Application of a disposal facility post-closure criticality model to Oklo zone 2 - 8-09

For post-closure safety the Radioactive Waste Management Directorate (RWMD) has undertaken research to understand the features and processes that ensure a low likelihood of criticality, and 'what-if' postclosure criticality scenarios for a geological disposal facility (GDF). A 'what-if' scenario for GDF criticality is the mobilisation of fissile material from a number of waste packages and its subsequent localised accumulation. Should sufficient fissile material accumulate, a quasi-steady-state (QSS) criticality could result, where an increase in reactivity from continued accumulation is balanced by a temperature increase. The QSS model has been developed to simulate such a criticality. A postulated QSS criticality could continue for many millennia so building confidence in the model is difficult. However, the Oklo reactors operated for similar durations around two billion years ago, so a study has been undertaken to compare the available information with a QSS model simulation of this natural analogue. The paper describes the QSS model approach and its application to an Oklo reactor. The results are found to be in agreement with observational evidence for burn-up, temperature and duration, building confidence in applying the QSS model to the 'what-if' scenario for GDF criticality. (authors)