Time-dependent unavailability of aging standby components based on nuclear plant data

The time-dependent unavailability of periodically tested and repaired aging components which have first failure densities satisfying the normal and the Weibull distributions with aging threshold are investigated using a generalized formula for surveillance testing and repair. The formalism is applied to the case of periodic, good-as-old surveillance testing with good-as-new repair. Actual aging data are used. The results show that aging can have significant impacts on the availability and reliability of a component and the system containing the component. More specifically: a) the time dependent unavailability of aging components can go through large swings with varying magnitude in time before stabilizing to the standard saw-tooth behavior of periodically tested components with constant failure rates, and b) approximating the unavailability by a constant failure rate can lead to large differences even when an average unavailability is calculated.