Optimizing the Modeling Performance for Safety Assessments of Nuclear Waste Repositories By Approximating Two-Phase Flow and Transport by Single-Phase Transport Simulations

The long-term safety performance of a potential deep geological repository for high-level and intermediate-level long-lived nuclear waste is studied through a numerical simulation program that requires simulation tools capable of modeling appropriately the phenomenologies of interest in the repository and its environment. Because of the complexity of the modeled layout, the numerous physical processes involved, and the simulated times (up to one million years), the computational needs are very high. TOUGH2-MP is a very suitable tool for modeling the impact that the heat and gas generated in the emplacement areas may have on the evolution of the fluid pressure and on the saturation fields in the repository's drifts and shafts as well as in the host rock itself. The module EOS7R also gives the possibility to compute a coupled radionuclide transfer. Regarding computational efficiency, it is of interest to decouple the transport from the hydraulic calculation for three main reasons. First, this allows the hydraulic calculation to be used once for several transport computations of a performance analysis and safety assessment study, which is expected to lead to a substantial gain in CPU time. Second, it allows optimization of the discretization separately for both hydraulic and transport calculations. Third, it allows combination of the TOUGH2 hydraulic and other codes modeling radionuclide transport, which allows consideration of phenomenologies that are not available in TOUGH2. This work shows how to establish a sequential approach between TOUGH2 and another code. It presents the conditions of use of such an approach, in terms of performance and the impact of the temporal discretization on the results.