Beyond emergency response: extending international cooperation frameworks for the post-accident management of the marine environment after a nuclear accident.

International cooperation is a central component of nuclear accident management. Although feedback on the Fukushima accident has highlighted the long-term economic, social and cultural consequences of such an event, the management of the post-accident phase remains less studied than the emergency phase and is not supported by an equivalent legal framework for international cooperation. This article aims to discuss whether, and to what extent, the international cooperation mechanisms primarily designed for the emergency phase could be extended to the post-accident phase, taking into account a hypothetical accidental release of radionuclides into the sea from a French metropolitan land-based nuclear power plant, as happened with the Fukushima accident. To address this question, the study considers the relevant legal instruments derived from nuclear law and the law of the sea, at both the international level, including bilateral agreements, and the European level, distinguishing their respective objectives and modalities of cooperation. The analysis shows that no explicit legal obstacles prevent the extension of these mechanisms beyond the emergency phase, whether regarding assistance, information exchange, or measures aimed at reducing and controlling marine pollution. However, the applicability of certain provisions depends on their interpretation, and others would require adaptation to effectively address long-term radioactive contamination, such as existing data aggregation mechanisms established under regional seas conventions. Finally, effective implementation of such extended cooperation would benefit from dedicated international post-accident management exercises specifically integrating marine environment management, as well as from intercomparisons of decision-support tools, especially modelling approaches, reflecting operational implications of divergent technical results.