On probability models for the unavailability analysis of safety systems under aging phenomena

This paper presents and discusses three probability models for approaching aging of a component. NRC and IAEA views of aging phenomena are reviewed an d discussed. Repair is approached in the first model by stochastic point pr ocesses, and a statistical dynamical model is employed to allow for Bayesia n forecasting. A discussion on repairable systems terminology is presented because much controversy may be found concerning this feature. A queueing m odel is then presented for a component considering its age as a supplementa ry variable. Presented also are numerical and exact solutions (for some cas es). Aging is modeled by Weibull and lognormal distributions for failure ti mes. Repair is approached by exponential distributions. The device of stage s is applied to the same problem, and results are obtained for a Weibull fa ilure time distribution. Typical means and variances for the times to failu re are considered, and combinations of stages are checked. Alternative solu tions by failure rates discretization are generated to check the validity o f the developed models. An important issue concerning these models is discu ssed, that of appropriate failure data for each of them. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.