A. Safety case in the framework of Project Entsorgungsnachweis and the General Licence Application - a comparison (NTB 24-10)

Safety case in Project Entsorgungsnachweis and in the General Licence Application – a comparison

Purpose of the safety case

Both Project Entsorgungsnachweis and the current safety case for a repository for both HLW and L/ILW at the Haberstal site have demonstrated the post-closure safety provided by a deep geological repository from the hazards presented by the waste.

Safety concept

The current safety concept, i.e., NAB 24-18 Rev. 1 (Nagra 2024s), is similar in Project Entsorgungsnachweis and in the present safety case. However, the definitions of the pillars of safety and the ways in which they contribute to the safety functions of the repository have been further developed and clarified.

Repository layout and engineered barrier design

Although the safety concept is largely the same, the details of the engineered barrier system and repository design have evolved since Project Entsorgungsnachweis. Of particular significance to safety assessment are:

• The use of dead-end emplacement rooms for both HLW and L/ILW, compared with the tunnels with access from two sides used for the disposal of HLW in Project Entsorgungsnachweis, which provides additional assurance of low water flows along the emplacement rooms.

• The closure system, including the system of repository seals and backfill, has been revised to ensure, for example, that there are no safety-relevant interactions between the HLW and L/ILW repository sections, and to ensure adequate storage volume for repository-generated gas, avoiding build-up of potentially damaging gas pressures.

• In Project Entsorgungsnachweis, the use of a concrete liner in the HLW emplacement tunnels was assumed to be unlikely and the possibility was not accounted for in the safety assessment. In the present provisional design, a concrete liner is now foreseen for the HLW drifts, as well as for the L/ILW emplacement caverns. This has motivated a more detailed evaluation of cement-clay interactions.

• In Project Entsorgungsnachweis access would be provided, during construction and operation, by a spiral ramp. A vertical shaft, also used for construction during enlargement of the facility, would provide ventilation and the required emergency escape facility.

Safety arguments

The arguments for the post-closure safety, including arguments for both the intrinsic quality of the system and the quality of the safety assessment, are essentially the same in the present safety case as they were in Project Entsorgungsnachweis but are better supported by an improved assessment basis and a more thorough safety assessment, as indicated above.

Assessment basis (general)

Since Project Entsorgungsnachweis, the assessment basis has benefited from several further years of RD&D and site characterisation work by Nagra, providing a firmer basis for the arguments supporting the present safety case, including experiments, models and databases used to assess the performance of repository components, the repository system as a whole and the radiological consequences of safety scenarios (see below). The FEP database, while still serving primarily as a completeness check, has also been extensively reviewed, revised and reorganised.

Waste inventory and repository type

Project Entsorgungsnachweis considered a repository for the disposal of SF, HLW and long-lived ILW (primarily resulting from fuel reprocessing) at the “Zürcher Weinland” site. It was assumed that there would be a separate repository for the disposal of L/ILW including ATW arising from the operation and decommissioning of Swiss NPPs, from medicine, industry and research and from those operations in reprocessing that produce only low-level technological waste. The present safety case considers a repository for both HLW and L/ILW including ATW at the Haberstal site.

Geological understanding of the site / siting region

Project Entsorgungsnachweis basically relied on information from one deep borehole at Benken and a 3D seismic survey of the ZNO siting region. For the post-closure safety case presented here 4 deep boreholes in NL and 8 more in the other siting regions provide a wealth of information on rock properties. Additional 2D and 3D seismic surveys complete the picture. Drilling into the quaternary sediments adds information on the most recent geological evolution of northern Switzerland for better understanding of possible future evolutions. A more detailed comparison of the two geosyntheses can be found in NTB 24-17 (Nagra 2024i).

Reporting structure

A reporting structure has been adopted for the present safety case that reflects the safety assessment methodology. This includes, in addition to the present post-closure safety report which is a synthesis report of the overall safety case, dedicated reports on performance assessment, safety scenario development and the analysis of radiological consequences. In Project Entsorgungsnachweis, the performance assessment (then termed “system evolution”), safety scenario development (then termed “the safety concept and the identification of assessment cases”) and the analysis of radiological consequences (then termed “evaluation of the performance of the disposal system”) were documented primarily as chapters of the main post-closure safety report (Nagra 2002), though with many supporting reports including a report on FEP management for the Opalinus Clay safety assessment (Nagra 2003b). The current structure allows these safety assessment processes to be documented in more depth, while keeping the post-closure safety report itself concise and accessible to a general technical audience.

Safety assessment methodology

The main processes of safety assessment, i.e., performance assessment, safety scenario development and the analysis of radiological consequences, feature in both Project Entsorgungsnachweis and in the present safety case. However, workflows for each of these processes and the interfaces between them have been further developed and illustrated diagrammatically. Specific developments include:

• The explicit link that is now made between the development of the repository concept and design and the safety assessment processes.

• The greater emphasis on performance assessment as a key element of the methodology.

• The more systematic performance assessment methodology, based around a set of claims, arguments and evidence at the system-component level and at the total system level, with each claim supported, where possible, by wide ranging arguments and evidence and linked to one or more safety functions.

• The definition and use of quantitative performance indicators, indices and targets to assess the potentially detrimental phenomena and uncertainties and the margins by which the safety functions are provided, in support of safety scenario development.

• The more extensive and systematic use of probabilistic methods for uncertainty assessment in both the performance assessment and in the analysis of radiological consequences.

Safety scenarios

While the safety scenarios of both the Project Entsorgungsnachweis and the present safety case cover the potential evolution paths of the repository, their exact formulation differs, accounting for the more profound assessment basis and the optimised safety assessment methodology. Specific differences include:

• The inclusion of gas-mediated radionuclide transport in the evaluation of all safety scenarios (in Project Entsorgungsnachweis, gas-mediated transport was considered as an alternative safety scenario).

• Alternative safety scenarios solely linked to uncertainty in geological characterisation and evolution, namely linked to either hydraulically active confining geological units or to an undetected or reactivated fault.

• The more systematic definition of hypothetical “what-if?” cases that test system robustness, based on the degraded performance of each of the pillars of safety.

• More profound assessments of the “what-if?” case of excavation of the repository by glacial and non-glacial erosion in the far future.

Process understanding and models

Improved process understanding and greater characterisation of the geological setting has been accompanied by a more thorough analysis of these features and processes in both the performance assessment and in the analysis of radiological consequences. Examples include:

• More profound characterisation and deeper process understanding for radionuclide retention and transport in the Opalinus Clay and the confining geological units.

• More detailed and comprehensive THM modelling, allowing a better understanding of the rate of repository-generated gas and of the impact of repository-generated heat on the surrounding barriers.

• A thorough examination of cement/clay interactions, including the development of arguments that such interactions will not lead to pore clogging or other detrimental effects.

• The inclusion of the confining geological units in all radionuclide transport calculations (in the Reference Conceptualisation for Project Entsorgungsnachweis, the geological barrier was conservatively restricted to a transport path segment through the unperturbed host rock).