Effects of Residual Water on Storage Canister Internal Components

Abstract

This report describes an evaluation of the spent nuclear fuel cladding and canister internal materials' corrosion/oxidation due to (inadvertent) residual free water inside a dry storage canister post-dryout. Drying of spent nuclear fuel (SNF) and the impact of (inadvertent) residual water in a SNF canister is being addressed under the DOE-NE Spent Fuel and Waste Science and Technology (SFWST) program to ensure the safe extended dry storage and transportation of commercial SNF. A long-standing understanding has been that the amount of residual water in a storage canister after vacuum drying is not expected to be much more than trace amounts of 0.43 gram mole (NRC, 2010). However, recent findings from the High Burnup Demonstration project (Bryan et al., 2019a-c), and an Integrated Research Project (Knight, 2018) show that residual free water well above the amount of approximately 0.43 gm-moles that had been assumed for a 3 torr rebound pressure, may remain within an SNF canister following prototypic drying. In fact, an NRC-NMSS sponsored study considered residual water amount as high as 55 moles (CNWRA, 2013). Considering the recent findings and the NRC-sponsored work, this present work was conducted to study the effects of unspecified amount of residual water on oxidation and corrosion of canister internals, i.e., it was assumed that the residual water amount is not limiting. It was assumed that the spent nuclear fuel content in the canister is not exposed to the internal environment, i.e., the packaged canister does not contain any damaged (breached) fuel. The overall approach consisted of explicit accounting of both spatial and temporal variations of thermal conditions in a generic storage canister in the corrosion/oxidation of the materials. Radiolysis of the residual water is expected to result in the formation of hydrogen peroxide, both in liquid and vapor phase of the residual water. Literature information suggests that vapor phase hydrogen peroxide would decompose into oxygen and water; this reaction will be catalyzed by the cladding surface. On other hand, hydrogen peroxide in liquid water would persist. It was assumed that radiolysis would yield approximately 0.1 mole of hydrogen peroxide for each liter of residual water or 0.1 M concentration. Considering this, it was considered for the corrosion evaluation that hydrogen peroxide concentration in liquid phase residual water ranges between 0 to 2 M. Corrosion rates of various internal component materials were obtained as a function of temperature and hydrogen peroxide. Cladding materials' oxidation rates, reported in the previous study (Shukla, et al, 2019), were used. Additional analysis also indicated that cladding oxidation could only occur when temperatures are high enough for the residual water to be in vapor phase. Cladding oxidation rates and corrosion rates of the internal components were integrated with the spatial and temporal thermal conditions inside the canister for the storage period of 300 years. The specific components were those for the f...