ML031980611

Results of the Alvin W. Vogtle Generating Plant Units 1 and 2 Sdp Phase 2 Notebook Benchmarking Visit

ML031980611
Person / Time
Site:	Vogtle
Issue date:	07/16/2003
From:	Reinhart F NRC/NRR/DSSA/SPSB
To:	O'Reilly P, Richards S NRC/NRR/DIPM/IIPB, NRC/RES/DRAA
Wilson P, NRC/NRR/DSSA/SPSB, 415-1114
References
Download: ML031980611 (23)

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July 16, 2003 NOTE TO: Stuart Richards, Chief Inspection Program Branch Division of Inspection Program Management Office of Nuclear Reactor Regulation Patrick D. OReilly Operating Experience Risk Applications Branch Division of Risk Analysis and Applications Office of Nuclear Regulatory Research FROM: Mark F. Reinhart, Chief /RA/

Licensing Section Probabilistic Safety Assessment Branch Division of Systems Safety and Analysis Office of Nuclear Reactor Regulation

SUBJECT:

RESULTS OF THE ALVIN W. VOGTLE GENERATING PLANT UNITS 1 AND 2 SDP PHASE 2 NOTEBOOK BENCHMARKING VISIT During March, 2002, NRC staff and contractors visited the Southern Nuclear Company in Birmingham, Al to compare the Alvin W. Vogtle Generating Plant Units 1 and 2 Significance Determination Process (SDP) Phase 2 notebook and the licensees risk model results to ensure that the SDP notebook was generally conservative. The Vogtle PSA did not include external initiating events; so no sensitivity studies were performed to assess the impact of these initiators on SDP color determinations. In addition, the results from analyses using the NRCs draft Revision 3i Standard Plant Analysis Risk (SPAR) model for Vogtle were compared with the licensees risk model. The results of the SPAR model benchmarking effort will be documented in next revision of the SPAR (revision 3) model documentation.

The benchmarking visit identified that there was a good correlation between the Phase 2 SDP Notebook and the licensees PSA. The results indicate that the Vogtle Phase 2 notebook was generally more conservative in comparison to the licensees PSA. The revision 1 SDP notebook will capture about 87% (results matched or overestimated the licensees PSA by one order of magnitude) of the risk significance of inspection findings. A summary of the results of comparisons of hypothetical inspection findings between the SDP notebook and the licensees PSA are as follows.

CONTACT: Peter Wilson, SPSB/DSSA/NRR 301-415-1114

S. Richards 2 P. ORielly 2.6% (1 of 39 cases) Non-conservative; underestimation of risk significance (by two orders of magnitude) 2.6% (1 of 39 cases) Non-conservative; underestimation of risk significance (by one order of magnitude) 7.7% (3 of 39 cases) Conservative; overestimation of risk significance (by two orders of magnitude) 30.8% (12 of 39 cases) Conservative; overestimation of risk significance (by one order of magnitude) 56.4% (22 of 39 cases) Consistent risk significance One characteristic of the licensees PSA that contributed to a large percentage of the overestimates obtained by the Rev. 1 SDP notebook was the licensees use of initiating event frequencies that were smaller than those of the Rev. 1 SDP notebook. For example, the licensees frequencies for small LOCA and medium LOCA were 4.97E-4/year and 3.98E-5/year, while the notebooks credits for them were 3 and 4, respectively.

The two under estimates were due to modeling differences between the notebook and the Vogtle PSA. See attachment A for further details.

The Rev-1 SDP notebook was improved as a result of the benchmarking activity. The number of cases that the Rev-1 SDP would match that of the updated licensees PSA has increased from 12 to 22. The number of over estimations dropped from 26 to 15 cases. However, the number of underestimations increased from 1 to 2.

The licensees PSA staff was very knowledgeable of the plant model and provided very helpful comments during the benchmark visit.

Attachment A describes the process and results of the comparison of the Vogtle SDP Phase 2 Notebook and the licensees PSA.

Attachment:

As stated

S. Richards 2 P. ORielly 2.6% (1 of 39 cases) Non-conservative; underestimation of risk significance (by two orders of magnitude) 2.6% (1 of 39 cases) Non-conservative; underestimation of risk significance (by one order of magnitude) 7.7% (3 of 39 cases) Conservative; overestimation of risk significance (by two orders of magnitude) 30.8% (12 of 39 cases) Conservative; overestimation of risk significance (by one order of magnitude) 56.4% (22 of 39 cases) Consistent risk significance One characteristic of the licensees PSA that contributed to a large percentage of the overestimates obtained by the Rev. 1 SDP notebook was the licensees use of initiating event frequencies that were smaller than those of the Rev. 1 SDP notebook. For example, the licensees frequencies for small LOCA and medium LOCA were 4.97E-4/year and 3.98E-5/year, while the notebooks credits for them were 3 and 4, respectively.

