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• The Open Phase Condition (OPC) is defined as an open circuit of one or two of the three phases of any power circuit needed for the normal operation of electrical systems. • The OPC of the nuclear power plant (NPP) may cause station blackout (SBO) and pose a great risk to the integrity of the reactor core. • It was confirmed that there was a valid difference in OPC detection between the types of Korea's standard NPPs. • The goal of this study is to secure the integrity of the reactor core by detecting the OPC of the NPP's power system early. Open Phase Condition (OPC) is a failure in which one (or two) of the three-phases of the power system is maintained open, and the OPC of the nuclear power plant (NPP) may cause station blackout (SBO) and pose a great risk to the integrity of the reactor core. Recently, OPCs have occurred in power systems at NPPs around the world, but many problems have been reported that failures are not detected by the current protection systems. Accordingly, the NPPs in Korea, which have a power system similar to that of overseas NPPs, also need to review the risks to OPCs. In this paper, the occurrence of OPCs was simulated for Korea's standard NPPs using ETAP, and the current protective relays were analyzed and compared for each scenario. In addition, based on these analysis results, countermeasures against OPCs are proposed in order to secure reactor core integrity. It is considered that this paper can be used as a technical background material necessary for improving the on/off-site power protection system of NPPs and establishing related detailed regulatory guidelines in the future. [ABSTRACT FROM AUTHOR]

• The change in the internal/external environment of the system can have an effect on the performance of PECCS. • This study has conducted the effect evaluation on performance issues as part of the performance evaluation methodology for PSS development. • Major performance issues (such as modeling of PECCS pressure, wall condensation in the system, and check valve) were identified through impact evaluation using the system analysis code, and the effects of major performance issues on the system were analyzed. • In this paper, the evaluation of performance issues has led to the identification of the main factors to consider in PECCS modeling. In this study, the effect of performance issues on the Passive Emergency Core Cooling System (PECCS), a type of Passive Safety System (PSS), was evaluated. The PECCS relies on natural phenomena such as gravity, density difference, and pressure difference to operate. Its driving force is lower compared to that of the active safety system, making it necessary to evaluate its performance in various transient situations. Major performance issues were identified through effect evaluation using a conceptual problem for the PECCS, and their effects on the system were analyzed using the system analysis code MARS-KS. Therefore, there are concerns about whether the PSS can sufficiently perform safety functions in various situations. In Korea, performance evaluation methodology for PSSs is being developed to dispel these concerns. As part of the methodology development, this study conducted the effect evaluation on performance issues. The study identified several major performance issues for the PECCS, including those related to pressure and heat loss of piping within the system, uncertainties in tank wall condensation modeling, and checks valve modeling. The evaluation of performance issues has led to the identification of the main factors to consider in PECCS modeling, which include: 1) the geometry and pressure drop of PECCS, 2) the occurrence of wall condensation in pipes and tanks, and 3) the behavior of check valves. Proper modeling of these factors is important in achieving accurate analysis results and preventing potential issues, such as premature check valve closure or incomplete coolant injection. In conclusion, the results of this study are expected to contribute to deriving safety analysis modeling guidelines for PSSs and design considerations for NPPs that utilize PSSs by improving the completeness of the developing performance evaluation methodology. [ABSTRACT FROM AUTHOR]

• Development of long term cooling safe shutdown analysis methodology using passive safety system. • Long term cooling safe shutdown performance analysis for the SMART reference plant. • Sensitivity analyses of the passive system failures on the long term cooling safe shutdown performance. Passive safety systems have been adopted in reactor design to enhance safety especially in SMRs after the TMI-2 accident in 1979. The passive system has high operational reliability with natural driving forces such as gravity, natural circulation and stored energy. In recent years, development of innovative SMRs has emerged as a global R&D initiative for sustainable energy to challenge the carbon neutrality of climate change. The passive safety system makes reactor design simple and thus enhances the reliability of the reactor. As advanced reactors adopt passive safety systems, regulatory requirements and guides are required. Thus, development of regulatory guide and detailed regulatory review methodology for the passive safety systems is needed. In this paper, long term cooling safe shutdown acceptance criteria and performance analysis methodology are proposed for the regulatory standard design approval for the SMART100 integral reactor with fully passive system which has been jointly developed by KAERI in Korea and K.A.CARE in Saudi Arabia. The long term cooling safe shutdown performance analyses are performed until 72 hours to evaluate the methodology, and sensitivity analyses are also performed to provide a technical basis for the regulatory independent review for the long term cooling performance of the SMART100 passive safety systems. [ABSTRACT FROM AUTHOR]