Risk-benefit analyses of SG tube maintenance based on probabilistic fracture mechanics

As an application of probabilistic fracture mechanics (PFM), a risk-benefit analysis was performed for the purpose of optimizing maintenance activitie s of steam generator (SG) tubes used in pressurized water reactors (PWRs), The probabilities of the SG tube leakage and rupture are defined as risks i n this study. A model was made modifying pe-PRAISE (Piping Reliability Anal ysis Including Seismic Events) to evaluate the risks during 60 year operati ons due to stress corrosion cracking (SCC) of the tubes under various maint enance strategies for SG tubes. In the risk analysis, parameters such as inspection accuracy, inspection in terval, sampling inspection and crack propagation law were selected for sen sitivity analysis. Based on the risk analysis, a risk-benefit analysis was conducted when implementing two maintenance strategies taking both costs an d revenues for 60 year operations into account. In the risk-benefit analysi s, the expected cost of leakage or rupture was calculated by multiplying 'p robability of leakage or rupture' by 'expected loss of leakage or rupture a ccident'. To justify whether it is worthwhile implementing the maintenance strategies or not, the net present value (NPV) was calculated as an index, which is one of the most fundamental financial indices for decision-making based on the discounted cash flow (DCF) method. The results demonstrated that in the risk analysis, the risks are influence d significantly by the crack propagation law, accuracy of inspection and sa mpling inspection. In the risk-benefit analysis, it was suggested that inve stment to improve inspection accuracy would reduce the total costs of 60 ye ar operations significantly and increase the NPV. Although the analysis was mainly conducted for SG tubes made of Inconel 600 mill anneal (MA) material, the analysis was also carried out for Inconel 6 90 thermal treatment (TT) material, making assumptions on its crack initiat ion and crack propagation law. In addition, the effect of introducing maint enance criteria, namely, operation with a crack justified by certain criter ia, on NPV was evaluated. (C) 2001 Elsevier Science B.V. All rights reserve d.