Improvement of plant reliability based on combining of prediction and inspection of crack growth due to intergranular stress corrosion cracking

摘要

Improvement of plant reliability based on reliability-centred maintenance (RCM) is going to be implemented in nuclear power plants. RCM is supported by three types of maintenances: risk-based maintenance (RBM); time-based maintenance; and condition-based maintenance. RBM is supported by suitable combinations of prediction, inspection and maintenance (repair of elemental defects in their propagation stage). The combination of prediction and inspection is one of the key issues to promote RBM. Early prediction of IGSCC occurrence and its propagation should be confirmed throughout entire plant systems which should be accomplished by inspections at the target locations followed by timely applications of suitable countermeasures such as water chemistry improvements. At the same time, transparency and traceability as well as accuracy are strongly required for the prediction. A set of empirical formulas for evaluation of electrochemical corrosion potential (ECP) and crack growth rate due to IGSCC was applied for evaluation of IGSCC risk as well as IGSCC occurrence and propagation. The details of IGSCC formulas have been reported by various researchers along with the expected accuracy of prediction to prepare for ensuring their transparency and traceability. Based on prediction results, primary points for inspections will be determined. Unfortunately, it is still difficult to evaluate some key parameters in the plants, e.g., residual stress. From the inspections, accumulated data will be applied to confirm the accuracy of the IGSCC formulas and their computer codes, to tune some uncertainties of the key data for prediction, and then, to increase their accuracy. The synergetic effects of prediction and inspection on application of effective and suitable countermeasures are expected. In this paper, the procedures for the combination of prediction and inspection have been introduced.