The two under estimates were due to modeling differences between the notebook and the Vogtle PSA. See attachment A for further details.

The Rev-1 SDP notebook was improved as a result of the benchmarking activity. The number of cases that the Rev-1 SDP would match that of the updated licensees PSA has increased from 12 to 22. The number of over estimations dropped from 26 to 15 cases. However, the number of underestimations increased from 1 to 2.

The licensees PSA staff was very knowledgeable of the plant model and provided very helpful comments during the benchmark visit.

Attachment A describes the process and results of the comparison of the Vogtle SDP Phase 2 Notebook and the licensees PSA.

Attachments: As stated CONTACT: Peter Wilson, SPSB/DSSA/NRR 301-415-1114 Distribution: SPSB: r/f W. Rogers RIII Accession#ML031980611 G:\SPSB\wilson\vogtlebench.wpd NRR-096 OFFICE SPSB SPSB:SC SPSB: RII NAME PWilson: nxh2 MReinhart WRogers DATE 07/15/03 07/16/03 07/15/03 OFFICIAL RECORD COPY

SUMMARY

REPORT ON BENCHMARKING TRIP TO ALVIN W. VOGTLE ELECTRIC GENERATING PLANT UNITS 1 AND 2 G. Martinez-Guridi Brookhaven National Laboratory (BNL)

Energy Sciences and Technology Department Upton, NY 11973 May 29, 2003 Attachment A

Table of Contents Page

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Summary Results From Benchmarking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Proposed Revisions to the Rev. 0 SDP Notebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 Specific Changes to the Rev. 0 SDP Notebook for the Vogtle Electric Generating Plant, Units 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2 Generic Change in IMC 0609 for Guidance to NRC Inspectors . . . . . . . . . . . . 13 3.3 Generic Change to the SDP Notebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4. Discussion on External Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5. List of Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 List of Tables Page Table 1. Summary of Benchmarking Results for Vogtle Units 1 and 2 . . . . . . . . . . . . . . 5 Table 2. Comparative Summary of the Benchmarking Results - Vogtle Units 1 and 2 . . 9

-iv-

1. INTRODUCTION A benchmarking of the Alvin W. Vogtle Electric Generating Plant Units 1 and 2, Significance Determination Process (SDP) Risk-Informed Inspection Notebook was conducted during a plant site visit on March 10-12, 2003. Rudolph Bernhard and Peter Wilson (NRC), supported by Gerardo Martinez-Guridi (BNL), participated in this benchmarking exercise.

In preparation for the plant site visit, BNL staff reviewed the Rev. 0 Vogtle SDP notebook and evaluated a set of hypothetical inspection findings using the Rev. 0 SDP notebook, plant system diagrams and information in the licensees updated PRA.

The major activities performed during this plant site visit were:

1. Discussed licensees comments on the Rev. 0 SDP notebook.

2. Obtained listings of the Risk Achievement Worth (RAW) values for basic events of the internal events PRA model.

3. Identified a target set of basic events (hypothetical inspection findings) for the benchmarking exercise.

4. Performed benchmarking of the Rev. 0 SDP notebook considering the licensees proposed modifications to this notebook.

5. Identified overestimates and reviewed the licensees PRA model to determine the underlying reasons. Additional changes to the SDP notebook were proposed, as appropriate.

Chapter 2 presents a summary of the results obtained during benchmarking, Chapter 3 discusses the proposed revisions to the Rev. 0 SDP notebook, and Chapter 4 discusses the results from both internal and external events. Finally, Attachment 1 shows a list of the participants in the benchmarking activities.

2.

SUMMARY

RESULTS FROM BENCHMARKING Summary of Benchmarking Results Benchmarking of the SDP Notebook for the Alvin W. Vogtle Electric Generating Plant Units 1 and 2 was conducted comparing the risk significance of the inspection findings obtained using the notebook with that obtained using the plant PRA. The benchmarking identified the hypothetical inspection findings for which the results of the evaluation using the notebook were under or overestimations compared to the plant PRA.

