Evolution of the Sealing System Porosity and its Impact on Performance

pdf NAB 23-21 Evolution of the Sealing System Porosity and its Impact on Performance(13.48 MB)

In Switzerland, the Nuclear Energy Law requires the disposal of all types of radioactive waste in deep geological repositories (KEG 2003). The basic feasibility of the safe disposal of radioactive waste in a deep geological repository in a clay host rock was demonstrated by Nagra in Project Entsorgungsnachweis (the term translates into English as “demonstration of disposal feasibility”) at the end of 2002. This feasibility study was based on the Opalinus Clay host rock option in the Zürcher Weinland area in northern Switzerland and was supported by a detailed safety case(Nagra 2002a, Nagra 2002b). At the time, two types of repositories were foreseen in Switzerland:

• a repository for the disposal of low- and intermediate-level waste (L/ILW) arising from the operation and decommissioning of Swiss nuclear power plants, from medicine, industry and research and from those operations in reprocessing that produce only low-level technological waste
• a repository for the disposal of high-level waste (HLW), comprising spent nuclear fuel (SF) and vitrified HLW, as well as long-lived intermediate-level waste (ILW), primarily resulting from fuel reprocessing

Project Entsorgungsnachweis considered only the repository for SF/HLW/ILW, referred to in the following as the HLW repository.

The choice of both Opalinus Clay and the Zürcher Weinland was the result of preliminary, safety-based evaluation of potential host rocks, in which other possible options were also evaluated but set aside by Nagra; host-rock and site selection were later carried out in a more systematic manner following the sectoral plan for deep geological repositories.

The sectoral plan for deep geological repositories (BFE 2008) specifies how sites for deep geological radwaste repositories are to be selected in Switzerland. The concept for the sectoral plan for deep geological repositories was developed by the Swiss Federal Office of Energy (SFOE) together with other agencies and organisations and was approved by the Federal Council in 2008. This procedure set out in the sectoral plan allows a transparent and fair choice of siting locations in three stages:

• Stage 1: Selection of geological siting regions
• Stage 2: Selection of at least two sites
• Stage 3: Selection of one site

In September 2022, as part of Stage 3 of the siting procedure, Nagra formally proposed the siting region Nördlich Lägern (NL), which lies within cantons Zürich and Aargau, as the site for a combined repository for all waste types, with Opalinus Clay as the selected host rock (Nagra 2022). Nagra is currently in the process of preparing a general licence application for this site, supported by a safety case, that it will submit at the end of 2024. The Federal Council and the Swiss Parliament will decide on the granting of the general licence and a national referendum can be called for on this decision.

The present report is one of several in support of the safety case for the general licence application that will address the requirement that the long-term behaviour of the repository barrier system must be understood (KEG 2003). The specific barrier addressed by this report is the sealing system, i.e., the sealing and backfilling of the underground openings and the access structures. The sealing system is designed to provide a strong and stable barrier to radionuclide transport. In addition, it provides a transport and storage system for repository-generated gas that prevents the build-up of potentially adverse gas pressures in the disposal areas, as well as mechanical stabilisation of the host rock following the degradation of the excavation support. It also reduces the probability of human intrusion into the repository. The sealing system must continue to perform these functions as it evolves over time due to a range of physical and, most importantly in the context of this report, chemical processes. The overarching aim of this report is to examine the ways in which the porosity of the sealing system could evolve in response to these processesand the resulting impact on sealing system performance, including, in particular, the impact on the fate of repository-generated gas.

The report reviews and summarises current understanding of the chemical processes that could lead to porosity changes, i.e., to mineral dissolution and, most importantly from the point of view of gas transport and storage, to precipitation of minerals in relevant porous media and the resultingimplications for the design and performance of the sealing system. Because it is critical to the occurrence of these chemical processes and to the evolution of gas pressures, the saturation history of the repository is also described, based on the results of two-phase flow modelling carried out as part of S tage 2 of the sectoral plan. In the course of ongoing work, these results will be updated and documented as part of the gas synthesis report ( Nagra NTB 24-23 planned) and of the performance assessment synthesis (Nagra NTB 24-22 planned), along with a description of modelabstraction and validation.

With currently available state-of-the-art numerical codes, it is not possible to simulate the sealing system porosity evolution in a holistic way, taking into account all physical and (bio-)chemical processes and their couplings at the necessary level of detail. For this reason, this report is based on a set of theoretical arguments and considerations supported by literature reviews on potentially relevant processes in combination with numerical simulations. These simulations investigate specific aspects of (sub-)system evolution by considering only selected processes and their couplings. The simulation codes that are used are validated in terms of their numerical correctness via numerical tests and code comparison (benchmarking) studies. Details of model conceptualisation and model validation are given in Nagra NTB 24-22 (planned) and in the reference therein. In terms of application, the results of numerical simulations are compared withpublished experimental and numerical studies on related systems. This approach is in line with IAEA Safety Glossary 2022 definition on “validation on process level” (IAEA 2022).

It is to be noted that porosity evolution and its impact on performance is assessed for a flexible sealing system design, which is part of a provisional engineered barrier system design developed specifically to support the general licence application by demonstrating that safe disposal in the NL siting region can be achieved. It is expected that these provisional designs will be reviewed and revised as necessary over the forthcoming years as part of the continuing licencing process.