Code development and verification for the review of long-term safety analyses

Long-term safety analyses need to be performed by the implementer to identify adequate siting regions in the course of the site selection process in Germany, regulated by the Site Selection Act (Standortauswahlgesetz - StandAG). The Federal Office for the Safety of Nuclear Waste Management (Bundesamt für die Sicherheit der nuklearen Entsorgung - BASE) as responsible federal authority has to review the implementer’s long-term safety analyses. To perform this duty at the required detailedness, and to identify potentially missing processes, it will be necessary to recalculate important aspects of the analyses by means of numerical computer programs. In addition, this will allow to assess the underlying uncertainties of the implementer’s long-term safety analyses from a regulatory point of view.

Numerical modelling requires a high degree of quality assurance. Therefore, simplified physical problems are needed to check the results of the computer programs against analytical solution of these problems or to assess the plausibility of the results.

At the BASE it is planned to further develop and use the open source programs PFLOTRAN [1] and FEHM [2] for the review of long-term safety analyses. PFLOTRAN 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. FEHM is used to simulate reactive groundwater and contaminant flow and transport in deep and shallow, fractured and unfractured porous media and allows for a coupling of the transport processes with geomechanical processes. In addition, the BASE develops its own multi-phase flow and transport program MARNIE2 [3] which allows flow and transport calculations including processes which are relevant in long-term safety analyses.

This contribution presents examples from the newly developed regression test procedure for MARNIE2 which allows to check plausibility and functionality of fundamental processes in the code when the source code undergoes changes due to code development. To compare the computer programs among each other a comparison with the analytic solution for the advective and diffusive transport of a radionuclide chain with four members including adsorption is presented. As an example for recent code development, first results for the compaction of salt backfill in PFLOTRAN are presented. Here, the further development of PFLOTRAN is motivated by the participation of the BASE in the task “performance assessment” of the DECOVALEX 2027 initiative.

Literature

[1] Nole, G.D. Beskardes, D. Fukuyama, R.C. Leone, H.D. Park, M. Paul, A. Salazar, G.E. Hammond and P.C. Lichtner: Recent Advancements in PFLOTRAN Development for the GDSA Framework (FY2023), SAND2023-07655, (SNL-NM), United States, 2023; and references therein.

[2] Zyvoloski: FEHM: A control volume finite element code for simulating subsurface multi-phase multi-fluid heat and mass transfer. Earth and Environmental Sciences Division, Los Alamos National Laboratory, LAUR-07-3359, 2007.

[3] Navarro, T. Beuth, G. Bracke, J. Eckel, G. Frieling, S. Hotzel, I. Kock, H. Seher, and T. Weyand: Weiterentwicklung und Qualitätssicherung von Modellierungswerkzeugen zur Durchführung und Bewertung von Sicherheitsanalysen im Standortauswahlverfahren, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, GRS-622: Köln, Februar 2021.