Further development and verification of computer programs for the review of long-term safety analysis

During the site selection process in Germany, regulated by the Site Selection Act (StandAG), the implementer has to identify adequate siting regions and perform long-term safety analysis for these regions. The Federal Office for the Safety of Nuclear Waste Management (BASE) as responsible federal authority has to review the implementers long-term safety analysis. To perform this duty at 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 to assess the underlying uncertainties of the implementers long-term safety analysis. Numerical modelling requires a high degree of quality assurance. Therefore, it is important 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 and 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 amongst others by the authors over the past two decades by means of the computer programs, TOUGH2-GRS and MARNIE(2). Both (co-)developed programs are thermo-hydraulic codes that compute transport phenomena in porous media and in the case of MARNIE(2) can be coupled to geochemical codes, see 1 and 2 for further details. 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 3. At the BASE it is planned to further develop and use the open source program PFLOTRAN in the future for the review of long-term safety analysis. PFLOTRAN 4 is an open source, multi-phase flow and reactive transport simulator designed to leverage massively-parallel high-performance computing to simulate subsurface earth system processes. Amongst other fields of applications, PFLOTRAN has been used to simulate uranium transport at the Hanford 300 Area and is also undergoing qualification for use in performance assessments at the Waste Isolation Pilot Plant (WIPP) 4. This contribution presents specific repository processes that are part of TOUGH2-GRS and MARNIE but have not yet been completely ported to PFLOTRAN. As an example, we show results from the verification process for anisotropic tortuosities for the diffusive transport of tracers. Another example is the implementation of an effective model for the compaction of salt in the TH part of PFLOTRAN. These works and the application of PFLOTRAN will strengthen the capabilities of BASE in the independent review process of implementers long-term safety analysis within the Site Selection process. Literature 1 M. Navarro: User Manual, TOUGH2-GRS Version 2, TOUGH2-MP-GRS Version 0, GRS-505: Kln, 2018. 2 M. Navarro, J. Eckel et al.: Weiterentwicklung und Qualittssicherung von Modellierungswerkzeugen zur Durchfhrung und Bewertung von Sicherheitsanalysen im Standortauswahlverfahren, GRS-622: Kln, 2021. 3 M. Navarro, J. Eckel et al.: Indikatoren zur Bewertung des Einschlusses und der Isolation mit exemplarischer Anwendung auf ein generisches Endlagersystem mit dem Wirtsgestein Tongestein, GRS-A-3985: Kln, 2019. 4 M. Nole et al.: GDSA PFLOTRAN Development (FY2021). United States: N. p., 2021.