A quantitative assessment of LCOs for operations using system dynamics

Limiting conditions for operations (LCOs) define the allowed outage times (AOTs) and the actions to be taken if the repair cannot be completed within the AOT. Typically plant shutdown is required. In situations where the risk associated with the action, i.e. the risk of plant shutdown given a failure of the safety system, may be substantial, a strategy is needed to control the plant risk. In this study the changing operation modes are evaluated quantitatively and dynamically using the tool of system dynamics. System dynamics has been developed to analyze the dynamic reliability of a complicated system. System dynamics using the Vensim software have been applied to LCOs assessment for an example system, the auxiliary feed water system of a reference nuclear power plant. Analysis results of both full power operation and shutdown operation have been compared for a measure of core damage frequency. The increase in core damage frequency is used as a measure in this study. A time dependent framework developed in this study has been shown to be very flexible in that it can be applied to assess LCOs quantitatively under any operational context of the Technical Specifications in Final Safety Analysis Report of the reference plant.