Safety Assessment Methodology
Nagra (2024): Safety Assessment Methodology. Nagra Technical Report NTB 24-19.
pdf NTB 24-19 Safety Assessment Methodology (5.27 MB)
This report presents a high-level description of the methodology used in the post-closure safety assessment, hereafter referred to as safety assessment, that supports the general licence application for a combined deep geological repository for low- and intermediate-level waste and high-level waste at the Haberstal site. Safety assessment is defined by Nagra as the process of gathering the evidence and arguments regarding the safety of the repository system during the post-closure phase.
The report first presents a broad perspective of the context in which the current safety assessment methodology was developed, including legal and regulatory requirements with respect to post-closure safety, as well as international developments and guidance on safety assessment and the safety case. The development of the safety and repository concept and a provisional repository design, needed to make the safety case for the general licence application, are also outlined. The key principles and features of safety assessment, including the management of uncertainties, are also presented.
Along with the post-closure safety case, the general licence application will include a justification for the siting proposal, based on a site comparison. This report provides a general overview of the “assessment basis” underlying both the safety case and the safety-related aspects of site comparison. The assessment basis, which this report is part of, includes the evidence, knowledge, assessment tools and methodologies developed or acquired in support of site selection and the safety case.
The report then describes the four main processes carried out in safety assessment, namely:
- performance assessment, in which the thermal, hydraulic, mechanical, and chemical evolution of the repository system is analysed,
- safety scenario development, which entails the identification and description of a set of safety scenarios that capture uncertainty in the initial state of the repository system and different ways in which it could evolve over time,
- analysis of radiological consequences, which consists of an evaluation of radionuclide release rates to the surface environment as a function of time, and
- demonstration of post-closure safety, in which evidence, arguments and results from the three previous processes are brought together in the post-closure safety report.
The focus of this report is especially on explaining how these processes are related to each other and how information flows between them.
Finally, the report highlights the similarities, parallels and general consistency between the safety assessment methodology and the methodology adopted in site comparison, including the relationship between:
- performance assessment for the post-closure safety case and the qualitative evaluation of the containment-providing rock zone carried out for each siting region in support of the site comparison, and
- the analysis of radiological consequences for the safety case and the site-specific safety analyses carried out for the different siting regions.
The present report, which is part of the assessment basis, is complemented by more detailed dedicated reports on each of the main processes of safety assessment. These describe the outcomes of the safety assessment processes, as well as methodological details not covered here.