Decision-making during urgent phase of a nuclear accident under extreme conditions

•We introduced a systematic decision-making process named ROSYNA, a real-time decision support system for a nuclear accident to be considered by the decision-makers during urgent phase of a nuclear accident under extreme conditions.•ROSYNA is a three-step approach i.e., evaluation of decision problem attributes, identification of situation and implementation of decision table.•A prototype applicability of ROSYNA was implemented to the Fukushima situation. We concluded that the decision problem attributes, which are the key element of this methodology need to be assessed using professional judgment based on the nature of the accident, the information that can be gleaned from the nuclear facility, radiation monitoring, and the prior understanding of such occurrences and pre-established response arrangements.