Although nuclear power has advantages of economic and environmental efficiency compared to other energy sources, it causes astronomical human and material damage when an accident occurs. As such, the most important aspect of nuclear power generation is safety since it uses nuclear fission energy. In a nuclear power plant, pumps and pump systems must be appropriately operated in the performance(flow and pressure) designed as important equipment in all processes or power plants, which must secure reliability of the pumps for safe operation of the nuclear power plant. It is especially necessary to ensure the reliability of safety-related pumps for safe shutdown of nuclear reactors, maintenance of safe shutdown conditions, and mitigation of accident consequences. In-service test (IST) is performed to check the safety function performance capability of pumps and valves for safe shutdown of nuclear power plants, maintenance of safe stop status, and alleviation of accident results, and to monitor and evaluate the degree of vulnerability over time. IST is also performed in accordance with the Nuclear Safety and Security Commission enforcement regulations of Nuclear Safety Act and the Nuclear Safety Commission notice.
The in-service test (IST) of a nuclear power plant is essential to confirm pump safety, and it is performed on various devices or systems several times a week not only when the nuclear power plant is in normal operation but also during the nuclear fuel loading period. This means that there are many potential danger factors that can cause transient or power generation stop of the power plant during test execution. The occurrence of a transient state or power generation stop due to the failure of in-service test results in direct losses, such as reduced power generation and reduced sales, as well as indirect losses in the increase of rental business by intensifying regulatory inspection
This study comprehensively investigated the failure cases of ISTs in the last 10 years (2010-2020) and the cases notified to take corrective measures or recommendations for improvement, due to non-conformity with regulatory agencies for safety-related pump in-operation tests during the regular inspection of nuclear power plant. The study classified the cases according to the causes prescribed by the law and analyzed the root causes of the failure causes of in-service test.
The results showed that 144 cases of inadequacy occurred over the past 10 years, in the order of nonconformity with device function (29.9%), nonconformity with procedures (27.8%), nonconformity with test/inspection or inspection (20.1%), and noncompliance with procedures (14.6%). Next, the study analyzed the inadequacy cases of the in-service test by dividing them into human factors and mechanical factors as the root cause, and the results showed that the number of inadequacy cases of in-service testing due to human factors (68.7%) was more than twice that caused by mechanical factors (31.3%). Human error was the biggest in the case of inadequate setting of reference value among the cases of inadequate in-service testing, and the setting of reference value after 2019 became the subject of intensive inspection in the in-service test.
As a preventive measure for cases of inadequate reference value setting, this study developed a reference value effectiveness evaluation table and evaluated the reference value effectiveness of 216 safety-related pumps in A, B, and C nuclear power plants. The result showed that 57.0% were satisfactory and 43.0% were unsatisfactory. For pumps with inadequate validity of the reference value, the risk factors of in-service test inspection point must be excluded in advance, and the performance curve must be rewritten or the reference value must be reset to secure a valid reference value. As shown in the study results, human error was analyzed as the biggest cause among the inadequate cases of in-service testing, which implies the necessary of more studies on how to reduce human error.