VGB POWERTECH 7 (2021) - International Journal for Generation and Storage of Electricity and Heat

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Equipment selection methodology of seismic probability safety assessment for nuclear power plants VGB PowerTech 7 l 2021 tion capacity (A m ), and the second is the high confidence of a low probability of failure (HCLPF). First, if A m is set as the screening criteria, it should not contribute to plant safety risk regardless of the results of the site’s PSHA, so a relatively high value should be set as the screening criteria, which can be an overly conservative analysis. Moreover, there is a weakness in that there is no difference in core damage frequency (CDF), which is a risk metric for SPSA between a power plant with a high probability of seismic occurrence on the site and a power plant with a low probability of seismic occurrence. Second, when HCLPF is set as the screening criteria, its value is the result of the convolution of the hazard curve derived by PSHA and fragility of a single piece of equipment. As an example, if 1.00E-05/yr, the convolution value computed in between the fragility of specific equipment and the site hazard curve is set as the screening criterion HCLPF, even if all of this equipment directly causes core damage, its CDF is 5.00E-07/yr, meaning less impact. In other words, HCLPF corresponds to a probability of equipment failure of 0.05 based on 95 % reliability; so conservatively, if the corresponding earthquake occurrence frequency is 1.00E-05/ yr, the final CDF corresponds to 5.00E-07/ yr. This will be analyzed in detail in the following sections; however, the disadvantage is that the SPSA results will be optimistic if seismic-induced failures are excluded on a single screening criterion because equipment that is critical to plant safety shutdown can be used in various seismic-induced initiating events and contributes to the CDF of each initiating event. In the case of the United States, ASME/ ANS RA-S-2008, which can be called a PRA standard, describes as follows [2]: –– DEVELOP seismic fragilities for all those structures, systems, or components, or combination thereof, identified by the systems analysis. –– If screening of high seismic capacity components is performed, DESCRIBE fully the basis for screening and supporting documents. For example, it is acceptable to apply the guidance given in EPRI NP- 6041-SL, Rev. 1, and NUREG/CR-4334 to screen out components with high seismic capacity. However, CHOOSE the screening level high enough that the contribution to core damage frequency and large early release frequency from the screened-out components is not significant. This provides a vague criterion that fragility analysis should be performed to reflect all SSCs in the SPSA model and screening can be applied, and this case should not have a serious impact on both CDF and large early release frequency (LERF). Next, guidance suggesting the US SPSA methodology recommends determining the detailed fragility analysis SSCs through the steps below [1]. –– Step 1: Screen Inherently Rugged Structures, Systems, and Components. –– Step 2: Assign Initial, Representative Fragility Values for Structures, Systems, and Components. –– Step 3: Create and Quantify an Initial Seismic Probability Risk Assessment Model. –– Step 4: Rank the Systems, Structures, and Components by Importance Measure. –– Step 5: Perform Detailed Fragility Calculations for the Risk-Important Structures, Systems, and Components. –– Step 6: Document the Seismic Equipment List Screening Process. All equipment except “inherently rugged structures, systems, and components” is considered in the SPSA model, so the evaluation result is quite conservative and the SPSA model must be established