In support of the disposal system safety case for a geological disposal facility (GDF) there is a requirement to consider 'what-if' hypothetical scenarios for post-closure nuclear criticality. Although all such scenarios are considered very unlikely, one 'what-if' scenario is the mobilization of fissile material from a number of waste packages and its slow accumulation within the GDF or the immediate surroundings. Should sufficient fissile material accumulate a quasi-steady-state (QSS) transient criticality event could result. A computer model has been developed to understand the evolution and consequences of such an event.Since a postulated QSS criticality could persist for many millennia, building confidence in the modelling approach is difficult. However, the Oklo natural reactors in Africa operated for similar durations around two billion years ago, providing a natural analogue for comparison. This paper describes the modelling approach, its application to hypothetical criticality events for a GDF, and how the model can be compared to Oklo. The model results are found to be in agreement with the observational evidence from Oklo, building confidence in the use of the QSS model to simulate postulated post-closure criticality events in GDFs.

In support of the Radioactive Waste Management (RWM) safety case for a geological disposal facility (GDF) in the UK, there is a regulatory requirement to consider the likelihood and consequences of nuclear criticality. Waste packages are designed to ensure that criticality is not possible during the transport and operational phases of a GDF and for a significant period post-closure. However, over longer post-closure timescales, conditions in the GDF will evolve.For waste packages containing spent fuel, it can be shown that, under certain conditions, package flooding could result in a type of criticality event referred to as 'quasi-steady-state' (QSS). Although unlikely, this defines a 'what-if' scenario for understanding the potential consequences of post-closure criticality. This paper provides an overview of a methodology to understand QSS criticality and its application to a spent fuel waste package.The power of such a hypothetical criticality event is typically estimated to be a few kilowatts: comparable with international studies of similar systems and the decay heat for which waste packages are designed. This work has built confidence in the methodology and supports RWM's demonstration that post-closure criticality is not a significant concern.

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The UK has had civil nuclear power for over half a century and a significant legacy of higher activity nuclear waste and spent fuel awaits disposal. UK Government policy is to construct a geological disposal facility (GDF) to dispose of these materials, safely containing them over hundreds of thousands of years, while radioactive decay reduces their potential harm. Scotland has developed its own policy. To ensure that the GDF is fit for purpose, we need to understand the long term effects of irradiation on materials as well as the interactions between the various components of a facility and the wastes. Radiochemical research will play an important role in helping us understand these processes and will enable us to implement the best strategies for long term disposal. Studies on radionuclide release from spent fuel and vitrified wastes, the effects of complexants on radionuclide behaviour, the impact of microbes on the mobility of radionuclides and the behaviour of colloids (see Box 1) and non-aqueous phase liquids (see Box 2) – all of these pose questions that need to be answered as we develop the safety case for a geological disposal facility. Bringing all this research effort together to build a comprehensive safety case for the final repository, one that reflects best practice from around the world, is one of the tasks assigned to a new organisation, Radioactive Waste Management. RWM was created in April 2014 as a wholly-owned subsidiary of the Nuclear Decommissioning Authority (NDA). It builds on the work of the NDA's former Radioactive Waste Management Directorate.