The Office for Nuclear Regulation (ONR) is the United Kingdom's (UK) independent regulator of nuclear safety and security. A key requirement of UK law and ONR's regulatory approach is that licensees build, operate and decommission nuclear sites ensuring that risks are As Low as Reasonably Practicable (ALARP). In line with international guidelines, ONR expects nuclear plants to show hazard resilience by means of layout optimisation and segregation of redundant and diverse safely systems. This generally involves the provision of suitably designed multi-hazard barriers. The design and qualification of hazard barriers against single internal and external hazard loads is not exempt from difficulties, but generally follows established methodologies that are documented within international standards and design guidelines. However, hazards can often occur in combination, and these can give more significant challenges to the design. A single event initiator e.g. an internal fire or seismic loading can lead to multiple hazards (secondary fires, dropped loads, pipe whip, jet impact, flooding and steam release). These loads can credibly combine on individual barriers with varying degrees of severity depending on hazard ranges, timing and plant geometry. The identification, screening and consequence assessment of hazard combinations is particularly challenging. This is due to not only the high number of potential combinations, but also due to the inherent uncertainty in hazard frequencies, event progression and potential consequences. Guidance on the assessment of hazard combinations in nuclear plant is notably scarce. Recent ONR experience in safety case assessment has shown the importance of coordinated, multi-disciplinary approaches in the development of resilient designs against combined hazards. In this paper, ONR Internal Hazards, External Hazards and Civil Engineering specialists provide a critical appraisal of the following aspects of combined hazards assessment: 1. Challenges in the identification and screening of hazard combinations; 2. Methods for characterisation of combined hazard loads and barrier design and substantiation; 3. Uncertainty and margins of safety. The paper presents expectations and recent challenges in evaluating resilience against combined hazards in the context of Nuclear New build installations which have recently undergone Generic Design Assessment (GDA) in the UK.
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