10B(n, α)7Li reaction-induced gas bubble formation in Al–B4C neutron absorber irradiated in spent nuclear fuel pool

Al–B ₄ C metal matrix composites have been utilized as a neutron absorber for the past two decades to manage the neutron flux and the criticality of spent nuclear fuel pools. Recently, premature surface corrosion has been found from an eight-year-old surveillance coupon although 40 years of service life was guaranteed based on separated corrosion tests and gamma irradiation tests. Suspecting the premature corrosion was assisted by radiation damage effects from energetic particles emitted from ¹⁰B(n, α) ⁷ Li reaction, nanoscale characterization and neutron attenuation tests were carried out on the surveillance coupons used in a spent nuclear fuel pool for 33, 52, and 99 months to investigate the degradation mechanisms and ¹⁰ B depletion. Highly radiation-damaged structures, such as numerous gas bubbles filled with helium and hydrogen in the aluminum matrix and microcracks in boron carbide particles, were observed from all absorber coupons with slightly reduced densities (<1%) matching with the porous structures. The neutron attenuation tests on 52 and 99 month-installed coupons showed ∼3.1% reduction of ¹⁰ B areal density, which also indicates the needs for further investigation on the long-term safety and performance of the absorber material.