Development of inspection and evaluation guidelines for light water reactor internals

Full text: In Japan, before a nuclear power plant reaches its 30 years of operation, the Japanese utilities carry out a 'study on plant life management'. Reflecting the results of that study into the maintenance, the utilities make efforts to maintain and improve the safety and reliability of their nuclear power plants. In this study, all safety related components are evaluated from the viewpoint of aging degradation, assuming a long-term operation. If a crack should be found at components such as reactor internals, which is deemed important for safety and are difficult to repair or replace may provide a serious impact on the plant operation and management. Reactor internals, for instance, made of austenitic stainless steel and nickel base alloy, are not completely free from aging degradation including stress corrosion cracking (SCC). Therefore, it is concluded in the study on plant life management that they are required continuous planned inspections to confirm their integrity while continuing plant operation. If an aging degradation such as SCC should be found at the reactor internals, a great amount of labor and time may be required for root cause investigation and analysis and subsequent repairs because it is very difficult to reach to the degraded portion due to structural, dimensional and environmental restrictions. Therefore, such situations may provide serious impact on plant operation and management. As to the reactor internals and other components with similar characteristics, it is strongly recommended that contingency plan should be prepared in advance. Thus, considering the significance of aging degradation, it is necessary to develop some standard rules for the inspection and evaluation of reactor internals. Such rules should specify when, where and how to inspect. They should also specify the evaluation method in case such degradation as a crack is found, and the repair method and extent if repairs are required. These standard rules must be reasonable and clarify the technical reasons or rationale to obtain the public acceptance. This is essential not only to plant operation management but also to fulfilling the accountability for the standard rules. Based on the understanding described above, the Thermal and Nuclear Power Engineering Society (TENPES) has organized a committee named 'The Committee on Inspection and Evaluation Guidelines for Light Water Reactor Internals' to develop the guidelines necessary for the inspection, evaluation and repair of PWR and BWR reactor internals. The outline of the guidelines developed by the TENPES committee is presented herein. (author)