Flux dependence of cluster formation in neutron irradiated weld material - small-angle neutron scattering experiments and rate theory simulation

Small-angle neutron scattering was applied to investigate the size distribution of irradiation-induced defect-solute clusters in a reactor pressure vessel weld material containing 0.22 wt% copper. In order to identify flux effects the material was exposed to neutron irradiations at two different levels of neutron flux in such a manner that the same value of neutron fluence was accumulated. We observed a pronounced effect of neutron flux on the cluster size, whereas the total volume fraction of the irradiation-induced clusters was found to be insensitive to the level of flux. The result is compatible with a rate theory model according to which the range of applied fluxes covers the transition from a flux-independent regime at lower fluxes to a regime of decelerating cluster growth. The issue of the effect of flux on the mechanical properties is also addressed.