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Aging Effects on Time-Dependent Nuclear Plant Component Unavailability: An Investigation of Variations from Static Calculations

Rob D. Radulovich, William E. Vesely, Tunc Aldemir

Nuclear Technology

Volume 112 / Number 1 / October 1995 / Pages 21-41


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In the nuclear industry, aging effects have been traditionally incorporated into probabilistic risk assessment studies by using a constant (static) unavailability (qs) averaged over time. However, recent work shows that because of aging, substantial deviations may occur in time-dependent nuclear plant component unavailability from that predicted by static models well within the plant lifetime. A methodology based on the standard extension of the classic renewal equation when repair is explicitly considered is used to investigate (a) the trends in the effects of aging on time-dependent component unavailability as a function of changing first failure density (FFD) and test parameters and (b) the circumstances for which static approximations may be inadequate to describe these effects. The investigation uses several point- and time-averaged unavailability measures based on time-dependent unavailability, such as before-test unavailability (BTU), average-interval unavailability (AIU) and year-average unavailability (YAU), and is restricted to periodically tested components whose FFDs satisfy the Weibull distribution with aging threshold. The results show that while point measures (e.g., BTU) can substantially differ from static unavailability and while all measures are sensitive to changes in the Weibull shape parameter b, aging threshold time t, and time between tests T, the differences between the time-averaged measures used (e.g., AIU, YAU) and the static unavailability were only found to be relatively significant for one case among more than 100 combinations of b, T, and T that were investigated. The differences are a factor of <2 for all other cases, which is within the uncertainty margin on the data used in the study. The results also show that qs may be an adequate unavailability measure for low values of b (i.e., b<2) and high values of T (i.e., T> 18 months) and may describe the late effects of aging on component unavailability irrespective of band T (i.e., beyond 25 yr of component age for the data under consideration).

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