Comparison of Performance Assessment Models and Methods in Crystalline Rock: Task F1 DECOVALEX-2023

Performance Assessment (PA) is important in ensuring the isolation and long-term containment of spent nuclear fuel from the geosphere. It plays a crucial role in evaluating the long-term safety and effectiveness of underground nuclear waste storage, considering factors such as radionuclide release rates, transport mechanisms, and the performance of engineered barriers. This paper presents the findings of DECOVALEX 2023 Task F, which aimed to compare various models and conceptual approaches used in PA of a generic deep geologic repository in crystalline rock. The objective was to explore the contribution of modeling choices to uncertainty in PA model outputs. The study highlights the importance of characterizing the crystalline rock properties and the engineered barrier system in PA.The so-called reference case, a simplified version of a PA focused on the transport of two conservative tracers from the deposition hole to the surface, neglecting waste package performance was used as an example. Seven international teams (Canada, Czech Republic, Germany, Korea, Sweden, Taiwan, and United States) developed and simulated the generic reference case, tracking tracer releases from waste package locations to the near field and ground surface. Quantities of Interest (QOI) such as remaining tracer in the repository and fluxes across the domain were compared. Technical and time constraints led some teams to exclude parts of the engineered barrier system which resulted in faster release of tracers and radionuclides from the repository region. Comparing all models highlighted the importance of explicitly including drifts, buffer, and backfill in the reference case models. The results also emphasize the utility of a diverse set of modeling approaches in building confidence with performance assessment analysis.