Development and quality assurance of computer programs for the review of long-term safety analysis

Abstract

Abstract. During the site selection process, regulated by the Site Selection Act (Standortauswahlgesetz – StandAG), the implementer has to identify adequate siting regions and has to perform long-term safety analysis for these regions. The Federal Office for the Safety of Nuclear Waste Management (Bundesamt für die Sicherheit der nuklearen Entsorgung – BASE) as the responsible federal authority has to review the implementer's long-term safety analysis. To perform this duty in the required depth, it will be necessary to recalculate important aspects of the analysis by means of numerical computer programs. In addition, this will allow assessment of the underlying uncertainties of the implementer's long-term safety analysis from a regulatory point of view. Numerical modelling requires a high degree of quality assurance. Therefore, it is of vital importance that the same problem is modelled with different computer programs and – if possible – by different teams of modelers. This strategy is known as the diverse modelling approach. The goal of this approach is to perform a cross-check of the computer programs in use as well as of the correctness of data handling, data interpretation, and model implementation. Comparing outcomes of different modelling teams also integrates complementary views and approaches, which can be beneficial when dealing with complicated problems. The diverse modelling approach forms the basis of regulatory modelling. It can also be implemented within organizations if more than one code is used to tackle the same task. The diverse modelling approach has been carried out at GRS gGmbH over the past two decades by means of the computer programs, TOUGH2-GRS and MARNIE(2). Both (co-)developed programs are thermohydraulic codes that compute transport phenomena in porous media and can be coupled to geochemical codes, see Navarro (2018) and Navarro et al. (2021) for further details. TOUGH2-GRS and MARNIE(2) were applied in various projects, such as preliminary safety analysis Gorleben (VSG, Vorläufige Sicherheitsanalyse Gorleben), ZIESEL and EVEREST. A recent example for the diverse modelling approach was the use of TOUGH2-GRS and MARNIE for the development of indicators for the safe confinement of radionuclides in a deep geological repository (Navarro et al., 2019). A high degree of quality assurance is also achieved by involving computer programs in benchmarks to compare results among different codes for well-defined problems. For the calculation of THM (thermo-hydraulical-mechanical) processes TOUGH2-GRS has been coupled with the geomechanical computer program FLAC3D and this approach has been used in the benchmark BenVaSim (Seher et al., 2019). In this contribution selected works related to TOUGH2-GRS and MARNIE(2) are presented. From this starting-point, ideas and concepts for the continuation of development and quality assurance of the two computer programs at BASE in the near future are outlined. These works will contribute to strengthen the capabilities of BASE in the independent review process of implementer's long-term safety analysis within the site selection process.