11. Further development of the safety case in future programme stages (NTB 24-10)

The Sectoral Plan for Deep Geological Repositories regulates the search for suitable siting regions for deep geological repositories in Switzerland, which is conducted in three stages (BFE 2008). The Sectoral Plan process concludes with the end of Stage 3, i.e., with the approval of the object sheet²⁹ by the Federal Council. At the same time, the Federal Council decides on the granting of the general licence.

After the site has been approved in the general licensing procedure, construction and operation licenses are required for constructing and operating the repository (based on nuclear energy legislation). After completion of the operating and monitoring periods, a permit for closure must be applied for and issued by the Federal Council.

The background for preparing the application for future licences is provided by experience gained in preparing the general licence application, experience from international projects and information on newly developed technologies. However, the following main points are expected:

• Construction licence: The construction licence showcases a detailed plan of the future facilities and a plan for closure. The construction licence also specifies the key elements for technical implementation. This includes the test areas, which, together with the main repository and the pilot repositories, form integral parts of the geological repository.

  With the framework of preparing the construction licence and its related permits, a stage-appropriate optimisation is carried out for the selection of the engineered barriers (disposal containers, backfilling, sealing) and the design of the underground repository architecture, as well as the technical implementation of the project.

  The construction licence application also provides the possibility to optimise and adapt the repository infrastructure.

• Operating licence: The operating licence specifies the definitive protection zone³⁰ of the deep geological repository and the permissible capacity of the facility. It also sets out measures for monitoring the environment and the stages of commissioning, the start of which requires prior clearance by the regulatory authorities. The operating licence also specifies requirements relating to limits for the activity of the waste to be disposed of. The emplace­ment of waste packages in a repository requires approval by ENSI (ENSI 2023). The sealing technology for any sealing elements that have not been tested in advance is tested at this point, and their functionality during repository operation is demonstrated. Technology that would allow, if needed, the pre-closure retrieval of waste packages without undue effort must also be demonstrated, including the introduction and removal of backfill materials.

  Once the operating licence has been granted, waste emplacement in the repository begins. Safety checks are carried out periodically during operation.

  The monitoring period begins once emplacement is complete. The application for de­commissioning of the facilities and structures at the surface is also prepared and submitted.

• Closure of the repository: After the end of the monitoring period, the Federal Council authorises closure work provided that the permanent protection of humans and the environ­ment can be ensured. After final closure, the Federal Council may order further, temporary monitoring. At this time, or after expiry of the monitoring period, the Federal Council determines that the repository is no longer subject to nuclear energy legislation.

  With the application for repository closure, the final decision on the preferred closure variant is made. For this, the repository closure plan and an updated project plan for the monitoring period are to be submitted.

Future work planned by Nagra will yield information that will allow further optimisation of the current provisional repository design and implementation plan, e.g., adapting it to the actual site-specific conditions that are encountered underground.

The waste producers have tasked Nagra with implementing safe deep geological disposal for all categories of radioactive waste in Switzerland at reasonable cost and in a timely manner. To fulfil this mandate, the safety of the deep geological repository must regularly be demonstrated to the authorities with a solid technical foundation and transparent documentation in line with the licensing procedure (KEG 2003).

The repository project must be realised within the framework of a holistically oriented optimisation process that is aligned with the following overarching optimisation goals and principles for post-closure safety and implementation:

• Ensuring post-closure safety for the protection of humans and the environment from ionising radiation without undue burdens for future generations.

• Ensuring safety and feasibility in connection with construction and operation; this includes occupational health and safety. Additional topics include security and safety in the case of incidents or accidents in terms of their impact on humans and the environment. Feasibility is defined as achieving reliable implementation of all required work steps with reasonable effort.

• Considering resources and economic efficiency, i.e., realising the project cost-efficiently and within an appropriate period.

While all major infrastructure projects must observe the latter three overarching optimisation goals and principles, the first refers specifically to deep geological disposal. This is highlighted in both ENSI Guideline G03 (ENSI 2023) and international standards as e.g., the IAEA Safety Standard SSR‑5 (IAEA 2011a).

Optimisation is carried out regarding all optimisation goals and principles. When evaluating potential variants and options, their proportionality in terms of time and costs, as well as the opportunities and risks that each of these present, are thoroughly considered. This includes an evaluation of the options available for increasing system robustness as well as future developments. Conflicting requirements are balanced. In individual cases, it may be necessary to determine which solution is the most advantageous overall with regard to safety (ENSI 2023).

In future stages of the disposal programme, the main drivers of the RD&D programme will be the need for optimisation, which must be carried out from the perspective of safety (the updating of the safety cases at each milestone), feasibility of implementation (confidence underpinning the feasibility of implementing the design at each milestone) and resources (assessing cost efficiency up to completion). In anticipation of the construction and operating licences, technological developments are also foreseen. These technological developments will affect all barrier components as well as their implementation.

The approval of future licences will necessitate considering those safety aspects pertinent to the corresponding licence under application. This will in turn determine how a safety case is further refined and presented.

For the general licence application, the repository and the barriers have been developed to the level of a conceptual design. However, forthcoming licences will demand progressively more detailed technical specifications. Along with a steadily improving understanding of the geological environment, e.g., due to construction activities, and along with technological progress and confirmation of engineering solutions, e.g., through testing facilities, the repository project will become increasingly specific. In parallel, options for optimisation will be reduced. This evolution will be reflected in the increasing level of detail at which arguments are discussed in the safety case.

Moreover, future safety cases will be expanded beyond post-closure safety, to include, in particular, the case for operational safety. This is necessary to meet the needs of IAEA SSR-5 (IAEA 2011a), which states that a safety case needs to consider both the operational and post-closure phases. It is necessary to ensure that actions during operation will not be detrimental to post-closure safety. Furthermore, operational aspects will be important for future design optimisation.