuclear reactor pressure vessel studied by three-dimensional atom probe and positron annihilation

The nanostructural evolution of irradiation-induced Cu-rich nano precipitates (CRNPs) and vacancy clusters in surveillance test specimens of the in-service commercial nuclear reactor pressure vessel steel welds of Doel-1 and Doel-2 are revealed by combining the three dimensional local electrode atom probe and the positron annihilation techniques. In both medium (0.13 wt.%) and high (0.30 wt.%) Cu welds, the CRNPs are found to form readily at the very beginning of the reactor lifetime. Thereafter, during the subsequent 30 years of operation, the residual Cu concentration in the matrix shows slight decrease while the CRNPs 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. The observed nanostructural evolution is shown to provide unique and fundamental information about the mechanisms of the irradiation-induced embrittlement of these specific materials.