  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.