Thirty-nine cases of hypothetical findings were evaluated. A summary of the results of the risk characterization of hypothetical inspection findings for Unit 1 is as follows:

2.6% (1 of 39 cases) Non-conservative; underestimation of risk significance (by two orders of magnitude) 2.6% (1 of 39 cases) Non-conservative; underestimation of risk significance (by one order of magnitude) 7.7% (3 of 39 cases) Conservative; overestimation of risk significance (by two orders of magnitude) 30.8% (12 of 39 cases) Conservative; overestimation of risk significance (by one order of magnitude) 56.4% (22 of 39 cases) Consistent risk significance Detailed results of benchmarking are summarized in Table 1 which consists of eight column headings. In the first two columns, the out-of-service components, including human errors, are identified for the case analyses. The colors assigned for significance characterization from using the Rev. 0 SDP notebook before incorporation of the licensees comments are shown in the third column. The licensees basic events for which the RAW was found, representing the hypothetical finding, is presented in the fourth column. The fifth and sixth columns show the RAW values and the associated colors, respectively, based on the licensees latest PRA model. The colors assigned for significance characterization from using the SDP notebook after incorporation of the licensees comments and the outcome of comparing the results between the SDP Rev. 1 notebook and the plant PRA are shown in the seventh column. Finally, the eighth column presents some comments about the evaluations.

A comparative summary of the benchmarking results is provided in Table 2. This table shows the number of cases where the SDP was more or less conservative, the SDP matched the outcome from the licensees PRA model, and the cases not modeled by the licensee. The percentages associated with these cases also are shown on this Table. The Rev. 1 SDP notebook was consistent (same color) in 56.4% of the inspection findings, 38.5% of overestimates, and 5.1% of underestimates.

Observations on the Licensees PRA One characteristic of the licensees PRA contributes to the percentage of overestimates obtained by the Rev. 1 SDP notebook. The licensees frequencies of several initiating events were smaller than those of the Rev. 1 SDP notebook. For example, the licensees frequencies for small LOCA and medium LOCA were 4.97E-4/year and 3.98E-5/year, while the notebooks credits for them were 3 and 4, respectively. Also, ATWS was evaluated by the licensee as an initiating event, such as a turbine trip, and failure of the reactor protection system, and other factors. The frequency of the initiating event and failure of the reactor protection system for the most dominant minimal cut set was about 4E-7/year, while the notebooks credit for ATWSwas6.

Discussion of Non-conservative Results by the Notebook The Rev. 1 notebook yielded two underestimates out of the 39 hypothetical findings evaluated:

operator fails to terminate SI following MSLB, and operator fails to trip RCPs. They are discussed next.

Operator fails to terminate SI following MSLB. The licensees PRA obtained yellow, and the notebook yielded green. The licensees model of MSLB considered that the operator had to terminate SI regardless of whether the MSIVs close or not. On the other hand, the notebooks model of MSLB considered that this operator action was not required if all steam paths were isolated using the MSIVs. Hence, in the notebooks model, the failure to terminate SI was in a sequence with the failure of all steam paths to be isolated, which has a credit of 2. This credit accounted for the difference in colors between the licensees PRA and the notebook.

Operator fails to trip RCPs. The licensees PRA obtained yellow, and the notebook yielded white.

The importance of this operator action in the licensees PRA came from the scenario of a total loss of NSCW. This total loss was not modeled in the SDP notebook because the notebook assumed that, given a total loss of RCP seal cooling due to a total loss of NSCW, a RCP seal LOCA that cannot be mitigated follows, regardless of whether the operator tripped the RCPs or not.

Discussion of Conservative Results by the Notebook The Rev. 1 notebook produced 15 overestimates, 3 by two orders of magnitude, and 12 by one order of magnitude. The three overestimates by two orders of magnitude were: battery charger of bus A fails, operator fails to switch over in LPR, and operator fails to recover AC power in < 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after a LOOP. They are discussed next.

Battery charger of bus A fails. The licensees PRA obtained green, and the notebook yielded yellow. This difference was because of different assumptions in treating this failure by the notebook and the licensee. There were two 100% battery chargers in each DC bus and one battery charger can handle all safety loads. On failure of one battery charger, the other battery charger automatically provides power to the DC bus. In addition, the licensee considered that the loss of the battery charger would be annunciated in the control room, and hence the failure of the charger would not cause the loss of its associated DC bus. On the other hand, the current SDP usage rules assume that without the battery charger the associated battery will discharge under normal loads and result in a loss of the DC bus, and require that each worksheet specified by Table 2 (of the notebook) for the equipment powered by the affected DC train to be solved considering this equipment unavailable.

