Safety Monitoring of Materials and Components of Nuclear Power Plants

Cluster dynamics (CD) is used to study the evolution of the size distributions of vacancy clusters (VC), self-interstitial atom (SIA) clusters (SIAC) and Cr precipitates in neutron irradiated Fe-9at.%Cr and Fe-12.5at.%Cr alloys at T = 573 K with irradiation doses up to 1.5 dpa and a flux of 140 ndpa/s. Transmission electron microscopy (TEM) and small angle neutron scattering (SANS) data on the defect structure of this material irradiated at doses of 0.6 and 1.5 dpa are used to calibrate the model. For both alloys a saturation behavior has been found by CD for the free vacancy and free SIA concentrations as well as for the number density of the SIAC for the doses above 0.006 dpa. The CD simulations also indicate the presence of VC with radii less than 0.5 nm and a strong SIAC peak with a mean diameter of about 0.5 nm, both invisible in SANS and TEM experiments. CD modeling of Cr precipitates has been made taking into account the deviation of this system from the ideal cluster gas. A specific surface tension of about 0.17 J/m2 between the α matrix and the Cr-rich α′ precipitate and the rate at which Cr monomers are absorbed about 7.94 m−1 have been found as best fit values for reproducing the long-term Cr evolution in the irradiated Fe-12.5%Cr alloys observed by SANS. The change of the composition of Fe-Cr precipitates due to irradiation has been found.