atw - International Journal for Nuclear Power | 06.2021

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atw Vol. 66 (2021) | Issue 6 ı November ENVIRONMENT AND SAFETY 34 | Fig. 3. Flowchart for the equipment selection methodology. 5 Case study for the equipment selection methodology In this paper, we compare and analyze the seismic PSA results of the reference nuclear power plant using the proposed methodology. Existing PSA results could not be considered, as detailed data on the walkdown could not be collected. However, even if the walkdown is excluded, the results are sufficient to demonstrate the effectiveness of the methodology proposed in this paper. The first step is the analysis of site-specific PSHA results. The review of the corresponding hazard curves indicates that the probability of earthquake occurrence is relatively low, and the probability of exceeding 0.2g on the SSE basis corresponds to 1.06E-04/yr, corresponding to the Region C medium risk region proposed in this paper. In terms of the likelihood of earthquake occurrence, it is a value that is approximately 54 % of the total power plants in terms of seismic hazard considered by Figure 2. Based on this, the plant needs to review equipment groups I and II subject to fragility analysis. Fragility analysis groups I and II include general vulnerable equipment including offsite power sources and yard tanks, as well as general active equipment. The next analysis is the plant seismic response analysis using internal events PSA results; 74 basic events were selected with an FV value of 0.005 or more and 253 basic events with a RAW value of two or more. Among them, 30 pieces of equipment that satisfy both FV and RAW are considered, along with 297 basic events. Excluding 20 human error basic events and 26 non-seismic basic events, 251 basic events are considered. In addition, except for valves, flow elements, flow transmitters, radiation transmitters, dampers and filters, which are inherently rugged SSCs, the total 136 basic events are derived. The 136 basic events derived can be divided into related systems and equipment types to achieve the following 14 critical system functions. Here, if we review the 14 important functions, we can confirm easily the unique operating characteristics of the reference plant. p Auxiliary feedwater (AF) supply p AF pump room cooling p Emergency power supply p Component cooling water supply p Diesel generator fuel supply p Diesel generator room cooling p Essential chilled water supply p High-pressure injection p Low-pressure injection p Plant control p Ultimate heat sink p Reactor containment cooling p Safety actuation signal p Ultimate heat sink pump room cooling In the end, cross-examining 14 critical systems and functions with fragility equipment groups I and II, the results shown in Table 11 are obtained. The number of equipment items derived through cross-examination is a total of 23, which is a very small result considering the overall equipment in nuclear power plants. How ever, when reviewing the pre viously analyzed SPSA results, it can be confirmed that all devices are considered important in the existing SPSA model except for the three pieces of equipment that initiate seismic-induced initiating events, so the methodology proposed in this paper is very efficient. It can be confirmed that it is reasonable. 6 Conclusion In this study, a methodology of the equipment selection for SPSA is proposed. The single HCLPF screening criterion which has been applied for SPSA reflects some of the site-specific PSHA results but does not reflect the plant design characteristics, so if the single HCLPF screening criterion is applied to the model based on this, an optimistic evaluation can be made. The methodology proposed in this paper has the following advantages: p Safety aspects: Single HCLPF screening criteria is not applied, and all equipment is not reflected in the model using general fragility data, so realistic and reasonable SPSA results are expected. Environment and Safety Equipment Selection Methodology of Seismic Probability Safety Assessment for Nuclear Power Plant ı Junghyun Ryu and Moosung Jae
atw Vol. 66 (2021) | Issue 6 ı November Group Category Equipment Mitigation Function Selected equipment I Electrical Equipment Offsite power – – Structures and Tanks Large flat-bottomed tank Auxiliary feed water supply Condensate storage tank Electrical Equipment 480 VAC Motor Control Center (MCC) / Switchgear / Bus (unanchored) High pressure injection Low pressure injection Diesel generator fuel supply Plant control Refueling water storage tank Refueling water storage tank EDG storage tank Load center Motor control center Electrical Equipment 5 kV+ AC Bus / Switchgear (Unanchored) Plant control Switchgear Electrical Equipment DC Motor Control Center (MCC) / Switchgear / Bus (Unanchored) Plant control DC Panel Electrical Equipment Panel (Electric or Instrument) (Unanchored) Plant control Electrical panel Pump & Compressors Diesel-driven pump (non-safety) – – Structures and Tanks Turbine building – – Structures and Tanks Other non-safety buildings – – Piping Buried piping (non-safety pipe) – – Piping Distributed piping system (welded steel pipe) – – II Electrical Equipment Cable tray – – Pump & Compressors Motor-driven pump Auxiliary feed water supply AF motor driven pump Component cooling water supply Essential chilled water supply Diesel generator fuel supply High pressure injection Low pressure injection Ultimate heat sink Component cooling water pump Essential chilled water pump EDG fuel pump HPSI pump LPSI pump NSCW pump Pump & Compressors Turbine-driven pump Auxiliary feed water supply AF turbine driven pump Electrical Equipment 480 VAC Motor Control Center (MCC) / Switchgear / Bus (Anchored) Plant control Piping Distributed piping system (cast iron pipe) – – Load center Motor control center Structures and Tanks Small tank Component cooling water supply Component cooling surge tank HX & HVAC Chiller Essential chilled water supply Chiller Piping Small LOCA (SLOCA) – – HX & HVAC Air handling unit (AHU) / Room Cooler / Fan AF pump room cooling AF room AHU Diesel generator room cooling Reactor containment cooling Ultimate heat sink pump room cooling EDG room AHU RCFC fan NSCW room AHU Electrical Equipment Panel (Electric or Instrument) (Anchored) Plant control Electrical panel Electrical Equipment Relay Chatter Failure (During Seismic Event) Plant control Electrical panel Pump & Compressors Large vertical, centrifugal pump (motor-driven, non-safety) – – Electrical Equipment Batteries Plant control Electrical panel Structures and Tanks Reactor internals and core assembly – – Piping Buried piping (safety pipe) – – Electrical Equipment Transformer (Indoor) – – Structures and Tanks Containment building / drywell – – Electrical Equipment Emergency Diesel Generator (EDG) Emergency power supply Emergency diesel generator ENVIRONMENT AND SAFETY 35 | Tab. 11. Selected equipment for fragility analysis. p Economics aspects: As the largest portion of the required manpower for SPSA is the detailed fragility analysis work, the methodology proposed in this paper is economically beneficial as the number of pieces of equipment subject to SPSA decreases. The equipment selection methodology proposed in this paper requires analysis of all three parts of SPSA, unlike the previously proposed methodology. This means that selecting equipment through consideration of only one part, such as PSHA, may lead to incorrect results. SPSA has its own uncertainty, so if one factor affects several steps, the uncertainty becomes very large. Therefore, the analysis of equipment not essential for SPSA can increase this uncertainty, so it can be said that it is necessary for a much more realistic analysis that is not considered in advance. In recent years, SPSA has evolved from a single unit criterion evaluation to an evaluation of multiple units operating at one site. In this case, the number of equipment items handled by the SPSA increases, resulting in Environment and Safety Equipment Selection Methodology of Seismic Probability Safety Assessment for Nuclear Power Plant ı Junghyun Ryu and Moosung Jae
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atw - International Journal for Nuclear Power | 06.2021

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