Evaluation of maintenance methods of PWR primary system pipes based on PFM

In regard to the maintenance of nuclear power plants (NPPs), inspection costs are reduced in the US by decreasing inspection elements, while maintaining safety using the method of Risk-Informed In-service Inspection (RIISI) considering core damage frequency (CDF). However, it is better to regard events that affect the reliability of NPPs, such as unplanned shutdown caused by damage to components, as significant even if CDF is extremely low. The loss of coolant accident (LOCA) caused by pipe damage accounts for a large percentage of the damage of components that causes core damage, and the evaluation of pipe damage to calculate CDF is usually conducted by the method of Probabilistic Fracture Mechanics (PFM). However, reports on studied that calculate the maintenance costs of pipes by PFM cannot be found in the main open domestic or foreign journals. Therefore, in this study, a maintenance costs evaluation model was made considering pipe damage probability based on PFM, inspection costs and repair costs of pipe welds, and the costs against damage in case of pipe damage for the PWR prmary pipes that are of particular importance for safety. Maintenance costs were calculated for various inspection methods using a model assuming that the inspection cost per weld is 900 thousand yen, and the cost against damage varies from 5 to 30 bilion yen for an operation period of 40 years. As a result, it was confirmed that whether In-Service Inspection (ISI) is conducted or not, an appropriate time of ISI in case ISI is conducted once, and whether ISI is conducted once or twice during the operation period can be judged roughly in terms of reliability and economic efficiency. (author)