Posiva’s safety concept for the geologic disposal of spent nuclear fuel is based on:
- the KBS-3 design of the geologic repository and
- the characteristics of the Olkiluoto site.
Posiva’s safety concept aims at making the best possible use of these two elements to fulfil the long-term safety principles. Posiva’s safety concept for the spent nuclear fuel repository is depicted in Figure 1-3. According to this concept, safety depends first and foremost on the long-term isolation and containment of radionuclides within the copper-iron canisters. A clay buffer protects the canisters from rock movements and potential detrimental substances, limits groundwater flow around the canisters and limits and retards radionuclide releases in the event of canister failure. Long-term containment within the canisters, in turn, depends primarily on the proven technical quality of the engineered barrier system ( ) and favourable near-field conditions for the canisters. The technical quality of the EBS is favoured by the use of components with well-characterised material properties and by the development of appropriate acceptance specifications and design criteria. Favourable and predictable bedrock and groundwater conditions are requirements for the natural barrier, i.e. the host rock.
The characterisation of the Olkiluoto site and the strategy for repository design initially focused on a volume of bedrock situated between 400 and 700 metres below the ground surface, where favourable and predictable bedrock and groundwater conditions are expected to be found. Therefore, the safety concept automatically includes the depth requirement from YVL D.5 (STUK 2014a):
“The disposal depth shall be selected giving priority to long-term safety, taking into account the geological structures of the bedrock as well as the changes occurring in hydraulic conductivity, groundwater chemistry and mechanical stability of the rock following the increase in depth. The repository for spent nuclear fuel shall be located at the depth of several hundreds of meters so as to adequately mitigate the impacts of above-ground natural phenomena, such as glaciation, and human actions” (YVL D.5, paragraph 414).
Besides providing a protective environment for the canisters, the siting and design of the disposal system ensure that the transport of radionuclides released from an initially defective or subsequently breached canister will be effectively retained and retarded by the other barriers. These are provided by the secondary set of safety pillars (shown in yellow in Figure 1-3, which comprises slow release from the spent fuel matrix, slow diffusive transport in the buffer, and slow transport in the geosphere.
The safety concept relies on a robust system design. A robust system design is a design that includes only components whose behaviour is relatively well understood and predictable. Furthermore, the performance of the system is relatively insensitive to possible imperfections in its implementation and to the unavoidable residual uncertainties in the understanding of its future evolution.
Figure 1-3. Outline of the safety concept for a KBS-3 type repository for spent nuclear fuel in a crystalline bedrock (adapted from Posiva 2003). Pillars and blocks shown in dark cyan colour indicate the primary safety features and properties of the disposal system. Yellow pillars and blocks indicate the secondary safety features that become important in the event of a radionuclide release from a canister.