Identification of CSSC Caused by Ageing and Degradation

PSA studies, that were developed for some NPPs, permit the using of the created models to perform many research tests, in order to optimize the structures, systems and components (SSCs) operation or to identify the NPP or systems weaknesses, due to specific or special factors. SSCs that influence decisively the NPP reliability are considered as critical. Also, for the accident conditions, the SSC, which have a major influence to the system availability or operability, are considered as critical. Many worldwide NPPs reached the life time or are very close to do that. Several SSCs have shorter life times than NPP’s life time. Ageing is one of the factors that decrease the SSC life time. Due to ageing, if are not replaced, some SSCs, or groups of redundant SSCs, become critical looking to safety. Some questions for what to do in the situation when a SSC must be replaced and the SSC specific manufacturer doesn’t exist, could also be put. The paper tried to solve the problem of SSC modeling by introducing of an ageing factor in SSC model. Fault tree (F/T) modeling approach is assumed. There are two possibilities for modeling: failure rates that are changed or specific MCS (minimal cut set) term modified by ageing. Risk analysis and PSA techniques are used as a basis for analysis. The paper includes: the steps to establish the systems or components that suffer ageing; methods to identify CSSC taking into account ageing; the events associated to ageing/degradation and presentation of method to determine the ageing related events, selection of the SSCs that are important for analyses; selection of the most significant ageing events; ranking of ageing events; association of events to these components in order to decide for the CSSC detailed analyses; ranking / ordering of the ageing related events; optimization of NPP systems design and operation considering ageing; impact of ageing to NPP operation/safety/safety margins and to manufacturer technical specifications. The paper presents a brief description of the most important aspects of the methods, used to analyze the ageing effects on appearing of CSSCs, taking into account the previous developed NPP PSA models and PSA modeling tools.