Operator fails to switch over in LPR. The licensees PRA obtained yellow, and the notebook yielded red (3). In general, for those scenarios in which the SDP notebook models LPR, the licensee modeled shutdown cooling. If shutdown cooling failed, then the licensee credited LPR.

Accordingly, in the licensees model, both shutdown cooling and LPR have to fail for core damage to occur. Hence, the licensees PRA obtained a color that was lower than the one from the notebook because of the additional credit that it took for shutdown cooling.

Operator fails to recover AC power in < 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after a LOOP. The licensees PSA obtained green, and the notebook yielded yellow. The SDP notebook considered that a RCP seal LOCA would occur after a station blackout (SBO) due to loss of RCP seal cooling, and that AC power had to be recovered before core uncovery, roughly estimated in about 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />. On the other hand, in the licensees PSA, the cause of a RCP seal LOCA resulted from a loss of RCP seal cooling mainly due to the loss of NSCW (or combinations of failures of components of NSCW with other components inside the plant) after the LOOP. Accordingly, recovering AC power did not help to prevent core damage after this LOCA. Hence, failing to recover AC power in less than 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after a LOOP had a low risk significance (green) in the licensees PRA.

The 12 overestimates by one color were: one accumulator fails, one pump of CCW fails, one pump of ECW fails, one Atmospheric Relief Valve (ARV) fails to open, one standby pump of NSCW fails, one primary block valve fails to close, one RHR pump fails, AMSAC fails, one PORV fails to close, operator fails to switchover in HPR, operator fails to conduct emergency boration after ATWS, and operator fails to recover one NSCW pump after loss of NSCW.

Changes Incorporated Following Benchmarking Resulting in Updating of Benchmarking Results Following benchmarking, we incorporated some additional changes to the Rev. 1 notebook as follows:

6 BNL requested the licensee to clarify the equipment and success criteria used for depressurization in scenarios of small LOCA. The licensee indicated that this depressurization was achieved by using 2/4 secondary ARVs or 3/3 steam dump valves.

This equipment and success criteria was used to update the following worksheets: SLOCA and LNSCW.

2. The credit for the action Operator trips RCPs was changed to operator action = 2 because the licensee estimated a human error probability (HEP) = 2.25E-2.

3. The ACCW provides cooling for the following components: RCP thermal barrier heat exchanger and the RCP bearing oil coolers, the CVCS letdown heat exchanger, the CVCS excess letdown heat exchanger, and the CVCS seal water heat exchanger. Loss of cooling to the RCP bearing oil coolers could result in overheating and failure of bearing.

Bearing failure, in turn, could cause the shaft to vibrate and thereby result in the potential for seal failure if the RCP was not tripped. Cooling to the RCP thermal barrier heat exchanger was lost, but RCP seal cooling would be provided by the centrifugal charging pumps. Accordingly, for a RCP seal LOCA to occur after a loss of ACCW (LACCW), the operator had to fail to trip the RCPs, or the CCPs failed to provide RCP seal cooling.

Hence, the most severe impact of a LACCW was a RCP seal LOCA, so this loss was covered by the worksheet of small LOCA, and the worksheet of LACCW was deleted.

4. Since the AFW pumps do not depend on ECW and, hence, do not depend on NSCW, the three AFW pumps were given credit in the worksheet for loss of 2 pumps in each train of NSCW (LNSCW).

Table 1: Summary of Benchmarking Results for Vogtle Units 1 and 2 Internal Events CDF is 1.71E-5/year RAW Thresholds are W = 1.06, Y = 1.59, and R = 6.85 No. Component Out of SDP Basic Event Name Internal Plant SDP Comments Service or Failed Worksheet RAW CDF Worksheet Operator Action Results Color Results (Before) (After)

