Evaluation of Hydrogen Generation in High Burnup Demonstration Dry Storage Cask

Abstract

This report provides a best-estimate evaluation of residual water content (post-dry out) in the High Burnup (HBU) LWR Spent Fuel Demonstration project TN-32 cask, and evaluates the radiolysis of the residual free water, and the physisorbed and chemisorbed waters on the surfaces of the fuel and cask internal contents. 1 The evaluation of radiolytic breakdown of those waters with gamma radiation causing the generation of hydrogen gas (H2) is made using available literature data and models. This evaluation is part of the overall materials performance evaluation of the SNF-in-canister system, and is part of the technical bases for their continued safe dry storage.The TN-32 cask contents included 32 HBU LWR spent fuel assemblies each with 264 fuel rods clad in zirconium alloys, aluminum neutron absorber components, and aluminum and stainless steel structural components. The residual free and surface (physisorbed/chemisorbed) waters are ascribed to water vapor in the free volume and to components' surfaces, respectively. The total potential radiolytic hydrogen inventory from the water vapor and from waters ascribed to surfaces has been calculated assuming all the water produced molecular H2. The residual water that is chemically incorporated into the bulk of a hydrated oxide, i.e., chemisorbed water, and its total potential hydrogen inventory has been calculated. These calculations are at the physical limit of material available and are used for a bounding assessment purpose only.An estimate was made of amount of hydrogen (H2) generated in the HBU cask free volume after 12 days and after 40 years with radiolysis of the residual waters. The oxygen from radiolytic breakdown of free and surface water (effective net reaction H2O = H2 + 1/2O2) is not built up in the canister, and rather is assumed to be consumed by oxidation reactions with the materials. The oxidation of materials in the canister, addressed in a previous report [Shukla et al, 2019] in the NE-SFWST campaign, is not discussed in this report. No oxygen (O2) generation is expected from radiolytic breakdown of chemisorbed water in hydrated oxides based on results of previous studies. Gettering of hydrogen and back reactions to reduce hydrogen concentration were not considered in this present work.

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

“…Key predictions in Ref. [d'Entremont et al, 2020] indicated that 1) short-term hydrogen generation would be dominated by radiolysis of water adsorbed to Zr-alloy surfaces due to energy transfer from the solid to the adsorbed water and 2) the amount of hydrogen generated by this mechanism would be heavily dependent on whether or not the surface-adsorbed water could be replenished by water vapor.…”

“…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 phase. The radiolytic hydrogen generation is empirically measured using in-situ gas sampling over the course of the irradiation to track the H2 and O2 concentrations (and relative humidity measurements) in the canister gas.…”

“…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.…”