Study on the Redundant Degree of Nuclear Power Plant Safety System

This article makes a clarification for the concept of the redundant degree of nuclear power plant safety system,which cannot be simply equivalent to the number of safety trains.The redundant degree of N+1 satisfies the single failure criteria which is a mandatory requirement,and the redundant degree of N+2 is an option for In-service Maintenance.The configurations and redundant degrees of safety systems of the main nuclear power models in the world are further introduced,and the specific relations between the redundant degree and operational flexibility are interpreted.On the basis of the study,the analysis and comparison is performed for two kinds of safety system configurations of three-loop pressurized water reactor,i.e.two trains with a common header and three independent trains.Both designs are of redundant degree of N+1,but are different in the way whether to consider the single failure of passive component(common header).The studies on nuclear safety codes,standards and utility requirements reveal that,the single failure of passive component should not be the decisive factor in selecting the two designs.With the comprehensive consideration of the safety benefit and economic cost,the design of two trains with a common header is more optimized and balanced.