Component 1 Class 1E AC bus A fails Red (2) (over 1ACBSAA02----F 205.8 Red (3) Red (3) by 1) 1ACBSAA02----M (match) 2 Diesel generator of bus Yellow 1DGDGG4001---A 1.997 Yellow Yellow A fails (match) 1DGDGG4001---M (match) 1DGDGG4001---X 3 Vital 125 VDC bus A Red (2) (over 1DCBSAD1-----F 43.73 Red (3) Red (3) fails by 1) 1DCBSAD1-----M (match) 4 Battery of bus A fails Red (4) (over 1DCBYAD1B----F 2.095 Yellow Yellow by 1) 1DCBYAD1B----M (match) 5 Battery charger of bus A Red (4) (over 1DCBCAD1CA---F 1.000 Green Yellow (over fails by 3) 1DCBCAD1CA---M by 2) 6 One pump of ACCW Yellow (over 1XCPMP4-001--A 1.435 White White fails by 1) 1XCPMP4-001--M (match) 1XCPMP4-001--X 7 One accumulator fails Yellow (over 1ATTKV6002---R 1.217 White Yellow (over by 1) by 1) 8 One MDP of AFW fails Red (4) (over 1AFPMP4003---A 2.697 Yellow Yellow by 1) 1AFPMP4003---M (match) 1AFPMP4003---X 9 TDP of AFW fails Yellow 1AFPTP4001---A 1.766 Yellow Yellow (match) 1AFPTP4001---M (match) 1AFPTP4001X 10 One pump of CCW fails Yellow (over 1CCPMP4-001--A 1.000 Green White (over by 2) 1CCPMP4-001--M by 1) 1CCPMP4-001--X 11 One condensate pump Green 1CDPM-----001M 1.000 Green Green fails (match) 1CDPM-----001X (match) 1CDPM001-----A

No. Component Out of SDP Basic Event Name Internal Plant SDP Comments Service or Failed Worksheet RAW CDF Worksheet Operator Action Results Color Results (Before) (After) 12 One pump of MFW fails Green 1FWPT004-----X 1.000 Green Green (match) (match) 13 One CVCS centrifugal Yellow (over 1CPPMCCPA----A 1.434 White White charging pump fails by 1) 1CPPMCCPA----M (match) 1CPPMCCPA----T 1CPPMCCPA----X 14 One boric acid transfer Green 1BAPMBAA-----A 1.000 Green Green pump fails (match) 1BAPMBAA-----M (match) 1BAPMBAA-----X 15 One pump of ECW fails Red (3) (over 1ECPM001-----A 1.094 White Yellow (over by 3) 1ECPM001-----M by 1) 1ECPM001-----X 16 One air compressor of Green 1IACM501-----A 1.000 Green Green IA fails (match) 1IACM501-----M (match) 1IACM501-----X 17 One Atmospheric Relief Green 1MSARVPV3000-D 1.000 Green White (over Valve (ARV) fails to (match) by 1) open 18 One Main Steam Green 1MSMSIV-3006AK 1.000 Green Green Isolation Valve (MSIV) (match) (match) fails to close 19 One standby pump of Red (4) (over 1SWPMP4-005--A 1.380 White Yellow (over NSCW fails by 2) 1SWPMP4-005--M by 1) 1SWPMP4-005--X 20 One PORV fails to open Yellow (over 1RCPORV0455A-D 1.489 White White by 1) (match) 21 One primary block valve White (over 1RCMV8000A---K 1.021 Green White (over fails to close by 1) by 1) 22 One primary safety White 1RCPSV8010A--D 1.066 White White valve fails to open (match) (match) 23 One RHR pump fails Red (4) (over 1LPPMRHRA----A 5.343 Yellow Red (4) (over by 1) 1LPPMRHRA----M by 1) 1LPPMRHRA----X

No. Component Out of SDP Basic Event Name Internal Plant SDP Comments Service or Failed Worksheet RAW CDF Worksheet Operator Action Results Color Results (Before) (After) 24 One SI pump fails White (over 1SIPMSIA-----A 1.002 Green Green by 1) 1SIPMSIA-----M (match) 1SIPMSIA-----X 25 One pump of TPCCW White (over 1TCPM001-----A 1.000 Green Green fails by 1) 1TCPM001-----M (match) 1TCPM001-----X 26 One pump of TPCW White (over 1TPPM501-----A 1.001 Green Green fails by 1) 1TPPM501-----M (match) 1TPPM501-----X 27 AMSAC fails White (over AMSAC 1.001 Green White (over by 1) by 1) 28 One PORV fails to close Yellow (over 1RCPORV0455A-K 1.562 White Yellow (over by 1) by 1)

Operator Actions 29 Operator fails to Red (4) (over OAB----------H 2.586 Yellow Yellow conduct Feed/Bleed by 1) OAB_SI-------H (match)

OAB_TR-------H 30 Operator fails to Red (4) (over OAR_HPATA----H 4.996 Yellow Red (4) (over switchover in HPR by 1) OAR_HPATB----H by 1)

OAR_HPML-----H OAR_HPSBO----H OAR_HPSG-----H OAR_HPSL-----H OAR_HPSLA----H OAR_HPSLB----H OAR_HPSSI----H OAR_HPSSO----H OAR_HPTR-----H

