Study On Advanced Fault Tree Analysis Method In Nuclear Power Plant Probabilistic Safety Assessment

Nuclear power is a very important kind of energy, and nuclear safety is significant in the nuclear industry. Probabilistic Safety Assessment (PSA) is one of most useful method of nuclear safety analysis, in which Fault Tree Analysis (FTA) is an indispensable tool.In the PSA of a Nuclear Power Plant (NPP), very large fault trees can be found. The efficient analysis of large fault tree is a very complex problem. Considering the characteristics of the NPP real-time risk analysis, further research on large scale fault tree pretreatment and calculation was done to develop a set of fast FTA algorithms which were able to meet the requirements of NPP real-time risk analysis. In this method, fast and sufficient fault tree normalization, simplification and modularization algorithms, several efficient ZBDD variable ordering and ZBDD parallel manipulation methods were developed to improve the efficiency.Based on the devolopment of RiskA by Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences·FDS Team, the greatest contributions of this paper are:(1) a parallel K/N gate processing method was proposed, which made the high-dimensional K/N gate processing fast;(2) some new supplements were added to the traditional fault tree simplification rules, which saved computer memory by30%～70%and improved the large fault tree analysis speed by1～10times than primary method in RiskA;(3) a dynamic adjustment strategy was proposed to improve the modularization rationality, which increased module number by0.5～1times for the same fault tree instance;(4) a new variable ordering method based on MCSs was proposed, which saved computer memory by20%and saved calculation time by more than50%comparing with primary method in RiskA;(5) a hybrid variable ordering method combining fault tree structure and MCSs was proposed, which further improved computing speed by about1～2times than MCS-based method while increasing a small amount of memory;(6) a parallel ZBDD manipulation method was proposed, whose efficiency is significantly higher than the serial algorithm, speeding up to4～6times than primary method in RiskA.All of these methods were implemented as a fast FTA engine within a PSA code package. The development of fast fault tree analysis algorithm can make up the defects on the problem of real-time processing large scale fault tree, and promote further development of FTA in the field of nuclear security.RiskAT was successfully applied in Third Qinshan Nuclear Power Plant (TQNPP) accident sequence quantitation. The results were correct, and the calculation speed was faster than similar business software, which provided important technical support for efficient analysis work of TQNPP.