10B(n, α)7Li Reaction-Assisted Corrosion of Al-B4C Metal Matrix Composite Neutron Absorber Irradiated in Spent Nuclear Fuel Pool

Al-B4C metal matrix composites (MMC) are neutron absorber materials widely used in spent fuel pool (SFP) to ensure the subcriticality with more densely stored fuel assemblies. The performance and safety of the absorber material was experimentally demonstrated considering only low level irradiation assisted corrosion at a lukewarm temperature (20 ~ 49 °C) of the coolant and 40 years of service time was guaranteed. Recently, however, Korea Hydro & Nuclear Power Co., Ltd (KHNP) has discovered white spots from the surface of the surveillance coupons that have been submerged under a Korean SFP for 8 years and 3 months.This unexpectedly premature corrosion was not evident from the accelerated corrosion tests equivalent to 20 years of storage time in typical SFP without irradiation. We have been speculated that the radiation damage induced by 10B(n, α)7Li reaction from boron-bearing particles, and also possibly from boron crud on the absorber surface, may have expedited the corrosion owing to ballistic and radiation-enhanced diffusivity, radiation-damaged porous surface, and locally elevated system temperature. In this study, the microstructures of various period (33, 52, and 99 months) installed surveillance coupons were characterized and highly radiation damaged structures were generally found: numerous helium bubbles in aluminum matrix were observed from longer time (52 and 92 months) stored coupons; microcracks in boron carbide were universal for all three periods. Since the severe microstructural evolution suggested potentially large boron depletion, the boron-10 areal densities of the coupons were measured from neutron attenuation test, which showed a stepwise reduction of the boron concentrations in the order of storage periods. Radiation damages and helium production rates were also deduced from the boron-10 depletion rate.