Regulating criticality safety: use of burn-up credit in the assessment of criticality risk

In Great Britain, the drop in reactivity that occurs when nuclear fuel is irradiated in a nuclear power reactor has typically not been claimed. Instead, criticality safety assessments have normally assumed that the fuel is unirradiated with no reduction in the fissile material present. Although this 'fresh fuel' approach is conservative, it leads to an overestimation of the calculated neutron multiplication factor (k_eff) and may lead to additional operational burdens being placed upon duty-holders. In future, it is possible that GB duty-holders may take credit for the reduction in reactivity, known as 'burn-up credit', that occurs when fissile material is consumed in a nuclear reactor. In order to provide authoritative and independent information on the key aspects of burn-up credit, the Office for Nuclear Regulation (ONR) commissioned a research project. This paper provides an overview of the key findings of this research and potential implications for the regulation and management of criticality safety in GB. A 'Regulator Question' set for use when assessing criticality safety cases is given that is intended to aid both regulator and duty holder in their assessments, which enables regulatory attention to be targeted proportionately on those areas of most importance