Flux dependence of cluster formation in neutron-irradiated weld material

Abstract

The effect of neutron flux on the formation of irradiation-induced clusters in reactorpressure vessel (RPV) steels is an unresolved issue. Small-angle neutron scattering wasmeasured for a neutron-irradiated RPV weld material containing 0.22 wt% impurity Cu.The experiment was focused on the influence of neutron flux on the formation ofirradiation-induced clusters at fixed fluence. The aim was to separate and tentativelyinterpret the effect of flux on the characteristics of the cluster size distribution. Wehave observed a pronounced effect of neutron flux on cluster size, whereas thetotal volume fraction of irradiation-induced clusters is insensitive to the level offlux. The result is compatible with a rate theory model according to which therange of applied fluxes covers the transition from a flux-independent regime atlower fluxes to a regime of decelerating cluster growth. The results are confrontedwith measured irradiation-induced changes of mechanical properties. Despitethe observed flux effect on cluster size, both yield stress increase and transitiontemperature shift turned out to be independent of flux. This is in agreement with thevolume fraction of irradiation-induced clusters being insensitive to the level of flux.