AGING EFFECTS ON TIME-DEPENDENT NUCLEAR-PLANT COMPONENT UNAVAILABILITY - AN INVESTIGATION OF VARIATIONS FROM STATIC CALCULATIONS

In the nuclear industry, aging effects have been traditionally incorpo rated into probabilistic risk assessment studies by using a constant ( static) unavailability (q(s)) 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 o n the standard extension of the classic renewal equation when repair i s explicitly considered is used to investigate (a) the trends in the e ffects of aging on time-dependent component unavailability as a functi on of changing first failure density (FFD) and test parameters and (b) the circumstances for which static approximations may be inadequate t o describe these effects. The investigation uses several point- and ti me-averaged unavailability measures based on time-dependent unavailabi lity, such as before-test unavailability (FFD) average-interval unavai lability (AIU) and year-average unavailability (YAU), and is restricte d to periodically tested components whose FFDs satisfy the Weibull dis tribution with aging threshold. The results show that while point meas ures (e.g., BTU) can substantially differ from static unavailability a nd white all measures are sensitive to changes in the Weibull shape pa rameter b, aging threshold time tau, and time between tests T, the dif ferences 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, tau, and T that w ere investigated. The differences are a factor of < 2 for all other ca ses, which is within the uncertainty margin on the data used in the st udy. The results also show that q(s) 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 componen t unavailability irrespective of b and T (i. e., beyond 25 yr of compo nent age for the data under consideration).