In 2011 the Swedish Nuclear Fuel and Waste Management Co (SKB) submitted a license application for construction of a geological repository for spent nuclear fuel according to the KBS-3 method, comprising of copper canisters, bentonite buffer, backfill and surrounding crystalline bedrock. The post-closure safety assessment of the repository, SR-Site, has been reviewed by the Swedish Radiation Safety Authority (SSM) for five years. During the review SSM commissioned the Southwest Research Institute (SwRI) to develop a model independent of SKB’s model to simulate the canister failure due to isostatic load. SwRI built up the model based on their understanding of SKB’s model description. From SwRI’s modelling it could be confirmed that SKB’s modelling results were relatively consistent with the description SKB provided of the performance assessment model, of the model assumptions, and of the model inputs for the isostatic load scenario.
The objective of this study is to investigate the consequences of extreme scenarios (here called bounding cases), assuming that all of the deposited canisters fail during different time spans after closure of the repository (e.g. from 1 to 300, 1 to 1 000, 1 to 6 000 and 1 to 100 000 years) using the earlier developed isostatic load model. The investigation is both an input to further research (see Need for further research below) and it can support SSM’s evaluation of regulatory fulfilment.
With the isostatic load model SwRI calculated the average total far-field radionuclide releases. The bounding case calculations for extreme scenarios resulted in doses less than the typical background radiation in Sweden (approximately 1 mSv/yr). The results can be used by simply scaling the results of the bounding case calculations to fit an estimated number of canisters from a scenario of interest.