(a) The relationship between failure probability of a weld and operation time. The failure probability decreases by ISI because cracks are detected and eliminated by repair or replacement. (b) The relationship between failure probability and ratio of welds to be inspected. When ISI with “Very high” accuracy was implemented, even with 10% low ratio of welds to be inspected, it can have the almost same failure probability in the case of ISI with lower accuracy of “Normal”.