Preparing Posiva's Post-Closure Safety Case Towards the Operational Phase

Posiva Oy is currently preparing a safety case to support the operating licence application (OLA) for the spent nuclear fuel disposal facility under construction at the Olkiluoto site in south-western Finland. The methodology to prepare the safety case documentation will consider the latest updates in the regulations; lessons learned from Posiva's previous safety case, TURVA-2012, submitted in the context of the construction licence application (CLA) in 2012; the feedback received from the Radiation and Nuclear Safety Authority (STUK) on the CLA, including several specific requirements for the next safety case; and new challenges related to the implementation of repository construction and operation. This calls for a higher level of maturity in both the safety case itself and in the design on which the safety case is based. Since the safety case work will inevitably take several years, it is necessary to introduce requirements, design and data freezes at the beginning of the safety case production process. The design freeze is based on the information and requirements available at the start of the safety case work, but updates can be expected as the design matures and is optimized for industrialization and operation. A change management process is set up to facilitate the assessment of the impacts of the proposed changes on the safety case results. The input data used in the safety assessment and their possible updates will be managed by using of a central database. The uncertainties in the initial state of the components of the disposal facility will be tackled by implementing an analysis of potential deviations in these components at the time of installation. Deviations are then screened and implemented in scenario formulation. Defining a range of initial state parameter values and deviations for the installed components introduces some flexibility in design and increases the robustness of the safety case. (author)