Influence of cascade damage on helium diffusion and bubble nucleation

Possible effects of the spontaneous production of 3D vacancy aggregates in collision cascades on helium diffusion and bubble nucleation is examined. For this, a set of simple rate equations combined with deterministic growth/shrinkage equations for the defect clusters is proposed to describe the evolution of cascade damage during the early transients. Within this framework, the bubble density and its temperature dependence is analysed in terms of diatomic nucleation assumed to occur homogeneously as well as in cascade induced vacancy aggregates (CIVAs). For He diffusion between CIVAs two very different mechanisms are considered, namely the fast self-interstitial/He replacement mechanism and the slow vacancy mechanism for substitutional He. In the case of fast He diffusion, the production and decay of sufficiently large CIVAs would result in extremely strong temperature dependencies of the He diffusivity and bubble density below 0.5Tm (Tm: melting temperature). In the case of slow He diffusion on the other hand, bubble nucleation in cascade is unlikely to make any significant contribution to the bubble density. (orig.)