Measurement of Hydrogen Generation from SNF Cladding Surrogates in a Lab-Scale “MiniCanister”

Residual water in a spent nuclear fuel (SNF) canister, including both free water and water physisorbed or chemisorbed on surfaces, can break down under irradiation resulting in generation of hydrogen gas (H2). SNF cladding surfaces provide large surface area for adsorbed water, and interactions between water and cladding with its attendant oxides are expected to strongly impact the H2 generation rates and total yield. For example, aluminum research reactor fuel cladding can hold significant amounts of chemisorbed water in the form of aluminum (oxy)hydroxides, while zirconium cladding may accelerate radiolysis via energy transfer from ZrO2 to physisorbed water. Experimental data on radiolytic H2 generation under conditions expected in dry storage is important for predicting the evolution of SNF-in-canister conditions to ensure safety during long-term dry storage. For this purpose, a lab-scale radiolysis testing apparatus was developed for repeated in-situ sampling of the gas inside a miniature steel canister (“mini-canister”) that fits into a 60Co gamma irradiator. This system enables monitoring of radiolytic yield from a single sample and initial atmosphere over an extended irradiation. Radiolysis testing of aluminum cladding surrogates with adherent aluminum (oxy)hydroxide films was conducted for three different drying conditions (one vacuum-only and two heated). The surrogate samples were exposed to gamma irradiation with absorbed doses up to 15 MGy. Testing included two “upset conditions”: the addition of a large “spike” of hydrogen from another source to one canister and post-irradiation heat treatments intended to release trapped hydrogen in the other two canisters. Similar testing of zirconium samples has begun recently to interrogate the radiolytic yield of H2 from Zr surfaces with different oxide loadings, physisorbed water, and water vapor in the canister atmosphere. This data is expected to help in selection of drying approaches for aluminum-clad fuel and improve predictions of canister gas conditions over time for both SNFrelevant material systems.