Measurement of radiolytic hydrogen generation from surrogate zr-based materials in a "mini-canister"

Abstract

This report describes a test campaign to investigate radiolytic hydrogen generation from surrogate materials resembling the (Zr-based) cladding of commercial spent nuclear fuel (SNF) with an inventory of residual water post-dryout. The radiolytic hydrogen generation rates from residual waters in the SNF-incanister system are important for predicting the in-canister environment for sealed SNF dry storage canisters over time and the impact to continued safe dry storage.Under gamma radiation, radiolytic breakdown of water remaining in a canister (free, physisorbed, or chemisorbed) causes the generation of hydrogen gas (H2). SNF assemblies, clad in zirconium alloys, provide a large surface area to host surface-adsorbed water (physisorbed water). Other components in the cask, including stainless steel and aluminum structural components and aluminum neutron absorbers, also contribute to the total water inventory and radiolytic H2 generation in an SNF canister.Water adsorbed to the ZrO2 on the cladding surface may experience accelerated radiolysis compared to free water due to energy exchange with the oxide. In addition, surface water decomposed by radiolysis may be replenished by water vapor in the canister gas, providing a mechanism for accelerated radiolysis of the free water as well. The previous simplified analysis [d'Entremont et al., 2020] predicted that radiolysis of physisorbed water on the Zr-alloy components would dominate the short-term H2 generation due to these factors. Therefore, the yield associated with water radiolysis on the Zr surfaces (physisorbed water, with potential replenishment from water vapor) is the focus of the current testing.The lab-scale testing is conducted on non-radioactive surrogates, namely assemblies of zirconium coupons, irradiated inside a miniature steel canister ("mini-canister") designed to allow intermittent, insitu sampling of the canister gas during irradiation. The mini-canister approach for irradiation with in-situ monitoring was used in recent and on-going studies [Verst and d'Entremont, 2021] to measure radiolytic hydrogen generation from aluminum SNF cladding surrogate materials. This proposed testing forms part of the overall materials performance evaluation of the commercial SNF-in-canister system and is part of the technical bases for their continued safe dry storage.The test plan draws on the previous radiolysis testing on aluminum as well as the previous analysis, "Evaluation of Hydrogen Generation in High Burnup Demonstration Dry Storage Cask" [d'Entremont et al., 2020], which provided a best-estimate evaluation of residual water content (post-dryout) in the High Burnup (HBU) LWR Spent Fuel Demonstration project TN-32 cask and evaluated the expected radiolysis of the residual water, including free, physisorbed, and chemisorbed water in the sealed cask, based on literature data and models.The experiments focus on the contribution associated with the Zr-alloy components, i.e., surface-adsorbed water as well as interactions with free water vapor present in the gas p...

Milestone 1.2.12: The Fate of Oxygen Radical Species in Corroded Aluminum Alloys under Irradiation

“…22 Other studies explicitly stated that no O2 generation was detected in their systems. 17,[27][28] Earlier work by this team also found no radiation-induced O2 generated from corroded aluminum coupons. 12 Based on these results, and given the detection limits of the techniques employed, it seems unlikely that a significant amount of O2 is generated upon the irradiation of the aluminum hydroxides and oxyhydroxides present in ASNF.…”