Displacement cascade bombardment of delta-hydrides in alpha-zirconium

Abstract

One of the factors affecting the in-pile performance of Zr-based alloys is the precipitation of hydrides once H concentrations exceed the terminal solubility limit. H transport and hydride precipitation/dissolution is commonly modeled in codes such as BISON, but most of the experimental data supporting these models has been collected on unirradiated materials. As such, there is considerable uncertainty as to the influence of irradiation effects. In this work, molecular dynamics simulations of displacement cascades were performed on δ-hydrides to elucidate: 1) the extent of H dissolution following cascade impacts and 2) any alterations to defect production characteristics when compared to cascades in bulk Zr. The immediate amount of H dissolved in a high-energy cascade impact is notable, but a considerable fraction of the dissolved H atoms are rapidly re-absorbed into the hydride at reactor-relevant temperatures. The amount of dissolved H also decreases with increasing hydride size. When considering the expected volume fractions of hydrides, it is not expected that the irradiation-induced H dissolution rate will significantly affect the availability of H in the Zr lattice. In terms of defect production, cascades which overlap δ-hydrides produced an order of magnitude more stable defects than equivalent-energy cascades in bulk Zr. Vacancy defects are predominantly contained within the hydride structure while interstitials clusters are found adjacent to the hydride surface. Interstitials are strongly repelled by the hydride structure which may drive the expulsion of cascade-generated interstitials to the hydride surface and impede athermal recombination. Thus, the interatomic potential used in this work predicted a significant alteration to the defect survival efficiency and a stark production bias in the availability of mobile defects in bulk Zr following hydride-overlapped displacement cascades.