The Likelihood of Criticality Following Disposal of SF/HLW/HEU/Pu

The Radioactive Waste Management Directorate (RWMD) is responsible for implementing geological disposal of the UK’s higher-activity radioactive wastes. RWMD’s research into geological disposal considers safety during waste transport to a disposal facility, during waste disposal operations, and once the facility has been closed. The wastes for disposal comprise a wide range of materials and include some fissile radionuclides. The presence of fissile materials requires an assessment of the potential for criticality. After disposal, the engineered barrier system of a geological disposal facility (GDF) will ensure that criticality is prevented for such time as the waste packaging affords a high level of containment. However, as waste packages begin to degrade, fissile and other materials may be mobilised and this could affect the potential for criticality. Therefore, the possibility for the evolution of conditions in a GDF to lead to criticality requires consideration. RWMD established the Likelihood of Criticality research project to develop, document and communicate the qualitative and quantitative arguments required to evaluate the probability of nuclear criticality after closure of a GDF. The evaluation of the probability of criticality has been underpinned by consideration of the features, events and processes (FEPs) that could affect nuclear reactivity after GDF closure. The FEP analysis led to the construction of postclosure criticality scenarios and these scenarios have been evaluated using a range of approaches, from high-level judgments about scenario credibility, to modelling of evolving conditions in a GDF to determine if critical systems could develop. Understanding the radioactive waste inventories, the GDF concepts for the different waste types and geological settings, and the expected evolution of conditions in the different GDF concepts, as well as associated uncertainties, has been of fundamental importance to the criticality FEP and scenario analysis.