A review of the treatment of criticality in post-closure safety assessment of radioactive waste disposal

Any safety case for a radioactive waste disposal facility in England and Wales will include consideration of the potential for post-closure nuclear criticality if the waste includes fissile material. In order to fulfil its responsibility to assess such a safety case, the Environment Agency will need to be able to understand and assess the treatment of post-closure criticality. The project reported here forms the basis for the Environment Agency's development of a capability to analyse post-closure criticality issues. The report will also help the Environment Agency, in consultation with other regulatory organizations, to provide an effective input to the process of evaluating proposed fissile contents of waste packages. The report presents a review of key post-closure criticality studies undertaken in radioactive waste disposal programmes around the world (including the Waste Isolation Pilot Plant and Yucca Mountain projects in the US, the Konrad repository in Germany, and proposed deep geologic repositories in the UK, Canada, and Sweden), and also examines the treatment of criticality safety in several waste handling and storage facilities. In general, these studies emphasise the need for post-closure criticality to be considered as part of a repository safety case, although most post-closure criticality studies suggest that criticality is a low-probability event with low consequences in terms of overall repository safety. The review has led to the identification of five post-closure criticality scenarios that are typical of the scenarios studied in post-closure criticality assessments, although the specification of scenarios is not unique. The scenarios relate to the concentration of fissile radionuclides under different conditions at various locations in a disposal system: criticality in an intact waste container; criticality in a leaking waste container; criticality in one or more collapsed waste containers; criticality in the near-field; and criticality in the far-field