No. Component Out of SDP Basic Event Name Internal Plant SDP Comments Service or Failed Worksheet RAW CDF Worksheet Operator Action Results Color Results (Before) (After) 31 Operator fails to Red (3) (over OAR-LPMLA----H 5.562 Yellow Red (3) (over switchover in LPR by 2) OAR_LPLL-----H by 2)

OAR_LPMLB----H OAR_LPSL-----H OAR_LPSLC----H OAR_LPSLD----H 32 Operator fails to recover White 1HR-1 1.083 White White AC power in < 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> (match) (match) after a LOOP 33 Operator fails to recover Yellow (over XHR-61 1.004 Green Yellow (over AC power in < 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> by 2) XHR-62 by 2) after a LOOP XHR-63 34 Operator fails to White OAD_SGR------H 1.093 White White depressurize RCS (match) (match) using SGs to less than setpoint of relief valves of SG after SGTR 35 Operator fails to White (over OA-OBR-------H 1.002 Green White (over conduct emergency by 1) by 1) boration after ATWS 36 Operator fails to recover Red (4) (over OA-OSW-------H 3.028 Yellow Red (4) (over one NSCW pump after by 1) by 1) loss of NSCW 37 Operator fails to trip Red (3) (over OA-ORC-A-1---H 4.200 Yellow White (under RCPs by 2) by 1) 38 Operator fails to isolate White OAI_SG-------H 1.094 White White affected SG (match) (match) 39 Operator fails to Green (under OAT----------H 2.032 Yellow Green (under terminate SI following by 2) by 2)

MSLB

Table 2: Comparative Summary of the Benchmarking Results - Vogtle Units 1 and 2 SDP Notebook SDP Notebook SDP Notebook Before (Rev. 0) After (Rev. 1) gives...

Number of Percentage Number of Percentage Cases Cases Underestimate by 1 2.6 1 2.6 two colors Underestimate by 0 0 1 2.6 one color More conservative 19 48.7 12 30.8 by one color More conservative 5 12.8 3 7.7 by two colors More conservative 2 5.1 0 0 by three colors Matched 12 30.8 22 56.4 Total 39 100.0 39 100.1 (1)

Note:

1. The total percentage is not exactly 100 because of rounding of the individual percentages.

3. PROPOSED REVISIONS TO THE REV. 0 SDP NOTEBOOK Based on insights gained from the plant site visit, a set of revisions are proposed for the Rev. 0 SDP notebook. The proposed revisions are based on the licensees comments on the Rev. 0 SDP notebook, better understanding of the current plant design features, consideration of additional recovery actions, use of revised Human Error Probabilities (HEPs) and initiator frequencies, and the results of benchmarking.

3.1 Specific Changes to the Rev. 0 SDP Notebook for the Vogtle Electric Generating Plant, Units 1 and 2 The NRC staff participating in the benchmarking and the licensee provided several comments on the Rev. 0 SDP Notebook. In addition, several major revisions that directly impacted the color assignments by the SDP evaluation were discussed with the licensee and their resolutions were identified in the meeting. Several significant changes that had an impact on the evaluation of the notebook were incorporated during the visit, including revised HEPs and initiator frequencies. The proposed revisions are discussed below:

1. The worksheet for Loss of IA (LIA) was removed from the notebook because all findings involving the compressors of IA are green regardless of the duration of the finding.

2. The worksheet for Loss of One Train of Control Building (CB) ESF Electrical HVAC (HVAC) was removed from the notebook because it causes the same impact as a loss of one 125 VDC bus (LBDC).

3. Table 1 and Table 2. Added footnote indicating that the licensee models two initiating events related to NSCW: Loss of 2 Pumps in Each Train of NSCW and Total loss of NSCW. The former is modeled by a worksheet in this notebook, and the latter leads to core damage directly. Total loss of NSCW has a frequency of 5.52E-5/year. The NSCW provides cooling for the following components: EDGs, CCW heat exchangers, ACCW heat exchangers, ECW chillers, and ECCS (RHR, SIP, CCP) pump motor coolers and CS pump motor coolers.

ACCW can continue for a short time (approximately half an hour) before system failure occurs.

The ACCW provides cooling for the RCP thermal barrier and to the RCP bearing oil coolers.

Hence, the result of a total loss of NSCW is a RCP seal LOCA that cannot be mitigated because the mitigating equipment is unavailable.

