Irradiation enhanced degradation of Al-B4C metal matrix composite neutron absorber in spent nuclear fuel dry storage

Abstract

Aluminum-boron carbide (Al-B4C) metal matrix composite (MMC) is a type of neutron absorber that has been widely used in spent nuclear fuel (SNF) storages to maintain the system subcriticality below the regulatory limit. This use of the neutron absorber will likely be expanded and extended since the construction of final SNF repository has been delayed in many nuclear energy countries. The longer-term degradation of Al-B4C MMC in wet and dry SNF storages hence becomes a more important nuclear safety issue these days. Recent experimental studies reported premature degradation of the absorber surveillance coupon used for ~8 years in spent nuclear fuel pools (SPF) which clearly indicated some discrepancy between the results of qualification tests and in-service degradation of Al-B4C absorber. Numerous pit corrosions, gas bubble accumulation near B4C particles, and faster decrease of 10B areal density were all unexpected from the qualification test which had concluded 40 years of service life for the absorber in SNF pool based on separately conducted γ-ray irradiation and corrosion tests. Findings from these studies may have suggested the need for improvement or at least review of current test methodology to account for synergistic effect of 10B(n, α)7Li reaction-induced radiation damage and surface corrosion of the absorber. In this context, the degradation behavior of Al-B4C MMC in dry SNF storage environments, which has been largely underestimated and unexplored, was experimentally studied by conducting high-temperature humid gas corrosion tests on the ion-beam irradiated absorber specimens. At various dose levels (0.01, 0.1 and 1 dpa) and temperatures (room temperature (R.T.), 150, 270, and 400 °C), in-situ high-temperature helium ion-beam irradiation was conducted on a commercial Al-B4C MMC neutron absorber, MAXUS®, to emulate the radiation damage induced by 10B(n, α)7Li reaction during dry SNF storage. Post ion-beam irradiation characterization using TEM confirmed the formation of numerous helium bubbles (8,347 ± 1,512 #/μm2) within Al alloy matrix even with the lowest dose (0.01 dpa) and slightly elevated irradiation temperature (150 °C). This instantaneous microstructure evolution could be attributed to the low melting point of Al alloy (~660 °C) and thus resulted in high homologous system temperature (Tirr/Tm = ~0.45). Helium bubble size increase and number density decrease with irradiation temperature elevation were observed from all tested dose levels, however, most evident in the 1 dpa case; from R.T. to 400 °C, average bubble size increased from (1.639 ± 0.386) nm to (12.908 ± 2.530) nm and number density decreased from (56,327 ± 12,122) #/μm to (556 ± 99) #/μm2. Spatial distribution-wise, helium bubble formation and growth were concentrated at interfacial boundaries and more severe with higher irradiation temperature, typically in an elongated shape similar with the elliptic bubbles observed from the interface between Al matrix and B4C particles in the surveillance coupons. High-temperature humid gas corrosion test on as-received/irradiated MAXUS® absorber samples utilizing argon gas containing 27 g/m3 of humidity was conducted at 400 °C for up to 500 hours, and the samples were withdrawn from the test section after 100 and 300 hours for the analysis. Microstructure analysis revealed preferential growth of corrosion layer at the surface of the Al alloy matrix along the boundaries of aluminum oxide precipitates. The progressive intergranular corrosion behavior along the interfacial boundaries formed a ~500 nm-thick corrosion layer which is presumed to be aluminum boehmite (AlOOH) which is also likely to be formed at B4C particle interface. The bubble growth was observed from the irradiated absorber samples after 500 hours of high-temperature corrosion test which could be attributed to the helium bubble agglomeration under elevated temperature and the participation of hydrogen gas atom since electron energy loss spectroscopy (EELS) confirmed the presence of hydrogen in the bubbles surrounded by aluminum hydroxide layer. Overall, experimental results of this study may indicate that the porosification of the absorber matrix could be more significant than conventional predictions and initiated from the early stage of dry SNF storage mainly due to two following reasons: 1) relatively higher system temperature of the dry storage than SNF pools combined with lower melting points of Al alloys; 2) continuous production of helium atoms via 10B(n, α)7Li reactions from B4C particles. Further indication was that such type of porosification could be concentrated at B4C particle interfaces, and hence the rate and extent of the absorber corrosion could be faster and more severe than conventional corrosion tests adopting non-irradiated or only γ-ray-irradiated absorber specimens. From the irradiation-induced porosification and corrosion layer formation, the localized deterioration in Al alloy matrix is expected along the B4C particle interface, potentially leading to the detachment of B4C particle fragments during the long-term use of Al-B4C MMC in SNF dry storage; this type of absorber degradation may need to be concerned particularly in the non-clad neutron absorbers.