The high-operating-temperature fuel duty, 18-month and longer-cycle lengths, and higher burnups have put increased demands on PWR fuel during recent years. As a result of these demanding operating conditions, the corrosion and dimensional stability of PWR fuel assembly skeletons are being challenged. To understand the in-reactor performance of fuel assembly skeletons, two hot cell programs were initiated on high duty fuel. Specifically, two Zircaloy-4 skeletons and one ZIRLO skeleton were dimensionally characterized and destructively examined to understand the effect of corrosion on the dimensional stability of guide tubes and grids. While previous papers have dealt with the effects of fluence and crystallographic texture on the irradiation growth of zirconium alloys, this paper focuses on the effect of corrosion (oxidation and hydrogen uptake) on the dimensional stability of ZIRLO and Zircaloy-4. Laboratory tests were performed to quantify potential dimensional changes resulting from corrosion. Accelerated autoclave tests of ZIRLO and Zircaloy-4 in 589 K water containing 700-ppm lithium revealed that dimensional changes were well correlated to hydrogen content, not oxide thickness. In addition, the dimensional changes for both guide tube and strip (grid) autoclave specimens showed similar dependency upon hydrogen content and no significant dependency on crystallographic texture. The hot cell evaluation of the irradiated ZIRLO and Zircaloy-4 structural components revealed increasing dimensional changes of grids and guide tubes from the bottom to the top of the assembly. This variation was strongly correlated with increasing hydrogen content as suggested by the autoclave tests. For a given alloy (i.e., ZIRLO or Zircaloy-4), the grid dimensional change is similar to the guide tube diameter change, suggesting that the effects of hydrogen are generic and not related to processing or starting material geometry. Comparison of ZIRLO and Zircaloy-4 dimensional data reveals greater stability of ZIRLO as a result of lower hydrogen uptake, a lower fluence component to growth, and a lower hydrogen component to growth. ZIRLO structural components, thimbles and grids, show an enhanced degree of dimensional stability with the combined effects of irradiation and corrosion/hydriding.

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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.

This report presents a test plan to investigate radiolytic hydrogen generation in spent nuclear fuel (SNF) canisters containing Zr-based cladding materials. The initial primary contributor to the generation of radiolytic hydrogen is estimated to be the residual physisorbed water on the ZrO2 film, post-dryout. Over the longer term the primary hydrogen source is associated with the aluminum hydroxides and water vapor.The proposed testing to be conducted in FY22 would consist of lab-scale testing on non-radioactive surrogates of oxidized ZIRLO tubing and unalloyed zirconium using a miniature steel canister ("minicanister") designed to allow intermittent, in-situ sampling of the canister gas during irradiation. The minicanister approach for irradiation with in-situ monitoring system was used in a recent and on-going study Verst, 2020a &Verst et al., 2021] to measure radiolytic hydrogen generation from aluminum SNF cladding surrogate materials. This proposed testing will form 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 proposed test plan draws on previous radiolysis testing focused on aluminum-clad spent nuclear fuel (research reactor fuel) as well as the previous analysis, "Evaluation of Hydrogen Generation in High Burnup Demonstration Dry Storage Cask" [d'Entremont et al., 2020b], 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.Under gamma radiation, radiolytic breakdown of water remaining in a canister (free, physisorbed, or chemisorbed) causes the generation of hydrogen gas (H2). SNF fuel assemblies, clad in zirconium alloys, provide a large surface area to host surface-adsorbed water (physisorbed water). Various 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, water decomposed by radiolysis may be replenished by water vapor in the canister gas, providing a mechanism for accelerated radiolysis of the free water. The previous simplified analysis [d'Entremont et al., 2020b] predicted that radiolysis of physisorbed water on the Zr-alloy components would dominate the short-term H2 generation due to these factors.The proposed test plan therefore focuses 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 will be empirically measured using in-situ gas samplin...

SRNL-STI-2020-00147 Revision 0 chemisorbed water. This rate would continue until the hydrogen from the chemisorbed water is exhausted or an equilibrium is established with back reactions. Additional radiolysis testing in calendar year 2020 (CY20) is being planned to allow better discrimination of physisorbed vs. chemisorbed contributions to H2 yield and the dose regimes over which they dominate. Data for thermally-conditioned ("dried") hydrated oxides will also be developed. The completion of the CY20 testing will lead to identification of the "steady-state" H2 generation rates that would be input to the ASNF-in-canister performance model to provide the best estimate of the evolution of gas composition over decades of dry storage.