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Object kinetic Monte Carlo was employed to study the effect of dose rate onthe evolution of vacancy microstructure in polycrystalline tungsten underneutron bombardment. The evolution was followed up to 1.0 displacement per atom(dpa) with point defects generated in accordance with a primary knock-on atom(PKA) spectrum corresponding to 14-MeV neutrons. The present study includes theeffect of grain size (2.0 and 4.0 $\mu$m) but excludes the impact oftransmutation or pre-existing defects beyond grain boundary sinks. Vacancycluster density increases with dose rate, while the density of vacanciesdecreases. Consequently, the average vacancy cluster size and the fraction ofvacancies in visible clusters decrease with increasing dose rate. The densityof vacancies and vacancy clusters decrease with grain size such that theaverage size of the clusters remains similar. However, the average size islarger for larger grains at dose rates < 4.5 x 10-7 dpa/s. The trend of vacancyaccumulation as a function of dose, dose rate, and grain size is similar tothat obtained with the High Flux Isotope Reactor (HFIR) PKA spectrum. However,the amount of vacancy accumulation and the vacancy microstructure are quitedifferent. Compared to the HFIR case, we find that even though the dose ratesare 2.5 times higher, the density of vacancies and the average vacancy clustersizes are lower. In addition, a void lattice forms only for the lowest two doserates (4.5 x 10-8 and 4.5 x 10-9 dpa/s). In contrast, a void lattice formed atall dose rates studied using the HFIR PKA spectrum. We discuss in detail thefactors that lead to these different microstructures.

Object Kinetic Monte Carlo Simulations of Radiation Damage in Bulk Tungsten Part-II: With a PKA Spectrum Corresponding to 14-MeV Neutrons