2. Contextual and regulatory framework (NTB 24-10)

Radioactive waste needs to be managed (see e.g., SSR‑5 IAEA 2011a). It can, in principle, be safely stored in surface facilities. However, because these facilities need maintenance and care, their safety is inevitably reliant on continued societal stability. Societal stability may, for example, be linked to political or financial stability, uncertainties which are far greater than those associated with the evolution of conditions deep underground in geological formations that are suitable to host a repository.

As a long-term waste management option, deep geological disposal has the positive attribute that, if the site and design are chosen appropriately, neither the societal stability needed for govern­mental and regulatory control, nor any specific controls on activities occurring at the surface of the Earth are prerequisites for post-closure safety. In fact, geological disposal provides long-term passive safety. Passive safety means that safety does not require any active measure to be under­taken following closure of the repository. Thus, no burden is placed on future generations to maintain and control the repository once it has been closed, although such control is certainly possible if society decides in favour of it.

Geological disposal contains the waste for a prolonged period while its associated radioactivity decays naturally. It is designed to prevent radioactivity from reaching the surface environment in amounts that could cause harm to humans or the environment. Placing the waste in a deep rock formation also enhances security in that it reduces the possibility of sabotage or theft.

The proof of existence of suitable rock formations, together with the findings of safety assess­ments conducted for a wide range of sites and designs world-wide (see, e.g., OECD/NEA 2017), lend support to the feasibility of safe geological disposal.

Comprehensive international principles and guidelines for dealing with radioactive substances and for deep geological disposal establish a robust framework for the Swiss waste management programme and are presented in detail in NAB 24-18 Rev. 1 (Nagra 2024s). In the following, the most relevant aspects are summarised.

The International Commission on Radiological Protection (ICRP) has contributed significantly to the framework with its system of radiation protection, updated in ICRP Publication 103 (ICRP 2007b). This system is built on the principle of dose limitation. Furthermore, ICRP Publication 81 (ICRP 1998) emphasises that future generations should receive at least the same level of protection from radioactive waste as the current generation. This principle involves applying current quantitative dose and risk criteria to estimated future doses or risks for appropriately defined critical groups. 

At a high level, safety principles and provisions are laid down in the International Atomic Energy Agency (IAEA) Joint Convention on the Safety of Spent Fuel and on the Safety of Radioactive Waste Management (IAEA 1997), to which Switzerland is a signatory. Safety standards and safety guides establish more detailed guidelines and requirements: IAEA Safety Standard SF-1 (IAEA 2006) underlines the fundamental safety objective to protect humans and the environment from harmful effects of ionising radiation. This standard articulates ten associated safety principles that provide the foundation for requirements and measures for the protection of humans and the environment against radiation risks and for the safety of facilities and activities generating those risks. More specific safety guides are concerned with the safe development of deep geological repositories, referred to here as geological disposal facilities (SSG-14; IAEA 2011b) and with the safety case and safety assessments (SSG-23; IAEA 2012). Furthermore, specific safety requirements are given in SSR‑5 (IAEA 2011a).

Internationally recognised principles that underlie the disposal concepts and designs for deep geological disposal include passive safety and a stepwise and iterative development of the repository for the optimisation of protection. While many detrimental phenomena and uncertainties can be avoided, reduced, or their effects mitigated by optimisation, some uncertainty will inevitably remain. It is by following the principle of robustness that geological disposal systems are sited and designed in such a way that post-closure safety can be demonstrated irrespective of the remaining uncertainty. The Nuclear Energy Agency (NEA) of the Organisation for Economic Cooperation and Development (OECD) has stated that “robust systems are characterised by a lack of complex, poorly understood or difficult to characterise features and phenomena, demonstrated quality control, and an absence of, or relative insensitivity to, detrimental phenomena arising either internally within the repository and host rock, or externally in the form of geological and climatic phenomena that introduce processes with the potential to compromise safety” (OECD/NEA 2013).

Regarding the design of a disposal facility, IAEA SSR-5 (IAEA 2011a) states that “the disposal facility and its engineered barriers shall be designed to contain the waste with its associated hazard, to be physically and chemically compatible with the host geological formation and/or surface environment, and to provide safety features after closure that complement those features afforded by the host environment”. Safety is provided by means of multiple safety functions, which provide defence-in-depth, according to IAEA SSR-5 (IAEA 2011a):

• “the host environment shall be selected, the engineered barriers of the disposal facility shall be designed and the facility shall be operated to ensure that safety is provided by means of multiple safety functions”,

• “a safety function may be provided by means of a physical or chemical property or process that contributes to containment and isolation, such as: impermeability to water; limited corrosion, dissolution, leach rate and solubility; retention of radionuclides; and retardation of radionuclide migration” and

• “adequate defence-in-depth has to be ensured by demonstrating that there are multiple safety functions, that the fulfilment of individual safety functions is robust and that the performance of the various physical components of the disposal system and the safety functions they fulfil can be relied upon, as assumed in the safety case and supporting safety assessment”.

