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).