Technischer Bericht NTB 08-05

According to the conceptual part of the Sectoral Plan for Deep Geological Repositories (BFE 2008), the first step in the site selection process requires the waste producers to submit proposals for geological siting regions for repositories for low- and intermediate-level waste (L/ILW) and high-level waste (HLW). The proposals prepared by Nagra on behalf of the waste producers for stage 1 of the Sectoral Plan procedure are justified and documented in Nagra (2008b).

The Sectoral Plan states that the siting proposals have to be prepared in five steps. In the first step, the waste inventory, including reserves for future developments in the nuclear power programme, is allocated to the L/ILW and HLW repositories. Based on this allocation, the barrier and safety concepts for the two repositories are defined in the second step. Quantitative and qualitative requirements on the geology are then derived with a view to evaluating the geological siting possibilities. This applies to the time period under consideration, the space required by the repository, the properties of the host rock (depth, thickness, lateral extent, hydraulic conductivity), the long-term stability of the geological situation, the reliability of geological information and engineering suitability.

Steps 3 to 5 cover the evaluation of the geological siting possibilities. In the third step, the large-scale geological-tectonic situation is assessed and large-scale areas for further consideration are identified. The fourth step involves selecting the preferred host rock formations within these large-scale areas. In the fifth step the rock configurations, i.e. the spatial arrangement of the preferred host rocks within the large-scale areas under consideration, are evaluated and geological siting regions are identified.

This report complements Nagra’s report documenting the proposals for geological siting regions (Nagra 2008b). It explains the allocation of the waste to the L/ILW and HLW repositories, defines the barrier and safety concepts and justifies the repository-specific requirements on the geology (steps 1 and 2). These requirements are used in the stepwise narrowing-down procedure to select the geological siting regions. The report does not take the form of a conventional safety analysis in that, in particular, it does not contain a safety assessment of concrete geological repositories at specific sites in specific host rocks. Instead, the fundamental requirements are derived using guiding safety considerations and experience. Site-specific safety analyses will be part of the subsequent stages 2 and 3 of the Sectoral Plan process.

The key results are presented in the following.

Waste allocation 

According to the Federal Office of Energy (BFE 2008), the waste properties to be considered for waste allocation, repository design and identification of geological siting regions are inventory, half-lives and the activity and radiotoxicity of the safety-relevant radionuclides and their evolution with time. Waste volumes, material properties and their potential influences on the host rock, heat production, content of potentially gas-producing components (metals, organics) and content of complexants also have to be considered. The description of the waste properties forms the starting-point for the waste allocation. The waste is divided into the categories high-level waste (HLW), alpha-toxic waste (ATW) and low- and intermediate-level waste (L/ILW), as specified in the Nuclear Energy Ordinance (2004). The description shows clearly that, with respect to all properties, HLW differs significantly from ATW and L/ILW. For this reason, as reflected in disposal concepts to date, HLW is disposed of in a separate repository with a specifically designed barrier system.

The ATW and L/ILW differ in terms of specific radiotoxicity, specific activity and specific heat production, in terms of both absolute values and evolution with time. However, many of their other properties are very similar, particularly the material inventory. 

In principle, a combined repository for all ATW and L/ILW would be conceivable. Experience shows that such a facility constructed in a suitable host rock in a favourable geological setting has the potential to fulfil the safety requirements specified by the authorities. On the other hand, experience also shows that calculated doses are dominated by just a few of the ATW and L/ILW waste types. If these dominant waste types could be disposed of elsewhere, the requirements on the geology could be reduced while the level of safety would remain the same; this has the effect of increasing the possibility of finding suitable siting regions. For these reasons, the existing concept (a HLW repository with a facility for long-lived intermediate-level waste (ILW) and a L/ILW repository) has been maintained, with the aim of allocating the dosedominating ATW and L/ILW to the ILW facility. This allows to reduce the safety-related requirements on the geology for the L/ILW repository. Based mainly on generic dose calculations, the ATW and L/ILW are allocated to the two repository types as they differ only slightly with respect to material inventory and gas generation rates and both the HLW repository (including the ILW facility) and the L/ILW repository are designed in such a way that repository-induced influences (originating from the waste) do not significantly affect longterm safety.

