A review on the defense-in-depth concept and the flex strategies in different countries after Fukushima accident

In order to enhance the defense in depth for nuclear safety after the Fukushima nuclear accident, U.S. Nuclear Energy Institute put forward the concept of Diverse and Flexible Coping Strategies and the corresponding FLEX support guidelines for the special scenarios of Extended Loss of Alternating current Power and Loss of Ultimate Heat Sink caused by Beyond-Design-Basis External Event. Subsequently, the idea of the FLEX strategy was widely accepted and spread widely. The introduction of the concept of FLEX strategy into the defense in depth was the biggest improvement for nuclear safety in the recent decade. This paper has reviewed the concept of traditional defense in depth and its weakness that led to the Fukushima nuclear accident, which led to the development motivation for the FLEX strategy. The research progress of the FLEX strategy in different countries in the past ten years has been reviewed. Based on the literature, and the above-mentioned review, some recommended future work has been given.