ABSTRACT
In recent years, many authors have proposed alternative approaches to modelling the effect of ageing and test and maintenance activities. Some authors have proposed approaches to modelling the unavailability of safety-related components associated with standby-related failures that explicitly addresses all aspects of the effect of ageing, maintenance effectiveness and test efficiency. Recently, other authors have proposed a new reliability model for the demand failure probability that explicitly addresses all aspects of the effect of demand-induced stress, maintenance effectiveness and test efficiency. In this context, the paper presents a whole time-dependent unavailability model for a safety component, regarding time-dependent unreliability contributions for each failure mode addressing ageing, degradation stress by demand and effectiveness of maintenance and testing as well as the downtime contributions related to maintenance and testing activities. An application case of a motor-operated valve of a pressurized water reactor nuclear power plant is included. A set of sensitivity cases are presented to show how this model can be used, for example, in the planning of maintenance and surveillance test activities with the aim of minimizing equipment unavailability.