Thus, the safety case is built on multiple types of evidence and lines of argument for the performance of individual barriers, including their contributions to the safety functions, and for the performance and robustness of the disposal system as a whole.

Besides following international principles and guidance (see Section ‎2.2 above), Switzerland’s waste management programme benefits from the experience gained from work for deep geological repositories abroad. Of particular interest are more advanced programmes that also work towards disposal in clay rock. France, for example, recently applied for the construction licence for a repository in Callovo-Oxfordian clay rock. Canada is currently investigating two siting regions and host rocks, one of which is an argillaceous limestone in southern Ontario and the other a crystalline batholith in northwestern Ontario. Finland and Sweden are the most advanced with respect to the implementation of deep geological repositories, which are hosted in crystalline rock. Relevant work relating to post-closure safety assessment carried out by these programmes is listed in Tab. ‎2‑1.

Tab. ‎2‑1:Some recent safety cases for deep geological disposal, their main objectives in terms of the project milestones they support and main aspects with relevance to the present safety case

Requirements for the documentation of the general licence application and for the final stage of the site selection process are set out in ENSI 33/649 (ENSI 2018). Chapter 5 of that document states that, for the safety case supporting the general licence application, the safety analysis carried out for the proposed site in the site comparison is to be supplemented by a comprehensive scenario analysis and analysis of radiological consequences. Excavation of the repository by erosive processes is specifically mentioned as a scenario to be evaluated.

The protection objective and the criteria by which the post-closure safety of a deep geological repository is to be evaluated are those set out in ENSI Guideline G03 (ENSI 2023). The objective of deep geological disposal is to ensure long-term protection of humans and the environment by means of a system of staged, passively functioning engineered and geological barriers, termed the multi-barrier system. It is acknowledged that absolute containment of all radioactive substances over very long periods of time is impossible, and the multi-barrier system has therefore to be designed in such a way that the release of radionuclides through the engineered and geological barriers to the biosphere remains so low that the protection of humans and the environment is ensured. Specific elements to be addressed in the safety assessment, according to ENSI guideline G03, are presented in Tab. ‎2‑2, along with the sections in this report where each element is covered. Further requirements for safety assessment and the safety case are also set out in the guideline, and their implementation in the present safety assessment methodology is described in NTB 24‑19 (Nagra 2024t).

Tab. ‎2‑2:Elements to be addressed in safety assessment, according to (ENSI 2018) and sections in the present report and key supporting reports where these are covered

Quantitative protection criteria in the form of individual effective dose and risk limits are specified in the ENSI guideline G03 (ENSI 2023). These are as follows.

• Any future evolution of a deep geological repository must not lead to the release of radionuclides causing an individual dose exceeding 0.1 mSv per year, or not cause the risk value according to criterion (b) in paragraph 2.15 of IAEA Safety Standard SSR-5 (IAEA 2011a) to be exceeded⁴.

• The IAEA Safety Standard SSR-5 gives a risk constraint of the order of 10^–5 per year, where risk is understood as the probability of fatal cancer or serious hereditary effects.

• During the assessment period, the radiological consequences of inadvertent human intrusion into the deep geological repository have to be assessed on the basis of criteria (c) and (e) as set out in paragraph 2.15 of IAEA Safety Standard SSR-5⁵.

• Here, SSR-5 states that, if intrusion leads to a calculated annual dose of less than 1 mSv to those living around the site, “ ... then efforts to reduce the probability of intrusion or to limit its consequences are not warranted” whereas, if the calculated annual dose is in the range 1 – 20 mSv, “... then reasonable efforts are warranted at the stage of development of the facility to reduce the probability of intrusion or to limit its consequences by means of optimisation of the facility’s design”.

• After the end of the time period for assessment (see Section ‎4.3), the effects on the surface must not be significantly higher than the average current radiation exposure of the Swiss population⁶.

Compliance with the protection criteria for the post-closure phase has to be shown in a safety case. Protection criteria for the operational phase are also set out in ENSI Guideline G03 (ENSI 2023). These are addressed in the separate report NAB 24‑02 (Nagra 2024f).