INTEGRATED EVALUATION OF MAINTENANCE OPTIMIZATION FOR PIPES IN NUCLEAR POWER PLANTS BASED ON PROBABILISTIC FRACTURE MECHANICS

As an application of probabilistic fracture mechanics (PFM) and a financial method, risk-benefit analyses were performed for the purpose of optimizing maintenance activities of steam generator (SG) tubes used in pressurized water reactors (PWRs). Parameters such as inspection accuracy, inspection interval, sampling inspection, replacement of SGs and maintenance criteria (operation with verified cracks) were selected for sensitivity analyses. In the analysis of the operation introducing maintenance criteria, the effect of quantitative accuracy of the inspection (sizing performance) was also taken into account. Although the analyses were mainly conducted for SG tubes made of Inconel 600 mill anneal (MA) materials, the analyses were also performed for Inconel 690 thermal treatment (TT) materials with making assumptions on their crack initiation probabilities and crack propagation laws. To justify whether or not it is worth while implementing the selected maintenance strategies in terms of an economic point of view, 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. For the purpose of optimizing the plant operation including the influence of the level of public acceptance, change of the leakage cost by frequency of accidents is modeled.