The Price of Risk Reduction: Optimization of Test and Maintenance Integrating Risk and Cost

The primary purpose of surveillance testing and maintenance (T&M) is to assure the reliability of standby safety systems and components in nuclear power plants. The optimal schedule of safety equipment outages due to testing and maintenance is one of the parameters of the plant safety. Early optimizations of single component test intervals were based on minimizing the risk 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 the costs may affect the T&M optimization. The costs are expressed as a function of the selected risk measure. The timeaveraged 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. The testing strategy sequential versus staggered is compared. Results of case study analysis have shown that relatively high system unavailability reduction is gained on the account of relatively low total T&M costs increase. The contribution of component aging to risk is, in general, negligible.