ML021150266
| ML021150266 | |
| Person / Time | |
|---|---|
| Site: | Saint Lucie  |
| Issue date: | 04/24/2002 |
| From: | Reinhart F Division of Systems Safety and Analysis |
| To: | Carpenter C, O'Reilly P NRC/NRR/DIPM, NRC/RES/DRAA |
| Wilson P, NRR/SPSB, 415-1114 | |
| References | |
| Download: ML021150266 (18) | |
Text
April 24, 2002 NOTE TO: Cynthia Carpenter, 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/Signed by M. Caruso for Licensing Section Probabilistic Safety Assessment Branch Division of Systems Safety and Analysis Office of Nuclear Reactor Regulation
SUBJECT:
RESULTS OF THE SAINT LUCIE UNIT 2 SDP PHASE 2 NOTEBOOK BENCHMARKING VISIT During February, 2002, NRC staff and a contractor visited the Florida Power and Light company headquarters to compare the Saint Lucie (STL) Unit 2 Significance Determination Process (SDP) Phase 2 notebook and licensees risk model results to ensure that the SDP notebook was generally conservative. STL Unit 2's 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 the review of the STL Unit 2 SDP notebook, it was found that some changes to the SDP worksheets were needed to reflect how the plant is currently designed and operated. Thirty hypothetical inspection findings were processed through the SDP notebook. Results from this effort indicated that the total risk impacts modeled in the SDP notebook were underestimated by 7 percent, overestimated by 37 percent, and adequately estimated by 57 percent. The reviewers found that if eleven fixes were made to the SDP notebook, the results would be 0 percent underestimation and 3 percent overestimation of risk impacts.
Attachment A describes the process and results of the comparison of the STL Unit 2 SDP Phase 2 Notebook and the licensees PSA.
If you have any questions regarding this effort, please contact Peter Wilson.
Attachments: As stated CONTACT: P. Wilson, SPSB/DSSA/NRR 301-415-1114
April 24, 2002 NOTE TO: Cynthia Carpenter, 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/signed by M. Caruso for Licensing Section Probabilistic Safety Assessment Branch Division of Systems Safety and Analysis Office of Nuclear Reactor Regulation
SUBJECT:
RESULTS OF THE SAINT LUCIE UNIT 2 SDP PHASE 2 NOTEBOOK BENCHMARKING VISIT During February, 2002, NRC staff and a contractor visited the Florida Power and Light company headquarters to compare the Saint Lucie (STL) Unit 2 Significance Determination Process (SDP) Phase 2 notebook and licensees risk model results to ensure that the SDP notebook was generally conservative. STL Unit 2's 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 the review of the STL Unit 2 SDP notebook, it was found that some changes to the SDP worksheets were needed to reflect how the plant is currently designed and operated. Thirty hypothetical inspection findings were processed through the SDP notebook. Results from this effort indicated that the total risk impacts modeled in the SDP notebook were underestimated by 7 percent, overestimated by 37 percent, and adequately estimated by 57 percent. The reviewers found that if eleven fixes were made to the SDP notebook, the results would be 0 percent underestimation and 3 percent overestimation of risk impacts.
Attachment A describes the process and results of the comparison of the STL Unit 2 SDP Phase 2 Notebook and the licensees PSA.
If you have any questions regarding this effort, please contact Peter Wilson.
