After the Fukushima accident, the safety requirements of nuclear power plants have been further improved. As an important design requirement, the design extension conditions need to be fully considered in the design of advanced nuclear power plants. Design extension condition list is an important aspect. The VVER nuclear power plant has also made many design improvements on the basis of practice. According to the requirements of design extension condition analysis, combining the design characteristics of VVER nuclear power plants, through carrying out PSA analysis of VVER nuclear power plants, PSA insights are used to systematically analyze the design extension conditions without significant fuel degradation (DEC-A). Such as SBO, ATWS, total loss of feedwater, total loss of heat sink, etc, have been analyzed and quantified to support design extension condition determination. Due to the need to identify important event sequences, the selection of the probability screening value of DEC-A is discussed and suggestions are given. In the probability screening value determination, the safety goal of nuclear power plant is considered, and also the accident sequences in PSA. In addition, the containment bypass event sequence has always been an important radioactive release scenario, which has a greater impact on safety, and is also discussed in the DEC-A analysis.

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