Technischer Bericht NTB 10-01
Beurteilung der geologischen Unterlagen für die provisorischen Sicherheitsanalysen in SGT Etappe 2 - Klärung der Notwendigkeit ergänzender geologischer Untersuchungen
The procedure for selecting repository sites for all categories of radioactive waste in Switzerland is defined in the conceptual part of the Sectoral Plan for Deep Geological Repositories (SFOE 2008). The Plan foresees a selection of sites in three stages, the third of which leads to a General Licence Application procedure that defines both the sites and the main features of the repositories. In Stage 1, Nagra proposed geological siting regions based on criteria relating to safety and engineering feasibility. These siting proposals were subsequently evaluated and approved by the responsible federal authorities in their statements. The Swiss Government (the Federal Council) is expected to decide on the siting proposals in 2011 based on these statements. After submittal of the siting proposals for Stage 1, Nagra has started preparations for Stage 2. Working together with the siting regions and the affected Cantons in the context of a participatory process, the objective of Stage 2 is to prepare proposals for the location of the surface facilities within the planning perimeters defined by the Federal Council in its decision on Stage 1 and to identify potential sites. Nagra also has to carry out a provisional safety analysis for each site and a safety-based comparison of the sites. Based on this, and taking into account the results of the socio-economic-ecological impact studies, Nagra then has to propose at least two sites for each repository type to be carried through to Stage 3. The proposed sites will then be investigated in more detail in Stage 3 to ensure that the selection of the sites for the General Licence Applications is well founded.
In order to realise the objectives of the upcoming Stage 2, the state of knowledge of the geological conditions at the sites has to be sufficient to perform the provisional safety analyses. Therefore, in preparation for Stage 2, the conceptual part of the Sectoral Plan requires Nagra to clarify with ENSI, at an early stage, the need for additional investigations aimed at providing input for the provisional safety analyses. The purpose of the present report is to document Nagra's technical-scientific assessment of this need so that ENSI can subsequently evaluate Nagra's assessment. The focus is on evaluating the geological information based on processes and parameters that are relevant for safety and engineering feasibility.
In evaluating the state of knowledge the key question is whether additional information (e.g. from future investigations) could lead to a different decision regarding the selection of the sites to be carried through to Stage 3. In order to set priorities based on safety, as required for Stage 2, it is necessary to use the characteristic dose intervals determined with the help of dose calculations, an evaluation of engineering feasibility and the results of a qualitative assessment. Accordingly, the state of knowledge is evaluated using test calculations to determine dose intervals, an assessment of the engineering feasibility and an evaluation of the qualitative assessment. The test calculations are used to derive dose curves for a wide spectrum of calculation cases for the different repository types in the geological siting regions under consideration. Besides the expected evolution (reference case), the calculations also cover existing uncertainties in the relevant processes and parameters. This allows the characteristic dose intervals required by the Sectoral Plan to be determined for the different siting regions for each repository type; these dose intervals are evaluated with respect to their unambiguousness concerning two aspects required by the Sectoral Plan: safety-related suitability and safety-related equivalence of the siting regions. For the purpose of evaluating safety, the state of knowledge is considered to be sufficient if clear, unambiguous statements can be made despite the fact that the parameter ranges are selected generously to allow for existing uncertainties and if these statements will not change if the uncertainties and associated parameter ranges are reduced through future investigations. Whether the information used to assess engineering feasibility is sufficient is also considered, along with an evaluation of the significance of uncertainties in the relevant processes and parameters. The analysis of the state of knowledge shows that, taking into account the investigations performed by Nagra since 2008 and those planned for the near future, the available information is sufficient for the provisional safety analyses and the safety-based comparison of the siting regions. For all siting regions, concrete statements can be made regarding suitability and equivalence of sites from the point of view of safety despite the deliberate selection of wide parameter ranges to take into account uncertainties; engineering feasibility has also been demonstrated. This means that, besides the work already completed or planned by Nagra, no additional investigations will be necessary for the provisional safety analyses to be performed in Stage 2 of the Sectoral Plan.
The report also describes the work already initiated and planned by Nagra for Stage 2 of the Sectoral Plan. It is envisaged that this work will contribute to reducing the uncertainties mentioned in this report, thereby at least partially reducing the corresponding parameter bandwidths. This work relates to the geometry (including structures) of the host rocks and the effective containment zones and to information on deposits of raw materials and on state parameters in the different siting regions, the properties of the host rocks and the effective containment zones (including sorption measurements), hydrogeological conditions and long-term evolution. The work carried out since submission of the documentation for Stage 1 of the Sectoral Plan and the initiated and planned activities comprise supplementary field investigations (participation in investigations in new boreholes drilled by third parties, seismics, mapping), analyses, laboratory programmes (including investigations on cores from new boreholes) and other studies. In addition to these activities that are focused on geology, investigations will be carried out on a wide range of topics to provide input for the provisional safety analyses, the evaluation of engineering feasibility and the safety-based comparison. In particular, this includes studies on repository layout and gas formation and release.
The state of knowledge will be reviewed once again by the authorities in Stage 2 when Nagra submits the provisional safety analyses and the safety-based comparison of the sites. The review by the authorities will contribute to the decision on the sites for which field investigations will be conducted in Stage 3 with the aim of obtaining more detailed information for identifying the sites for the General Licence Applications for the L/ILW and HLW repositories. If necessary, field work can be carried out at more than two sites for each repository type in Stage 3; this is compatible with the requirements set out in the conceptual part of the Sectoral Plan.