On the theory of void nucleation in irradiated crystals

Abstract

The theory of void nucleation in irradiated materials is critically analysed and further developed. It is shown that the existing theory overestimates the rate of empty void nucleation in ion-irradiated steels by several orders of magnitude and therefore requires revision for an adequate interpretation of ion bombardment tests. To extend the nucleation model to the conditions of reactor neutron irradiation, it is modified to take into account the effect of void stabilisation by transmutation gas (He) in steels or by fission gases (Xe, Kr) in fuels. It is shown that the presence of gas atoms in a solid solution strongly reduces the free energy of void formation, but does not affect the number of void nucleation sites, which is determined by the concentration of vacancies in the irradiated material. The model developed for nucleation of small gas filled voids (bubbles) is further modified to adequately consider their further growth, stabilisation (due to athermal resolution of gas atoms) and generation of a new population of larger bubbles. The obtained analytical expression for the nucleation rate is applied to a simplified analysis of experimental data on neutron irradiation of stainless steel in PWRs and FRs. For a more accurate analysis of these tests, it is recommended to implement the new model in a fuel performance code, additionally taking into consideration the migration and coalescence of small bubbles.