Cousins, Not Twins: Harmonisation and Divergence in National and International Understandings of ALARA Within Nuclear Safety Regulation

The global impetus for increased nuclear power is accompanied by the recognition that design standardisation and increased cross-border regulatory harmonisation are essential to achieving such ambitions. This includes establishing greater clarity in how international safety principles are operationalised within national regulatory regimes and aligned with domestic requirements. A persistent source of friction lies in the interpretative ambiguity surrounding the As Low As Reasonably Achievable (ALARA) principle. This study examines the issue through the analytical lens of the UK’s non-prescriptive nuclear licensing regime. Within this setting, ALARA is implemented alongside closely related national and international concepts (BAT, Optimisation, SFAIRP, and ALARP), which are frequently treated as equivalent in practice. The article presents a comparative analysis of these five near-synonyms, examining their regulatory genealogies, underlying justifications, and associated compliance expectations. Drawing on boundary theory, the analysis shows that these concepts function as boundary objects whose apparent equivalence masks divergent regulatory origins and disparate underlying rationales. The analysis demonstrates how false synonymy obscures important differences in how proportionality and reasonableness are embedded at international versus national levels, with consequences for compliance expectations and reactor safety design. Collapsing these into synonymy leaves duty holders navigating unresolved conceptual tensions within a non-prescriptive regime. Such an equivalence carries consequences for reactor safety design and, at a systems level, for the predictability and effectiveness of the licensing regime. The results emphasise the need for earlier and more structured engagement to tackle false presumptions and apparent consensus, and call for sustained international dialogue aimed at developing a verified, shared understanding of proportionality and reasonableness across regulatory contexts.