in advance to perform this analysis. In addition, since there are no specific screening criteria for detailed fragility analysis, there is a weakness in that the results may differ depending on the judgment criteria of the performing expert. As mentioned above, it is recommended that all SSCs should be included in the SEL and qualitatively selected and removed within the range that does not affect the entire CDF or LERF; however, no methodology can find a systematic method for this. 3. Sensitivity analysis for single HCLPF screening criteria The Nuclear Energy Institute (NEI) presents the screening criteria for performing Tab. 1. Equipment fragility data for sensitivity case [3]. Component Description A m βU βR HCLPF SEIS-BS-CRANE Seismic failure of polar crane damages reactor vessel causing core damage 1.34 0.3 0.3 0.50 SEIS-RC-RPV Seismic failure of Reactor Vessel 1.34 0.3 0.3 0.50 SEIS-RC-RPVINT Seismic failure of Vessel Internals causes ATWS 1.34 0.3 0.3 0.50 SEIS-SW-FO-TNK Seismic failure of ESW diesel fuel oil tank 1-SW-TK-1 1.25 0.24 0.37 0.45 SEIS-RH-PUMPS Seismic failure of MD RHR pumps 1.03 0.17 0.31 0.45 SEIS-EE-FO-TKS Seismic failure of underground EDG Fuel Oil tanks 1-EE-TK-2A/B 1.07 0.3 0.3 0.40 SEIS-EP-EDG Seismic Failure of the Emergency Diesel Generators (EDG-1, 2, 3) 1.07 0.3 0.3 0.40 SEIS-EP-LCC Seismic failure of Load Control Centers 1-EP-LCC-1H/J,1H/J-1 0.97 0.24 0.31 0.39 SEIS-EP-BATT Seismic failure of station batteries 1(2)-EPD-B-1A/B 1.02 0.19 0.42 0.35 SEIS-EE-DAY-TK Seismic failure of EDG fuel oil day tanks 1.08 0.45 0.24 0.33 SEIS-EP-XFRMR Seismic failure of 4kv-480v transformers 1(2)-EP-TRAN-1H/J and 1H/J1 0.83 0.24 0.25 0.37 SEIS-BC-BCHX Seismic failure of Bearing Cooling HXs results in flood of the Emergency Switchgear Room 0.77 0.25 0.22 0.35 SEIS-CC-CCHX Seismic failure of CCW HX results in flood of the Emergency Switchgear Room 0.68 0.32 0.18 0.29 SEIS-EP-MCC-CV Seismic failure of Motor Control Centers in Cable Vault and Tunnel 0.68 0.32 0.22 0.28 SEIS-EP-MCC-SB Seismic failure of Motor Control Centers in Service Building 0.69 0.23 0.36 0.26 SEIS-EE-FO-PMP Seismic failure of the EDG Fuel Oil Transfer Pumps 0.68 0.3 0.3 0.25 SEIS-FW-TDAFW Seismic failure of Turbine-driven Aux feed water pump 0.68 0.3 0.3 0.25 SEIS-CS-RWST Seismic failure of Refueling Water Storage Tank 1-CS-TK-1 0.55 0.3 0.2 0.24 SEIS-BS-TBLDG Seismic failure of the Turbine building causes loss of canal(SW) and leads to core damage 0.53 0.42 0.23 0.17 SEIS-EP-LOOP Seismic failure of offsite power 0.4 0.22 0.22 0.19 SEIS-CN-ECST Seismic failure of Emergency Condensate Storage Tank 1-CN-TK-1 0.32 0.2 0.07 0.20 SEIS-EP-AAC-DG Seismic failure of Alternate AC Diesel (batteries) 0.13 0.32 0.24 0.05 62
VGB PowerTech 7 l 2021 Equipment selection methodology of seismic probability safety assessment for nuclear power plants Tab. 2. Random failure and human error probability for sensitivity case [3]. Component Description Mean Error Factor Distribution Type 1EEEDG-FR-1EEEG1 EDG-1 Fails to run 1.90E-02 1.26 Log normal 1EEEDG-TM-1EEEG1 EDG-1 in Test or Maintenance 1.90E-02 1.21 Log normal 1SACMP-FS-1SAC2C Service Air Compressor 1-SA-C-2 fails to start 4.22E-03 1.12 Log normal 2EEEDG-FR-2EEEG1 EDG-2 Fails to run 1.90E-02 1.26 Log normal 2EEEDG-TM-2EEEG1 EDG-2 in Test or Maintenance 1.90E-02 1.21 Log normal 3EEEDG-FR-3EEEG1 EDG-3 Fails to run 1.90E-02 1.26 Log normal 3EEEDG-TM-3EEEG1 EDG-3 in Test or Maintenance 1.90E-02 1.21 Log normal AACEDG-TM-DG0M Alternate AC diesel in Test or Maintenance 3.07E-02 1.49 Log normal HEP-C-1AFW-SLO Operator fails to align alternate AFW source during seismic event - low g 2.64E-02 3 Log normal HEP-C-FTSESW-SLO Operator fails to start Emergency SW pump during seismic event - low g 2.54E-03 12.8 