Object Kinetic Monte Carlo Simulations of Radiation Damage in Neutron-Irradiated Tungsten Part-I: Neutron Flux with a PKA Spectrum Corresponding to the High-flux Isotope Reactor

Object kinetic Monte Carlo simulations were performed to study the impact of varying dose rate and grain size up to a dose of 1.0 dpa in pure, polycrystalline tungsten, subjected to a neutron irradiation having a PKA spectrum corresponding to the High Flux Isotope Reactor. The present study models defect cluster accumulation in tungsten, but does not consider the impact of transmutation or pre-existing defects beyond the grain boundary sinks, with varying grain size. With increasing dose rate, the vacancy cluster density increases, while the number density of vacancies decreases. Accordingly, the average vacancy cluster size and the fraction of vacancies that are part of visible clusters decreases with increasing dose rate. With increasing grain size, both the number densities of vacancies and vacancy clusters decrease, while both the fraction of vacancies in visible clusters and the average vacancy cluster size increase. This is caused by the pseudo-ripening of the vacancy clusters due to the longer-lived self-interstitial clusters in larger grains. The spatial ordering of vacancy clusters along {110} planes was observed for both grain sizes and all dose rates studied. Interplanar spacing increases with grain size; however, no clear dependence on dose or dose rate was observed. The results of this study show that 1D diffusion of self-interstitial clusters, while necessary, is not sufficient to form a void lattice, and that the diffusion of vacancies is also required. A methodology is suggested for choosing the simulation box dimensions so as to represent more faithfully the effects of one-dimensional migrating self-interstitial-atom clusters.