Provision of information to underpin the UK nuclear design bais criterion for naturally occurring external hazards

ONR is seeking to engage one of the Technical Support Contractors (TSC) appointed on Lot 2 of its Technical Support Framework. The TSC is required to compare the UK design basis criterion for naturally occurring external hazards with other practices (both in terms of adopting a probabilistic approach rather than deterministic, and in terms of the value of the probabilistic criterion itself - 10-4 per year). For example for seismic hazard analysis, US guide RG1.208 advocates using a performance based approach and selecting an appropriate fraction between the 10-4 and 10-5 mean hazard curves, rather than a single conservatively estimated curve. Other countries, particularly where earthquakes are more frequent or severe than in the UK, may elect to design to a given deterministic event. Even though the UK may be relatively seismically inactive, the current design basis criterion may lead to relatively stringent design conditions, ironically, partly driven by the uncertainty resulting from scarcity of severe earthquake data and the requirement to ensure a conservative evaluation.