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

Abstract

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 then 40 years of service time was guaranteed. Recently, however, white spots were discovered from the surface of the surveillance coupons with reduced boron-10 areal density. 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 urface, may have expedited the corrosion partially owing to ballistic atomic mixing and radiation-enhanced diffusivity, radiation-damaged porous surface, and locally elevated system temperature. In this study, the microstructures of various period-installed (33, 52, and 99 months) surveillance coupons were characterized. Highly radiation damaged structures such as numerous helium bubbles in aluminum matrix, microcracks in boron carbide, and correspondingly reduced density were commonly observed in all three period coupons. Since the severe microstructural evolution suggested potentially large boron depletion, the neutron transmission rates of the coupons were reevaluated by neutron attenuation test, which showed a stepwise reduction of the boron concentrations in the order of storage periods.