The paper describes at first the evolution of the Design Extension Condition (DEC) concept, that started in early 1990ies and was accelerated after the Fukushima accident in 2011. Nowadays the concept is elaborated in EUR document [1], WENRA Reference Levels [4], and finally in the IAEA Safety Standards [6][7]. Number of countries is implementing the DEC concept into the national legislation, safety assessment, design modifications and operation of NPPs. Typically, the DEC events are divided to 2 groups: DEC without core melt (DEC-A) and DEC with core melt (DEC-B).

The second part of the paper deals with safety assessment of DEC events with more detailed information about deterministic analysis of DEC-A events. Legislative and methodology basis, acceptance criteria, computer codes and their validation, list of scenarios to be analyzed, examples of analyses performed, and incorporation of results of new analyses into SAR are described step by step.

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