Switzerland has three commercial nuclear power plants, which provide around 30% of the country’s total energy and are expected to produce an estimated 1500 cubic meters of High-Level Waste (HLW)/Spend Fuel (SF) and 56,000 cubic meters of Intermediate-LevelWaste (ILW) nuclear wastes over their lifetime. To ensure the safety and long-term performance of the multi-barrier system, this project aims to model the Thermal-Hydraulic (TH) behavior of Opalinus Clay under elevated thermal flux for a post-emplacement period of twenty years.

A THmodel, using Finite Element Analysis (FEA) software, was developed and adapted according to Full-Scale Emplacement (FE) heating experiment in this project to characterize the thermalhydraulic response of the multi-barrier system, including: (1) temperature distribution, (2) pore and capillary pressure evolution, and (3) saturation changes. Six TH parameters of Opalinus Clay are selected for sensitivity analysis: permeability, porosity, thermal conductivity, specific heat, thermal expansion coefficient, and storage coefficient. The model further evaluates the system’s sensitivity to variations in key TH parameters of Opalinus Clay.

The results reveal that temperatures in the Excavation Damaged Zone (EDZ) and in the bentonite layer exceed the limits set by National Cooperative for the Disposal of RadioactiveWaste (NAGRA). The highest temperature in EDZ reached 100°C. Far-field Opalinus Clay will remain under 90°C during the entire twenty-year period. Temperatures of the repository unit will be even higher at the proposed storage site because of the horizontal bedding at Nördlich Lägern. The temperature will be lower in the case with SF canister but will be higher in the case with HLW canister. The system’s temperature is only sensitive to changes in the thermal conductivity of Opalinus Clay. Pore pressure fluctuates unexpectedly within the first 1000 days and decreases shortly after reaching amaximum before 2000 days. Very peculiarly, there are no significant differences between using either the maximum or the minimum values of the particular TH parameter; their results do not differ. The numerical model provides validation for the current repository design, confirming its capability to maintain far-field temperatures in Opalinus Clay below the critical threshold of 90 ◦C.

The model successfully reproduced qualitatively consistent temperature and pore pressure fields within the Opalinus Clay formation, while addressing current knowledge gaps regarding thermo-hydro (TH) coupled processes in the waste repository