Benchmarking a new TH2M implementation in OGS-6 with regard to processes relevant for nuclear waste disposal

Abstract

In this paper, thermo-hydro-mechanically (THM) coupled processes triggered during the construction, operation and closure of a deep geological repository for heat generating, high level radioactive waste are discussed based on a generic disposal concept. For this purpose, we are using the numerical non-isothermal two-phase–two-component flow in deformable porous media (TH2M) implementation (Grunwald et al. in Geomech Geophys Geo-energy Geo-resour, 2022) in the open-source software OpenGeoSys (Bilke et al. in Transport Porous Media 130(1):337–361, 2019, https://doi.org/10.1007/s11242-019-01310-1). THM coupled effects covered in this work focus on single and two-phase-flow phenomena, gas and heat generation as well as poro-elastic medium deformation. A suitable set of benchmarks covering aforementioned THM-effects, devised in the scope of the BenVaSim benchmarking project (Lux et al. in Synthesis report. BenVaSim—International Benchmarking for Verification and Validation of TH2M Simulators with Special Consideration of Fluid Dynamical Processes in Radioactive Waste Repository Systems. Tech. rep., 2021, https://doi.org/10.13140/RG.2.2.28998.34887) is chosen and one additional benchmark is presented, allowing for the demonstration and comparison of the OGS-6 TH2M implementation against results obtained by other well-established codes used in the field. Apart from the code comparison, the benchmarks also serve as means to analyze THM coupled processes in a repository based on very simplified geometries. Therefore, they can help to improve the process understanding, but any quantitative results should not be interpreted as predictions of the behaviour of a real repository. The results obtained in this work agree well with the results presented by the project partners in BenVaSim—both in single phasic, fully liquid saturated cases and in partially saturated two phase regions. Hence, the suitability of the OGS-6 TH2M implementation for the application in the field of radioactive waste management, supporting the safety case and analyzing the integrity of the geological and geotechnical barrier systems is demonstrated. Finally, a detailed discussion of observed phenomena in the benchmarks increases our understanding and confidence in the prediction of the behaviour of TH2M coupled systems in the context of deep geological radioactive waste disposal.