Stability of helium-vacancy clusters during irradiation

One of the major uncertainties in understanding cavity nucleation and growth is the degree of stability of Helium-Vacancy Clusters (HVCs) in an irradiation field. Such stability is a complex function of irradiation variables as well as material parameters. The goal of investigating the stability of HVCs is to understand and consequently to model cavity size distributions. The present research first investigations helium-vacancy binding energies, which are then used in analysing the stability of HVCs under irradiation. The stability studies establish the critical HVC size, that ensures growth under specified irradiation conditions. Using these models it is possible to understand the cavity size distribution of various simmulation facilities. The fine cavity size distribution seen in many High Flux Isotope Reactor (HFIR) experiments can be explained by the favorable irradiation conditions, which lead to 'spontaneous' nucleation of HVCs. In Experimental Breeder Reactor (EBR)-II experiments, on the other hand, the conditions for spontaneous nucleation of HVCs are not met, and nucleation is 'delayed', and occurs by stochastic fluctuations. (orig./RK)