ADVANCED MODELING AND EXPERIMENTAL VALIDATION OF COMPLEX NUCLEAR MATERIAL WASTE FORMS OF POTENTIAL TRANSPORTATION CONCERN

We present here computer modeling efforts to describe the time-dependent pressurization and gas-phase mole fractions inside sealed canisters containing actinide materials packaged with small (0.12 – 0.5 wt. %) amounts of water. The model is run using Chemkin software, and the chemical reaction mechanism includes gas generation due to radiolysis of adsorbed water, interfacial chemical reactions, and adsorption/desorption kinetics of water on PuO2 materials. The ultimate goal is to provide a verifiable computer model that can be used to predict problematic gas generation in storage forms and assure design criteria for short-term storage and transportation of less than well-characterized (with respect to gas generation) material classes. Our initial efforts are intended to assess pressurization and gas-phase mole fractions using well-defined 3013 container test cases. We have modeled gas generation on PuO2 with water loading up to 0.5 wt. %, at 300 and 525 K, for time frames of 3 years. Estimates of the initial H2 generation rates were determined using RadCalc and employed in the Chemkin model to assess time- and coverage-dependent system behavior. Results indicate that canister pressurization due to radiolysis is a relatively slow process, with pressure increases at 300 K of approximately 1.5 atm. for 5000 g of PuO2 packaged with 0.5 wt. % water. At higher temperatures (> 400 K), desorption of water into the gas phase largely dictates pressurization and the gas-phase mole fractions. These modeling efforts provide a predictive capability for potential gas generation behavior that when augmented and validated by surveillance information will provide a technical basis for safe storage and transportation