Log normal HEP-C-FTSIA-SLO Operator fails to start Instrument Air compressor during seismic event - low g 3.60E-03 3 Log normal PROB-RCCA-2 Fraction of Control Rods that fail to insert that exceeds emergency boration success criteria 5.50E-01 1 Uniform PROB-RCPSL-182 Probability of 182 gpm RCP seal leak 1.98E-01 1 Uniform REC-RHRHX-SLO Operator fails to align RHR during seismic event - low g 3.48E-01 1 Uniform fragility calculation [3], which is similar to the HCLPF screening criteria applied in Korea. To confirm the validity of the screening criteria presented in this report, we performed a sensitivity analysis using the top 50 cutsets of SPSA based on the data contained in this report. The sensitivity analysis consisted of SPSA models assuming 4th seismic acceleration intervals for convenience. As a quantification tool, software using minimal cutsets upper bound (MCUB) methodology was used, but over-estimated CDF was derived from high seismic acceleration interval and quantified using FTeMC [4] software with Monte Carlo simulation. The top 50 cutsets presented in the report consist of 22 seismic-induced failures, ten random failure events, and four human errors events, as shown in Ta b l e s 1 and 2 . These cutsets are also considered in the success sequence. In this paper, sensitivity analysis is carried out on two cases. First, we analyzed the changes in CDF according to the application of the screening criteria using the results of the convolution with the single piece of equipment; and second, we analyzed the changes in CDF when each piece of equipment with similar HCLPF size is not considered in the model. To utilize the results of the first, single piece of equipment convolute with seismic hazard curve. The convolution value was derived using PRASSE [5], which is used in the quantification analysis of seismic-induced initiating events with binary decision diagram method and Monte Carlo simulation. The results are shown in Ta b l e 3 . As Ta b l e 2 shows, the higher the HCLPF value, the smaller the convolution result, but it can be seen that there is a slight difference, depending on β R and β U . Table 4 shows the sensitivity analysis according to two screening criteria. When screening criteria 5.00E-5/yr is applied, four pieces of equipment are excluded from the model, and the CDF reduction is 0.5 % based on the base model, so there is no significant effect. With screening criteria 1.00E-4/yr, the change in the overall CDF is around 4 %, indicating that applying higher Tab. 3. Convolution result for sensitivity case. Component Case Convolution probability SEIS-BS-CRANE screening criteria can have a significant impact on the overall results. Therefore, the appropriate screening criteria was found to be effective because they might not have a significant impact on the entire CDF. HCLPF 3.11E-05 0.50 SEIS-RC-RPV Case 1 3.11E-05 0.50 SEIS-RC-RPVINT Under 5.00E-05 3.11E-05 0.50 SEIS-SW-FO-TNK 4.20E-05 0.45 SEIS-RH-PUMPS 5.89E-05 0.45 SEIS-EE-FO-TKS 6.60E-05 0.40 SEIS-EP-EDG Case 2 6.60E-05 0.40 Under 1.00E-04 SEIS-EP-LCC 8.07E-05 0.39 SEIS-EP-BATT 8.61E-05 0.35 SEIS-EE-DAY-TK 8.62E-05 0.33 SEIS-EP-XFRMR 1.14E-04 0.37 SEIS-BC-BCHX 1.38E-04 0.35 SEIS-CC-CCHX 2.08E-04 0.29 SEIS-EP-MCC-CV 2.16E-04 0.28 SEIS-EP-MCC-SB 2.26E-04 0.26 SEIS-EE-FO-PMP 2.33E-04 0.25 - SEIS-FW-TDAFW 2.33E-04 0.25 SEIS-CS-RWST 3.46E-04 0.24 SEIS-BS-TBLDG 4.69E-04 0.17 SEIS-EP-LOOP 6.56E-04 0.19 SEIS-CN-ECST 9.39E-04 0.20 SEIS-EP-AAC-DG 5.21E-03 0.05 Tab. 4. Sensitivity analysis result for screening criteria. Case Screening Criteria # of screening out component CDF - ΔCDF(%) Base model - - 6.38E-04/yr - Case 1 Under 5.00E-05 4 6.35E-04/yr 0.50 % Case 2 Under 1.00E-04 10 6.14E-04/yr 3.81 % 63
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