Loss of coolant accident analysis with consideration of aging effects

In the context of long term operation of nuclear power plants (NPPs) (i.e., 40-60 and potentially to 80 years), assessing the impact of aging of passive structures, systems, and components (SSCs) is important to preserve safety margins and to make decisions on reactor life extensions. A methodology has been previously proposed for the incorporation of aging modeling of passive components into a reactor simulation environment (e.g., RELAP 5, RELAP-7) to provide a framework for accounting for the impact of aging on plant risk. In this methodology, physical conditions of normal reactor operations and accident environments are explicitly modeled within the reactor system simulation framework including consideration of aleatory and epistemic uncertainties. A two-loop Monte Carlo approach is used to propagate input probability distributions representing the uncertainties throughout the physical model. An implementation of the overall methodology is presented to illustrate the feasibility of incorporating aging effects into the assessment of the risk of loss of coolant accident scenarios. The methodology is applied to cracks in the dissimilar metal welds of the pressurizer of a pressurized water reactor. These welds are potentially subject to stress corrosion cracking. The probabilities of leaks and ruptures are estimated over a time frame of 80 years of operating life which will cause to loss of coolant accident (LOCA). A practical tool for aging-induced failure probability calculation of passive components is introduced. The output of this tool could be used to modify existing PRAs.