Nuclear safety especially severe accidents risks are of great concerns of nuclear power plant. Design consideration of severe accident prevention and mitigation is generally required by various nuclear safety authorities worldwide. However, those requirements related to severe accidents consideration are somewhat different from country to country. Recently, the International Atomic Energy Agency (IAEA) updated and published a safety code on Specific Safety Requirement of Nuclear Power Plant Safety: Design (SSR-2/1). Meanwhile, the Chinese National Nuclear Safety Administration (NNSA) also revised and updated the safety code on Requirement of Nuclear Power Plant Safety in Design (HAF102). In these two codes, both IAEA and NNSA established some new requirements, among which two are of great concern. One is Design Extension Conditions (DEC) for consideration of those conditions traditionally called Beyond Design Basis Accidents (BDBA) in design of nuclear power plant, another is requirement of practically eliminating large release of radionuclide. These two new requirements are internally related, somewhat different and more restrict from those related to severe accident requirements set forth by Nuclear Regulatory Committee of United States (USNRC). Up to date, there are no specific guidelines about engineering implementation of those new safety codes. This paper present an overview of those requirements from IAEA, WENRA, NRC and China NNSA, followed by discussion of engineering approach for the implementation of the DEC requirement set forth by safety authorities.

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