Multi-state physics based aging assessment of passive components

Operation of nuclear power plants over durations of decades will lead to aging problems that will require effective surveillance and maintenance programs to assure the continued reliability of passive systems, structures, and components (SSCs). Physics based aging modeling is needed to assess the long term reliability of passive components since their operational failure data are limited and the implementation of more effective surveillance and maintenance programs are based on these limited data. Stress corrosion cracking (SCC) is one of the age-related degradation mechanisms for primary pressure boundary passive components. A multi-state model capable of representing progressive degradation is used to model SCC aging whose transition rates are functions of the thermal-hydraulic conditions under which the passive component operates. The multi-state model is converted to a semi-Markov process through the use of sojourn times. Plant physics parameter dependency of the transition rates is evaluated using RELAP/SCDAPSIM for the steady state thermal-hydraulic behavior assessment of an example pressurized water reactor. The semi-Markov model yields the cumulative probability distributions for all the states, as well as the expected residence times in the states, which then can be used for surveillance and maintenance planning. The model is implemented on an Alloy 182 weld of a pressurized water reactor pressurizer surge line. The results are compared to those obtained previously with transition rates independent of operational conditions.