A New Time-Dependent Unavailability Model Considering Equipment Ageing: A Multi-Objective Optimization Case Study Example

Managing a nuclear power plant (NPP) calls for evaluation and trading off the conflicting objectives of safe operation and economic burden. The reduction of NPP risk is strongly correlated to the reduction of NPP safety systems unavailability. The application of the probabilistic safety assessment in order to reduce safety systems unavailability is one of the prime goals in the nuclear safety analyses nowadays. Equipment ageing, on the other side, has gradually become a major concern in the nuclear industry. This paper presents a new approach for time-dependent unavailability modelling of safety equipment. The presented model incorporates the effects of ageing on equipment unavailability assessment as well. The model takes into account the effects of corrective maintenance and preventive maintenance on the calculated time-dependent unavailability. The core damage frequency, assessed with the new model, is selected to be the relevant plant risk measure within the study. The total incurred dose due to performing the test and maintenance activities on the safety equipment from one side and calculated plant risk from another are accounted as two objective functions in the process of genetic algorithm-based multi-objective optimization of the equipment surveillance test interval. The obtained optimal surveillance test intervals show that the risk-informed test and maintenance schedules differ from ones prescribed in the deterministically-defined technical specifications.