Availability improvement of standby safety component based on online status monitoring and test interval adjustment

Abstract

The safety of nuclear power plants (NPPs) has been treated as a major issue over the last few decades, and has been particularly emphasized in recent years. Safety systems that are fundamental to nuclear safety basically function to transfer residual heat from the core to the ultimate heat sink, and during this process, they control the flow path of coolant through the operation of various mechanical components such as motor-operated valves (MOV) and check valves (CV). In this context, for more reliable safety systems and safer NPPs, the availability of these critical standby components for coolant path control should be improved. In order to deduce appropriate strategies for the availability improvement and also to correctly analyze the effects of them, a realistic and general component unavailability model is developed, which takes into account aging effects, test duration, and repair duration. Based on this model, strategies which can improve the availability of standby component are suggested. The strategies are broadly categorized as online status monitoring, which monitors failures occurring during standby and operation, and test interval adjustment, which adaptively manages the test interval for each standby turn. To analyze the effects of these strategies quantitatively, their influences are formulaically reflected to the general component unavailability model. The feasibility of the proposed strategies is demonstrated via a case study for an MOV and a CV. As a result of the analysis, the average unavailability of the valves for their expected lifetimes can be greatly reduced to 51.28% and 12.36%, respectively, when compared to the cases with no improvement strategies. From the comparison between the MOV and CV cases, it is confirmed that the test interval adjustment strategy is meaningful as an auxiliary means for the online status monitoring strategy. The impedance analysis technique is suggested and validated as a monitoring technique for standby MOV. For the purpose of monitoring integrity, the use of impedance analysis for the motor is a novel application. This application is expected to be especially useful in areas with access restriction, such as NPPs, as motor impedance can be measured from a distance without any interruption to the motor on site. Moreover, it can be applied to all the components that are operated by motors, not only MOVs but also motor-driven pumps and so on. In order to apply the online status monitoring strategy to CVs during operation, a hybrid check valve is invented, which can be passively operated by pressure difference during normal situations and can also be actively operated by a motor. This hybrid check valve concept is confirmed to increase the availability of the CVs remarkably, as it can cover a large proportion of typical failures. Even though the proposed strategies are developed for the availability improvement of standby components, they can also be applied to other topics, such as preventive maintenance plans, dynamic probabilistic safety assessments (PSA), and risk-informed regulations, as the strategies provide realistic standby component availabilities in contrast to current statistical-only approaches.