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.

Since 2022 BASE operates the research project ERLa (Entwicklung und Verfizierung von Rechenprogrammen zur Bewertung von Langzeitsicherheitsanalysen) which will terminate by the end of 2025.

Within ERLa starting points were set to further develop and use the open source programs PFLOTRAN and FEHM 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. Moreover, BASE develops its own transport program MARNIE2 which allows flow and transport calculations including processes which are relevant in long-term safety analyses. Since numerical modelling and the development of the computer programs in use requires a high degree of quality assurance, verification has also been a key issue in the research project. To timely react on the requirements during the site selection process a workflow for the application of the afore mentioned programs has been set up including standard pre- and postprocessing techniques as well as  tools for sensitivity analyses. Beyond these technical topics a strategy has been developed considering the main aspects of research and development needed to perform modeling duties at BASE.

This contribution presents selected results of the research project ERLa which have a direct link to the review of long-term safety analyses within the site selection process. Research and development as executed in ERLa need to be carried out before their results like e.g. verified computer programs are actually needed. To this effect a well planned research which anticipates the needs of a regulatory assessment contributes to a time-efficient decision of the regulator which is one part of the topic ‘Time as a safety factor’.