Thermal Modeling of High-Level Nuclear Waste Disposal in a Salt Repository

Salt formations have received recent attention for geologic disposal of heat-generating, high-level nuclear waste (HLW). Existing investigations are summarized and expanded upon using analytical and numerical models to investigate simulated temperatures in the salt after emplacement of HLW. Analytical modeling suggests that temperature variations near canisters will be smooth, indicating that the system can be approximated by a coarsely discretized numerical model. Two multidimensional parameter studies explore canister configuration using characteristics from (a) defense HLW and (b) spent nuclear fuel (SNF) waste. Numerical modeling was conducted for a disposal concept consisting of emplacement of waste canisters on the floor of drifts and covering each with salt backfill. Results indicate that waste forms with U.S. Department of Energy (DOE) waste characteristics can be easily configured to maintain simulated temperatures far below 200 degrees C at spacings as close as 0.3 m (similar to 1 ft), the minimum feasible spacing that could practically be achieved. For SNF waste packaged into canisters with heat loads of 1500 or 1000 W with canister spacing of 6 m (similar to 20 ft) and 3 m (similar to 10 ft), respectively, simulated temperatures can be maintained below 200 degrees C; much higher maximum temperatures would result for designs with higher canister heat loads and smaller spacings. These results indicate that from a thermal loading perspective, in-drift disposal of HLW in salt deposits is feasible for DOE-managed waste as long as the maximum temperature is managed through proper selection of canister heat loads and spacings. The results will aid in the design of potential future field tests to confirm this conclusion.