A multiscale study of the production and recombination of closely correlated defects from irradiation-induced higher order recoils in beryllium

Abstract

Beryllium is considered as a neutron multiplier in the self-sustained fuel cycles of future fusion reactors like ITER and DEMO, thus the retention of tritium generated in transmutation events in beryllium is crucial for the operational safety of such devices. As tritium readily interacts with microstructural defects produced under neutron irradiation, its retention depends on the evolution of the microstructure. Thus, physics-based models to understand and ultimately predict the tritium retention behavior have to comprise the generation and recombination dynamics of such defects. Therefore, the generation and recombination of the most simple defects, i.e. closely correlated as well as separated Frenkel pairs, as produced at the very end of collision cascade branches are considered in this work. To that end, DFT calculations are performed to determine threshold displacement energies, typical closely correlated pair configurations, the volume of spontaneous recombination, and recombination energy barriers. On the basis of these atomic-scale results, rate-based recovery models are derived to simulate electrical resistivity recovery experiments. The proposed models suggest that the principal recovery peak of beryllium is associated with the onset of intra-basal self-interstitial diffusion. This association is found to hold for the principal recovery peak of zirconium as well and it is thus concluded it is likely to generalize to many hcp metals with smaller than ideal c to a ratios.