This article describes a technique to calculate the risk from failure of passive components over time, and demonstrates the technique by applying it to a weld in the auxiliary feedwater (AFW) system. It uses a modified version of the PRAISE computer code to perform a probabilistic structural analysis to calculate the probability that crack growth due to aging would cause the weld to rupture. It then uses the weld rupture probability as input to a modified existing PRA to calculate the change in plant risk with time. The results show an insignificant effect on plant risk because of the low calculated rupture rate of the weld in this particular calculation over 48 yr of service. A decreasing yearly rupture rate for this weld is calculated. This results from infant mortality; that is, most of those initial flaws that will eventually lead to rupture will do so early in life.

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