4. Table 2. Updated the plant internal event CDF (excluding internal floods) to 1.71E-5/year.

5. Table 2. Added footnote indicating that one train of Component Cooling Water (CCW) is normally running, and the other is in standby.

6. Table 2. 4.16 kV AC was removed from the support systems of the boric acid transfer pumps.

7. Table 2. Added footnote indicating that loss of one class 1E 4.16 kV bus does not cause a reactor trip. Accordingly, this loss is not modeled as an initiating event.

BNL #04334 May 14, 2003

8. Table 2. Added footnote indicating that the fuel oil day tank can support the EDGs without the fuel transfer pumps during 2.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. There is no gravity feed from the EDG fuel storage tank to the EDG fuel oil day tank.

9. Table 2. Added footnote indicating that without AC power, battery capacity is 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> with load shed.

10. Table 2. Added footnote indicating that there are two 100% battery chargers in each DC bus and one battery charger can handle all safety loads. On failure of one battery charger, the other battery charger automatically provides power to its associated DC bus. The inspection findings related to the batteries of DC buses 1AD1 and 1BD1 should be evaluated by assuming the loss of the associated DC bus when offsite power is not available (i.e., LOOP and LEAC worksheets), and increasing the frequency of loss of DC initiator by one order of magnitude. The inspection findings related to the batteries of DC buses 1CD1 and 1DD1 should be evaluated by assuming the loss of the associated DC bus when offsite power is not available (i.e., LOOP and LEAC worksheets).

11. Table 2. 4.16 kV AC was added to the support systems of the Essential Chilled Water (ECW).

12. Table 2. Added footnote indicating that the MSIVs fail closed on loss of DC or IA.

13. Table 2. 480 VAC was added to the support systems of the Nuclear Service Cooling Water (NSCW).

14. Table 2. Added footnote indicating that the Nuclear Service Cooling Water (NSCW) keeps running after a reactor trip or a safety injection signal. However, it is automatically tripped after a LOOP.

15. Table 2. Added footnote indicating that the primary block valves are normally open and have automatic re-closure on low pressure of the reactor coolant system.

16. Table 2. NSCW was removed from the support systems of the RHR/LPSI.

17. Table 2. In the row for RHR/LPSI, added footnote indicating that the CCW is only required for the RHRs heat exchangers.

18. Table 2. Added footnote indicating that room cooling is not required by the following systems:

AFW, NSCW, RHR/LPSI, and SI.

19. Table 2. The column Initiating Event was updated to account for the changes in the worksheets, as described in this document.

20. In all worksheets that use Feed/Bleed, the success criteria for this function was revised according to updated information provided by the licensee.

21. In all worksheets that use the ARVs as one of the paths for steam relief, the success criteria was re-arranged to make it clearer to the reader as follows: (AFW) to 2/4 SGs with (1/5 SG safety valves or 1/1 ARV) on each SG fed by AFW

22. Transients With Loss of PCS (TPCS). The ARVs were added as a path for steam relief.

23. Small LOCA (SLOCA), Stuck-open PORV (SORV) and Loss of 2 Pumps in Each Train of NSCW (LNSCW). Added footnote indicating that the licensee distinguishes two types of RCS Cooldown/Depressurization: when high pressure injection is available (normal cooldown) and when it is not available (rapid cooldown for LPI). The licensee uses the same equipment and success criteria for both types of RCS Cooldown/ Depressurization, but different human error probabilities (HEPs). For normal cooldown, the licensee estimated a HEP = 3.7E-3, and for rapid cooldown for LPI, the licensee estimated a HEP = 7.7E-3.

24. Stuck-open PORV (SORV). Added new event tree developed specifically for SORV. The sequence numbers in the worksheet were updated to match those in the new event tree.

25. Loss of Offsite Power (LOOP). Added operator using 1/1 EDG from other unit to the safety function Emergency AC Power (EAC). Added footnote indicating that the licensee estimated a HEP = 0.2 for cross-tying the EDGs from the other unit. To account for dependencies, we gave a total credit of 1 multi-train system to this function.

26. Loss of Offsite Power (LOOP). The ARVs were added as a path for steam relief.

27. Loss of Offsite Power (LOOP). In the Rev. 0 notebook, our understanding was that the licensee did not credit the use of the safety injection pumps for feed and bleed in the sequences where EAC failed and AC power is later restored. In this scenario, 2/2 charging pumps were required. The licensee indicated during the benchmarking visit that the success criteria for feed and bleed had changed to 1/2 charging trains and 1/2 PORVs. The event tree and worksheet were changed to implement the licensees current success criteria for feed and bleed.

