Loviisa and Olkiluoto LILW safety cases
The low and intermediate level radioactive waste (LILW) arising from the operation and decommissioning of the Loviisa and Olkiluoto Nuclear Power Plants (NPPs) will be disposed of in on-site repositories located at Hästholmen and Olkiluoto islands. The safety of the disposal is based on high-level principles of containment and isolation. The safety of the disposal after the closure of the repository is addressed in a safety case. The safety cases considered the evolution of the surface environment and the repository performance from their initial state at the closure up to hundred thousand (Loviisa) or one million years (Olkiluoto).
Case description
A safety case is a report portfolio demonstrating compliance with regulatory requirements regarding the long-term safety of the disposal. The safety case for Loviisa LILW repository 2018 considers the operational and decommissioning wastes arising from Loviisa nuclear power plant, consisting of two VVER-440 reactor units. The waste is disposed in an on-site repository at the Hästholmen island at the depth of about 110 m. The safety case for Olkiluoto LILW repository 2023 considers the wastes from Olkiluoto 1&2 BWR reactors and Olkiluoto 3 EPR. The waste is disposed in an on-site repository and its future extension for the decommissioning wastes at the Ulkopää Cape in the western part of the Olkiluoto Island at the depth of about 60 - 100 m.
Loviisa LILW repository
The safety case for Loviisa LILW repository 2018 consists of four primary and five supporting reports. The safety case was compiled in connection to periodic safety review for Loviisa LILW repository carried out in 2013 and decommissioning plan update in 2018. The safety case was founded on earlier safety cases compiled for Loviisa LILW repository, but the safety case methodology was updated to meet the revised regulatory requirements.
The assessment period was selected to be 100,000 years justified by the radioactivity of the waste. Based on the alternative evolutions of the disposal system, altogether four scenarios were formulated: base scenario, initial defect in welds, accelerated concrete degradation and large earthquake. The radionuclide releases and radiation doses were analysed in each scenario. In addition, the base scenario included calculation cases that studied the impacts of alternative conceptual assumptions or parameter selections.
The results in all scenarios indicate meeting the regulatory criteria set for the long-term safety. The results didn't differ dramatically between the scenarios, indicating the robustness of the repository system against perturbations. This was demonstrated in a what-if case that studied immediate and complete failure of containment provided by the reactor pressure vessel.
Olkiluoto LILW repository
Olkiluoto LILW safety case consists of four primary and two supporting reports. The safety case was compiled in connection to the periodic safety review for TVO’s existing LILW repository carried out in 2021. The starting point for the long-term safety considerations was the initial state of the disposal system at the time of the closure. The disposal system is conceptually divided into components (surface environment, bedrock, closure, waste caverns, concrete silo/basin, steel containers, concrete containers and low and intermediate level waste). The main features of the disposal system contributing to the long-term safety of the disposal were identified as part of the design basis. These features were developed into safety functions for those components of the disposal system that are specifically designed for the disposal, i.e. closure, waste caverns, concrete silo/basin, steel containers, concrete containers, and (solidified) intermediate level waste. The performance targets for these components were derived from the safety functions and the reference evolutions from the performance assessment.
The safety case considered the evolution of the repository and surface environment from their initial state at the closure up to one million years. The identified key uncertainties in the repository performance were systematically combined into six scenarios. The radiological impacts within these scenarios were studied in the radionuclide release, transport and dose modelling both using deterministic and probabilistic methods. Both the deterministic results and the probabilistic results were used in comparison with the regulatory constraints to build confidence on the safety of the disposal.
End results
Safety cases are a routinely updated and legally required documents for disposal facilities (repositories) in Finland. Therefore, the main working steps for the safety cases were relatively clear for Fortum even in the initial steps of the Loviisa safety case. Throughout the work, however, many possible points of improvement were recognized and new models were developed specifically for assessing the long-term safety of the disposal keeping in mind the extremely long timescales. These same models could be utilized in Olkiluoto safety case with small modifications to match certain characteristic differences in the wastes, repository designs and disposal sites.
The remaining uncertainties identified in the safety case provide a starting point for the future research activities to increase the confidence of the disposal, if considered necessary and reasonable. These are to be coordinated together with decommissioning planning and other waste management activities.
Otso Manninen
Director, Decommissioning and Disposal Services
+358 40 767 4168otso.manninen