The periodic safety review (PSR), for all operating nuclear power plants in Korea, has been conducted in accordance with SSG-25, a guideline suggested by the IAEA, The PSR is performed through the review of the regulatory body after the operator’s self-evaluation. In order to guarantee a high level of safety in consideration of the changed environment, such as operating experience (OE) and technology development, it should be comprehensively and integratedly performed, and it is also carried out every 10 years after the operation permit. However, in case that all or part of the reactor facilities have been permanently shut down, such as Kori Unit 1 and Wolsong Unit 1, Around a half of reactor facilities are not in operation. The periodic safety evaluation may not be conducted for unused parts if there is no safety hazard and if there are some difficulties for applying periodic safety evaluation. In considering that the biggest purpose of PSR safety (by PSR definition of KINS guideline) is to improve and accumulated factors such as aging deterioration, facility change, operation experience, and technological development for operating nuclear power plants. It refers to a comprehensive safety evaluation that is periodically performed during the period of operation of a nuclear power plant. It is necessary to review whether PSR should be performed for a nuclear power plant that is permanently shut down after nuclear power plant operation is terminated. Also, in IAEA SSR 2/2 Rev1, it is defined that PSR is performed during the nuclear power plant operation period. “Requirement 12: Periodic safety review, Systematic safety assessments of the plant, in accordance with the regulatory requirements, shall be performed by the operating organization throughout the plant’s operating lifetime, with due account taken of operating experience and significant new safety related information from all relevant sources”. Recently, Kori Unit 1 and Wolsong Unit 1 were decided to permanently shut down in June 2017 and December 2019, and are currently being prepared for decommissioning. According to the Wolsong decommissioning plan, decontamination and demolition will be completed by 2032. The PSR for permanent shutdown of Kori Unit 1 was submitted to the regulatory body in December 2018 and is under approval review. In the case of the permanent shutdown PSR of Wolsong Unit 1, the project will be launched in May 2023 and the PSR will be submitted to the regulatory body in May 2024. In the case of Wolsong Unit 1, it is necessary to operate the various systems, including the systems related to the spent fuel storage tank, even during the period of permanent shutdown. Such as the heavy water related systems used in common with Wolsong Unit 2, are essential operating systems. Based on Basic Subject Index (BSI), 112 out of 218 systems require operation, indicating that about 50% of systems require operation even after permanent shutdown. Decommissioning of systems and equipment will begin after the transfer to modular air-cooled canister storage (MACSTOR) by the end of 2025, and then in-depth discussions will be needed whether PSR evaluation is meaningful.

At Nuclear Power Plant (NPP), aging management is performed as part of the Periodic Safety Review (PSR) in accordance with the Nuclear Safety Act. The purpose of the aging management program (AMP) is to manage the integrity of structures, systems and components (SSCs) in NPPs over time and use. Through this, aging deterioration is mitigated to increase equipment life and secure long-term operation safety. Fuel Oil Chemistry is one of the AMPs. Through this program, aging management is performed for storage tanks, piping and other metal components that contact with diesel fuel oil. The program is focused on managing loss of material due to general, pitting, crevice, and microbiologically-influenced corrosion (MIC) and fouling that leads to corrosion of the diesel fuel tank internal surfaces. The fuel oil aging management method currently applied to NPPs in Korea measures the concentration of water and particulate contamination in the oil, analyzed the trend, and periodically cleans and inspect the inside of tanks. Among them, in monitoring MIC, a direct analysis and monitoring of the amount of microorganisms may be more effective. In this study, a method for improving the MIC monitoring system for diesel fuel oil systems was reviewed by reviewing reference documents including NUREG 1801 and examining the methods actually applied in US NPPs.