Benchmarking for verification and validation of TH2M simulators: Current results from the BenVaSim II project

Numerical simulations play an important role in predicting the long-term behavior (up to 1,000,000 years) of potential repositories for high level radioactive waste (HLW) in deep geological formations. To provide reliable and realistic predictions, the numerical simulators must incorporate multiple physical processes that may be strongly coupled with each other. Analytical solutions for the underlying differential equations often do not exist for such complex scenarios and therefore can only serve as limited verification tool for the simulators. However, comparative benchmarking of different simulators for the same problem statement can close this gap and ensure the correct implementation of the differential equations and their couplings.

The objective of the international project BenVaSim II is to verify the implementations of thermally-hydraulically-mechanically (TH²M) coupled processes, considering hydraulic two-phase flow in various simulators, i.e. TOUGH-FLAC, CODE_BRIGHT, COMSOL Multiphysics, OpenGeoSys 6, Oscar, FTK simulator, used by the participating institutions (TUC, GRS, ENSI, LBNL, BGR, BASE). The investigated scenarios range from one to three spatial dimensions, examine various (nonlinear) couplings between the individual processes, and study the influence of anisotropic thermal and mechanical material properties.

Here, we present results for selected scenarios such as the build-up of pore-gas pressure due to canister corrosion (1D), temperature induced fluid pressure changes as a result of time-dependent radiogenic heat production (2D and 3D) and the influence of anisotropic properties as expected for claystone as host rock (2D). In general, the results of the individual simulators demonstrate a high level of agreement. Nonetheless, they also reveal non-negligible differences. We analyze, in time and space, important process variables like temperature, solid matrix displacement, liquid saturation as well as liquid and gas pressure to explain the interaction between the coupled processes. Furthermore, we discuss, based on the analyzed variables, potential reasons for the observed deviations between the different simulators.

In conclusion, our study highlights the crucial role of inter-institutional and international collaborations in validating numerical simulators. These collaborations are particularly important when predicting the long-term behavior of potential repositories for HLW, a domain where accuracy and reliability are mandatory.