The proposal made by Nagra includes two variants, characterised by minimum requirements on the large-scale hydraulic conductivity of the host rock for the L/ILW repository of 10^-10 m/s and 10^-9 m/s respectively. As expected, the volume of waste allocated to the L/ILW repository is somewhat smaller for the 10^-9 m/s variant than for the 10^-10 m/s variant. Based on an assessment of the geological possibilities within Switzerland, which shows that there are sufficient suitable host rocks and effective containment rock zones with a large-scale hydraulic conductivity of 10^-10 m/s or better, this allocation variant is termed the reference allocation and the variant with 10^-9 m/s is termed the alternative allocation. In both cases, all the ATW is allocated to the HLW repository (ILW facility). For the reference allocation, somewhat less than 1% of the volume of L/ILW is also allocated to the HLW repository (ILW facility); in the case of the alternative allocation it is somewhat less than 10%.

Barrier and safety concept

The barrier concept describes the functions of the different engineered and geological barriers of the deep repository, based on a system of staged, passive safety barriers consisting of the waste matrix, disposal container, backfilling of the underground disposal chambers, backfilling and sealing of the underground structures, the host rock and any confining rock units and the overall geological situation.

The safety concept shows how the different engineered and geological barriers contribute to system safety and what safety functions they perform. The safety functions provide the physical separation of the waste from the human environment and ensure the required long-term stability, containment of the radionuclides, delayed release of nuclides and nuclide retention in the near-field and the geosphere, thus ensuring low release rates.

In the selected safety concept, both the engineered and the geological barriers (host rock, any confining units and their geological situation) contribute significantly to the barrier function of the overall system. In line with the requirements set out by the authorities, a system in which the containment and retention of radionuclides rely on the engineered barriers alone will not come into consideration.

The concept also describes the contribution to safety of the different components of the barrier system. For both repository types, by far the largest proportion of radiotoxicity will decay within the engineered barriers due to immobilisation of the radionuclides and radioactive decay. There is a further decrease due to decay during transport through the host rock, meaning that the proportion of radionuclides leaving the engineered and natural barriers represents only a tiny fraction of the original radiotoxicity. This means that the resulting doses are well below the protection objective specified by the regulatory authority.

Requirements on the geology

Specifying the requirements on the geology is carried out in two steps. In the first step, indicators are defined that adequately encompass the criteria set out in the Sectoral Plan and are used to identify geological siting regions. In the second step, the requirements and evaluation scales for the indicators are defined.

The starting-point for defining the indicators are the safety functions discussed above together with a set of overarching principles relating to reliable implementation of the geological repositories and the reliability of geological information. A set of indicators with associated requirements and evaluation scales are then defined, the application of which in the narrowing down process results in geological siting regions where repositories can be constructed that fulfil the safety functions and principles and ensure sufficient safety.

The requirements and evaluation scales for the indicators are defined using radionuclide release calculations, model calculations of the behaviour of individual barriers or properties, measured and empirical values and qualitative information.

Based on generic safety considerations and experience derived from earlier system and safety analyses, the following features are considered to be particularly important for the site evaluation:

• For identifying suitable large-scale geological-tectonic areas (step 3), the main emphasis is on the long-term stability of the geological situation (geodynamics and neotectonics, uplift and erosion) and the typical spatial conditions and their explorability (regional fault patterns and bedding conditions).

• For identifying potentially suitable host rocks and effective containment zones (step 4), the rock properties (particularly their stability (potential for karstification), the hydraulic conductivity and – for sediments – their self-sealing capacity), taking into account tectonic overprinting and the potential for a suitable geometry of the rock formations (thickness, minimum and maximum depth, lateral extent), as well as suitable geotechnical properties are decisive.

• For identifying suitable configurations (step 5), the focus is on the spatial geological conditions. These include thickness at suitable depth (minimum depth with respect to surface erosion and vertical glacial erosion and rock decompaction; maximum depth in terms of engineering requirements) and lateral extent (taking into account regional geological features), as well as the local geological-tectonic situation.

These features are integrated into the narrowing-down procedure; they are taken into account in the form of minimum requirements and stricter requirements and are used for the evaluation of the siting regions.