Conservatism in Dose Assessment and Modelling When Calculating Criticality Risks

The poster presentation provides an overview and key findings of work undertaken by the United Kingdom (UK) nuclear regulator into the use of conservatisms that can be applied in criticality safety cases to estimate criticality risks, and the potential implications of these findings for the regulation and management of criticality safety. When calculating the risk of criticality for fissile systems, conservative bounding assumptions must be made with regards those parameters that affect the reactivity of such fissile systems. Dutyholders balance these assumptions against operational requirements such that they are not overly restrictive and, for example, do not significantly hinder operations from taking place or disproportionately increase other risks. The Office for Nuclear Regulation (ONR), acting as the independent nuclear regulator across the UK, form regulatory judgements regarding dutyholders'safety cases and their use of these conservative assumptions. An overview of two examples of conservatisms that may be used in UK criticality safety cases to support the estimate of criticality risk are presented. These examples include work undertaken on behalf of ONR in the form of research papers into the use of 'burn-up credit' (used when assessing the criticality risk of spent nuclear fuel) and consideration of the effect temperature has on the criticality risk of fissile systems. Both research papers are published on the ONR website.