[en] Current nuclear power plants must be highly reliable with respect to safety and economy. Consequently, development of a new safety evaluation methodology is desired for the accurate evaluation of safety and economy. A nuclear power plant consists of many systems. This paper is concerned with the reliability evaluation of alternating systems. An alternating system usually consists of multitrains, and its operation alternates from one train to another. Examples of these are the nuclear service cooling water system, the component cooling water system, and the chemical and volume control system. The alternating systems exhibit strong dynamic time-dependent reliability characteristics because of the alternating operational mode and the technical specification requirements that are particularly difficult to analyze by static fault-tree techniques. The method used in this paper is based on the semi-Markov reliability analysis. The system performance measures evaluated are its contribution to the total risk of the plant (e.g., core damage probability) and to plant unavailability (reactor downtime). It is concluded that the methodology developed in this study can be applied to existing alternating systems for plant-specific evaluation of the various alternatives in technical specifications

[en] Check valve performance evaluation and diagnosis technology have been introduced. In performance evaluation, disc stability analysis, wear prediction and fatigue prediction technology are explained. In diagnosis technology, acoustic, magnetic, ultrasonic testing, radiography and thermography technology are explained. Finally, condition monitoring program which integrate performance evaluation and diagnosis technology are recommended based on risk-informed application

[en] Pressure locking occurs when the fluid in the valve bonnet is at a higher pressure than the adjacent piping at the time of valve opening by pressure trapping and bonnet heatup. Pressure locking is a phenomena which can cause the unseating thrust for a gate valve to increase from its typically static unseating thrust. This can result in the valve actuator having insufficient capability to open the valve. In addition, this can result in valve damage in cases where the actuator capability exceeds the valve structural limits. This paper discusses the primary mechanisms which cause pressure locking. These include sudden depressurization of piping adjacent to the valve and depressurization of fluid trapped in the valve bonnet due to heat transfer. This paper provides a methodology to calculate the unseating thrust for a valve which is pressure locking and applies to motor operated isolation valves of Residual Heat Removal System(RHRS)

[en] Weak Link Analysis (WLA) has been performed to assess the structural integrity of the Motor-Operated Valves (MOVs) according to GL 89-10 for Nuclear Power Plants (NPPs). More than six hundreds of the MOVs have been analyzed, and the most of the MOVs were evaluated by the conventional WLA method. From the experiences of the previously performed WLA, it was found that the most of the weak links are disc-stem T-head connections in opening mode and that the bearing stress on the disc is a dominant stress in opening mode. In these cases, Finite Element Analyses (FEAs) were performed to investigate the real stress state of the parts and to compare with the conventional WLAs. In the FEAs, fatigue analysis of the parts is considered by limiting the maximum von Mises stress and the maximum strain. On the other hand, the most of the weak links in closing mode are yokes including flanges. This paper discusses the FEA including elastic-plastic deformation of the yokes. In addition, the effects of the yoke flange contact were evaluated. It was found that the conventional WLA is sufficiently conservative and higher allowable thrusts are obtained by performing the elastic-plastic FEA. Also, the FEA result without considering flange contact is more conservative than that of considering contact

[en] The stem thrust required to unwedging a gate valve is influenced by the pressure and temperature when the valve is closed and by the changes in these conditions between closure and opening. 'Pressure locking' and 'thermal binding' refer to situations where pressure and temperature effects cause the unwedging load to be much higher than normal. A model of these phenomena has been developed. The effects of pressure and temperature are analyzed to determine the change in this disk-to-seat 'interference'. Flexibilities or stiffness of the disk and body strongly influence the unwedging thrust. Calculation and limited comparison to data have been performed for the RHR motor operated valve designs and scenario. Pressure changes can increase the unwedging thrust when bonnet pressure exceeds the pressure in the adjacent piping and temperature changes can increase the unwedging thrust when a temperature change after closure produces and increase in the disk-to-seat interference

[en] It is now recognized that current testing and maintenance requirements invoke too many inadvertent reactor trips and that operating staff must devote significant amount of time and effort to comply with the requirements. With this recognition, the value and the impact of the proposed changes in the allowed outage time (AOT) and surveillance test interval(STI) are evaluated for the alternating system. Because of the testing and AOT requirements, the alternating system exhibits semi-Markovian characteristics which change states in accordance with a Markov chain but take a nonexponentially distributed amount of time between changes. It is observed from the results that there is an optimal point that gives lowest core damage probability and that the optimal point depends on input parameters. With these results, we can conclude that the methodology developed in this study can be applied to the existing alternating systems to evaluate accurately the various alternatives in the technical specifications

[en] Station blackout is one of major contributors to the core damage frequency (CDF) in many PSA studies. Since station blackout sequence exhibits dynamic features, accurate calculation of CDF for the station blackout sequence is not possible with event tree/fault tree (ET/FT) method. Although the integral method can determine accurate CDF, it is time consuming and is difficult to evaluate various alternative AC source configuration and sensitivities. In this study, a comparison is made between static model and dynamic model and a new methodology which combines static model and dynamic model is provided for the accurate quantification of CDF and evaluation of improvement alternatives. Results of several case studies show that accurate calculation of CDF is possible by introducing equivalent mission time. (author)

[en] The purpose of this paper is to address the structural integrity of the stem disc T-head connection of the motor operated valve assembly by comparing the previously developed methodology with the Finite Element Analysis(FEA). From the experiences of the previously performed weak link analysis, the most of 'weak links' when valve is opened are stem-disc T-head connections, and the bearing stress on the disc is the dominant stress. The results of FEA are compared with that of the classical approaches of the weak link analysis. The higher allowable thrust be represented by performing the elastic-plastic FEA

[en] When the Seismic Margin Analysis(SMA) is conducted, the new structural load generation with Seismic Margin Earthquake(SME) is the time consuming work. For the convenience, EPRI NP 6041 suggests the scaling of the structure load. The report recommend that the fixed base(rock foundation) structure designed using either constant modal damping or modal damping ratios developed for a single material damping. For these cases, the SME loads can easily and accurately be calculated by scaling the spectral accelerations of the individual modes for the new SME response spectra. EPRI NP 6041 provides two simple methodologies for the scaling structure seismic loads which are the dominant frequency scaling methodology and the mode by mode scaling methodology. Scaling of the existing analysis to develop SME loads is much easier and more efficient than performing a new analysis. This paper is intended to compare the calculating results of two different methodologies

[en] A Single Point Vulnerability (SPV) may cause plant transients like reactor trip, turbine/ generator trip, orderated power to 50% or more. In order to improve plant reliability and performance by preventing unexpected plant transients, we, KEPCO-ENC and KHNP, are developing SPV monitoring program. To have a better result of the SPV identification and monitoring, we used a blended method which was comprised of qualitative and quantitative approaches. This method is described herein, representative results of SPV identification are presented