Challenge. Evaluate sites and key physical processes to support development of deep geologic repositories for the safe disposal of low-, intermediate-, and high-level radioactive wastes (LLW, ILW, and HLW)

Solution. Since 1982, INTERA has provided Nagra with a wide range of modeling and site characterization services to support the identification, evaluation, and development of geologic repositories for radioactive waste. While much of our early work involved hydrogeologic characterization (deep borehole testing, analysis, fluid logging, and geochemical sampling) of deep crystalline rocks (i.e., granites) and overlying sedimentary rocks (anhydrites and shales), INTERA’s more recent work has focused on modeling many of the near- and far-field processes that are critical to understanding long-term repository performance and safety. INTERA has conducted a wide range of process modeling to support Nagra’s Sectoral Plan for Deep Geological Repositories for LLW, ILW, and HLW. We have developed numerical models on different scales ranging from single waste canisters to detailed representations of emplacement caverns and tunnels and associated geometries of entire repositories and surrounding host rocks. In support of safety assessments for different repository types in the candidate clay-rich host-rock formations, INTERA has conducted thermo-hydrologic 2-phase flow and transport simulations of gas and heat in the repository and surrounding host rock to assess the effect of gas generated from corrosion and degradation of organic waste and heat generation from radioactive decay of radionuclides on pressure and temperatures, and the impact on the safety performance of the engineered barrier systems (EBS) of the repository and surrounding host rock. The simulations consider the entire life cycle of the repository which includes construction, operation, monitoring, and the post-closure phase, and describe the coupled processes associated with the early host-rock desaturation and depressurization, the re-saturation of the tunnels, and the generation of gas and heat from the radioactive waste. INTERA has also developed multiphase-multicomponent flow and transport models that incorporate different components in the gas and liquid phases to investigate processes including salt enrichment, water and oxygen consumption, and hydrogen generation through aerobic and anaerobic conditions. We have supported investigations on the coupled hydromechanical behavior of Opalinus Clay surrounding the emplacement tunnels under partially saturated conditions. Additional process analyses to support the repository engineering feasibility and safety assessments include the development of regional thermal-hydraulic models of candidate siting regions to investigate uncertainties related to regional geothermal flows, radiogenic heat emissions, fault reactivations, ice ages and permafrost, and the overall geomorphologic evolution of the region. We have also supported Nagra’s research and development program by conducting several design and prediction-evaluation analyses for experiments carried out in the Mont-Terri and the Grimsel underground research laboratories.