The price of risk reduction: Optimization of test and maintenance integrating risk and cost

Testing and maintenance (T&M) improve the reliability of safety systems and components in nuclear power plants, which is of special importance for standby systems. Early optimizations of single component test intervals were based on minimizing the risk, e.g. the time-average unavailability, without cost considerations. However, the appropriate development of T&M strategy depends not only on the T&M intervals but also on the resources (human and material) available to implement such strategies. Since these testing and maintenance activities are associated with substantial cost, they present an important domain, where risk reduction and costs can be balanced.The objective of this paper focuses on assessing how costs and component ageing may affect the T&M optimization in terms of minimal system risk. The costs are expressed as a function of the selected risk measure. The time-averaged function of the selected risk measure is obtained from probabilistic safety assessment, i.e. the fault tree analysis at the system level, extended with inclusion of time parameters related to T&M activities. Additionally, component ageing is taken into account while developing the system reliability model presented in this paper. The testing strategy is also addressed. Sequential and staggered testing strategies are compared. The developed approach is applied on a standard test system and the obtained results are presented. The results show that the risk-informed surveillance requirements differ from existing ones in technical specifications, which are deterministically based. The presented approach achieves a significant reduction in system unavailability accompanied with relatively small changes in total T&M costs.