Simulation of Radiation Damage and He Accumulation Induced by 10B(n, α)7Li Reactions in Al-B4C Neutron Absorbers Used in Spent Fuel Pools

Abstract

Faster than expected surface corrosion and a considerable decrease in the 10B areal density of a non-clad Al-B4C neutron absorber were recently reported from surveillance coupons used in a spent fuel pool for about 8 years, which is only approximately one-fifth of the guaranteed service life of the absorber. Such premature degradation was largely attributed to irradiation-assisted corrosion since numerous gas bubbles filled with He and H were discovered in the Al alloy matrix near B4C particles, and naturally 10B(n, α)7Li reactions were designated as the underlying mechanism for the porosification that may have expedited the absorber corrosion. In this study, the levels of radiation damage and He concentration in the micrographs published in recent experimental studies were estimated to design ion beam irradiation experiments having appropriate parameters to emulate the status of neutron-irradiated non-clad Al-B4C absorbers. TRITON, CSAS6, and SDTrimSP were coupled with modifications for the calculation of dpa and He concentration in irradiated neutron absorbers. The simulations yielded 3.74–6.71 dpa and 0.71–1.64 at% of He after 99 months of use and 3.82–8.39 dpa and 0.73–2.08 at% of He after 40 years of use. The simulations also demonstrated that the radiation damage and He concentration have a weak correlation with the particle diameter. In terms of radiation damage, these results are in good agreement with the reported experimental data, indicating that they can be referred to in the experimental design. The calculated He concentrations, however, may warrant modification to include leakage of implanted He atoms through irradiation-induced microcracks in the B4C particles. Because of the high diffusivity of He, full leakage of He from B4C particles to their boundaries with the Al alloy matrix was assumed for further estimations, which revealed that He concentration could be significantly elevated in the Al matrix near B4C particles.