Nuclear Safety Improvement Actions and Safety Requirements in Post-Fukushima Era

The severe nuclear accident at the Fukushima Daiichi Nuclear Power Plant (NPP) in Japan on March 11, 2011 had a huge impact on the world’s nuclear regulatory bodies and industry. This paper summarizes the lessons learned from the Fukushima Daiichi Nuclear Accident, which include inadequacy of sitespecific extreme external natural hazards evaluation, prolonged loss of power and ultimate heat sink, insufficiency of response and management of severe accident monitoring and mitigation means and etc. It is also important to aware of that the accident can occur simultaneously on several units at a multiunit plant. In the decade after the Fukushima nuclear accident, nuclear safety improvement actions and nuclear safety regulatory standard revisions were implemented by major nuclear power countries such as the United States, Japan, France, and China. In United States, the new regulation 10CFR 50.155 Mitigation of BeyondDesignBasis Events (MBDBE) was come into forces to further enhance the capability of NPP to address the BDBE. In Japan, a new independent regulatory body, Nuclear Regulatory Authority (NRA), was established and new safety standards were released in three aspects, including design basis safety standards, severe accident measures, and safety standards relative to earthquakes/tsunamis. In France, complementary safety assessments (CSAs) were performed on all nuclear facilities, the concept hardened safety core was prepared and longterm improvement measures are still undergoing. In China, comprehensive safety inspection and external hazard safety margin assessment on NPPs were carried out, the “Code on the Safety of Nuclear Power Plant Design” (HAF102) was revised in 2016, which is a highlevel requirement on design and new nuclear safety goal, safety philosophy, and safety requirements were established in general. In this paper, the nuclear safety philosophy and requirements were elaborated in detail in the fields of practical elimination of early or large radioactive release, category of accident conditions, defense-in-depth concept, mobile equipment configuration and etc. some of which are explicit in nuclear safety review practice while others may be needed to be further clarified. In summary, certain specific suggestions of NPP safety in the future were given with international requirements and practices, as well as the domestic NPP characteristics being considered. It is also recommended that the relevant safety guides and technical positions on specific requirements should be improved constantly, a complete and logical nuclear safety regulation regime should be established, a balanced combination of deterministic and probabilistic approaches should be adapted, the specific nuclear safety and assessment criteria should be consolidated in areas such as practical elimination of early or large radioactive release, the independence between all defenseindepth levels, design extension conditions with core melting, as well as the number and configuration of mobile equipment on the multipleunit plant site.