28. Steam Generator Tube Rupture (SGTR). The equipment and success criteria used by the operator to isolate the affected SG were added: 1/2 MSIVs associated with the affected SG and AFW discharge valves.

29. Steam Generator Tube Rupture (SGTR). Added footnote indicating that the licensee does not credit the affected SG.

30. Anticipated Transients Without Scram (ATWS). The success criteria for steam relief was added: 4/4 SGs with 4/5 safety valves opening on each SG.

31. Loss of 2 Pumps in Each Train of NSCW (LNSCW). The structure of the event tree was modified because the Rev. 0 event tree modeled a total loss of NSCW. After loss of 2 pumps in each train of NSCW, the licensee credits establishing operation of one pump of NSCW.

This recovery action provides cooling to one train of mitigating equipment, even though each NSCW pump has 50% capacity. If this recovery action fails, core damage follows due to an RCP seal LOCA that cannot be mitigated. If this recovery action is successful, but an RCP seal LOCA occurred because the RCPs were not tripped, this LOCA may be mitigated with one train of mitigating equipment.

32. Loss of 2 Pumps in Each Train of NSCW (LNSCW). The credit for the action Plant staff recovers one NSCW pump to provide cooling to one train of mitigating equipment was changed from operator action = 2 to operator action = 3 because the licensee estimated a HEP = 3.9E-3.

33. Loss of One 125 VDC Bus (LBDC). The ARVs were added as a path for steam relief.

34. Loss of One 125 VDC Bus (LBDC). Since one primary PORV is unavailable, and feed and bleed using the SI pumps requires two PORVs, these pumps were removed from the equipment used to implement feed and bleed.

35. Loss of One 125 VDC Bus (LBDC). Enhanced footnote 1 to indicate that the frequency of loss of 125 VDC bus 1AD1 is 4.5E-3/year, and the frequency of loss of 125 VDC bus 1BD1 is the same. Hence, the frequency of loss of 125 VDC bus 1BD1 or 1AD1 is 9.0E-3/year.

36. Loss of One 125 VDC Bus (LBDC). Added footnote indicating that the TDAFW pump steam supply valve (HV-5106) is normally closed, and it is required to open. It is powered by the 125 VDC panel 1CD1M which is powered by the 125 VDC bus 1CD1. Hence, after loss of 125 VDC bus 1AD1 or bus 1BD1, this valve may be powered by the 125 VDC bus 1CD1.

3.2 Generic Change in IMC 0609 for Guidance to NRC Inspectors Based on the lessons from this benchmarking, a recommendation for improving 0609 is as follows:

6 The rule 1.3 of 0609 indicates that For inspection findings that involve the unavailability of one train of a multi-train, normally cross-tied support system that increases the likelihood of an initiating event, increase the Initiating Event Likelihood by one order of magnitude for the associated special initiator. After observations during benchmarking of Vogtle and other plants, we suggest that for systems having two or more trains, this rule be modified so that the Initiating Event Likelihood for the associated special initiator is increased by two orders of magnitude when the unavailable train is normally in standby.

6 For the loss of a battery charger of a DC bus, the rules for SDP evaluation assume that the associated DC bus will be lost as a result of the failure of the battery charger because the associated battery will discharge under normal loads, and the rules require that each worksheet specified by Table 2 (of the notebook) for the equipment powered by the affected DC train be solved considering this equipment unavailable. On the other hand, the loss of the charger at Vogtle is annunciated in the main control room and hence this loss would not cause the loss of its associated DC bus. Accordingly, it is recommended that the rule for SDP evaluation of a battery charger be revised to account for the possibility that the associated DC bus will not be lost as a result of the failure of the battery charger. This issue also was observed while benchmarking the Seabrook and Farley plants.

3.3 Generic Change to the SDP Notebook No generic change to the SDP notebook was identified.

4. DISCUSSION ON EXTERNAL EVENTS The licensee does not have a PRA model for external events.

5. LIST OF PARTICIPANTS Rudolph Bernhard Nuclear Regulatory Commission/Region II Peter Wilson Nuclear Regulatory Commission/Office of Nuclear Reactor Regulation Anees Farruk Southern Company Young G. Jo Southern Company Owen M. Scott Southern Company Roger A. Hayes Southern Company John A. Schroeder Idaho National Engineering and Environmental Laboratory Gerardo Martinez-Guridi Brookhaven National Laboratory