Attachments: As stated CONTACT: P. Wilson, SPSB/DSSA/NRR 301-415-1114 DISTRIBUTION: SPSB r/f PWilson MReinhart G:\\SPSB\Wilson\Stlucie2bench.wpd Accession#ML021150266 NRR-096 OFFICE SPSB SC:SPSB NAME PWilson:nyc MReinhart DATE 04/22/02 04/24/02 OFFICIAL RECORD COPY
SUMMARY
REPORT ON BENCHMARKING TRIP to the St. Lucie Nuclear Power Plant, Unit 2 (February 25-27, 2002)
PRANAB K. SAMANTA Energy Sciences and Technology Department Brookhaven National Laboratory Upton, NY 11973-5000 March 2002 Attachment A
Table of Contents Page
- 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
- 2. Summary Results from Benchmarking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
- 3. Proposed Modifications to Rev. 0 SDP Notebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1 Specific Changes to the Rev. 0 SDP Notebook for St Lucie, Unit 2 . . . . . . . . . . 9 3.2 Generic Change in 0609 for Inspectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3 Generic Change to the SDP Notebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
- 4. Discussion on External Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
- 5. List of Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 List of Tables Page Table 1. Comparison Table for St. Lucie Unit 2 Benchmarking . . . . . . . . . . . . . . . . . . . . . 4 Table 2. Comparative Summary of the Benchmarking Results . . . . . . . . . . . . . . . . . . . . . 8
-iii-
- 1. Introduction A benchmarking of the Risk-Informed Inspection Notebook for St. Lucie, Unit 2 was conducted during a plant site visit on February 25-27, 2002. NRC staff (R. Bernhard, W. Rogers, and P.
Wilson) and BNL staff (G. Martinez-Guridi and P. Samanta) participated in this Benchmarking exercise.
In preparation for the meeting, BNL staff reviewed the SDP notebook for St. Lucie Unit 2 and evaluated a set of hypothetical inspection findings using the Rev. 0 SDP worksheets. In addition, a copy of the meeting protocol was sent to the licensee by P. Wilson of the NRC prior to the meeting.
The major milestones achieved during this meeting were as follows:
- 1) Licensees comments on the Rev. 0 SDP notebook were discussed and applicable modifications are considered in the benchmarking exercise.
- 2) Importance measures including the Risk Achievement Worths (RAWs) for the basic events in the internal event model for average maintenance was obtained from the licensee.
- 3. Benchmarking was conducted using the Rev. 0 SDP model and the revised SDP model considering the licensee inputs and other modifications that were judged necessary based on comparison of the SDP model and the licensees detailed model.
- 4) For cases where the color evaluated by the SDP notebook differed from that determined based on the RAW values generated by the updated licensees PRA, results of the licensees model including the detailed minimal cutsets were requested from the licensee. The cutsets were reviewed to understand the reason for the differences. Applicable changes were defined for the SDP model.
Following the modifications to the St. Lucie Unit 2 notebook as a result of the benchmarking exercise, it is observed that no underestimation is present for the hypothetical cases studied. In 4 out of 30 cases, a conservative result by one color, i.e., by one order of magnitude, is observed.
This is a significant improvement compared to the Rev. 0 version where some underestimates and a relatively large number of conservative results were obtained. Two of the conservative results may be attributed to licensees modeling and RAW calculations; results of the notebooks may be applicable for assessing the significance of the inspection finding. In addition, the benchmarking exercise identified a difference in assumption between PRA and SDP notebook modeling which contributes to differences in results. This aspect will be explained in the notebook for consideration by the user of the notebook.
- 2. Summary Results from Benchmarking This Section describes the results of the benchmarking exercise. The results are summarized in Table 1. Table 1 consists of six columns. The first column identifies the components or the case runs. The assigned colors from the SDP Rev. 0 worksheets without incorporating any modification from the benchmarking exercise are shown in the second column. The third column shows the internal RAW and the fourth column shows the associated colors estimated based on the Licensee generated RAW values from the latest PRA model. The fifth column presents the colors for the inspection findings based on the revisions of the SDP Rev. 0 worksheets judged applicable during benchmarking. The last column provides comments explaining the differences between the SDP and plant PRA colors.
Table 2 presents a summary of the comparisons between the results obtained using the St. Lucie, Unit 2 Notebook and the plant PRA. The results show that in 4 out of 30 cases, the notebook provides a color that is conservative by one order of magnitude. In the remaining 26 cases, the results match, i.e., both determine the same color.
The cases where a conservative color are obtained are discussed below. In addition, we discuss a difference in an assumption in plant modeling from the notebook that results in differences in results in selected cases as noted in Table 1.
Two of the four cases where the notebook color is conservative by one order-of-magnitude relate to the motor- and turbine-driven AFW pumps. The reason for the differences may be attributable to the common-cause modeling of the AFW pumps in the licensee model and associated RAW calculation. A common-cause failure probability of all three AFW pumps is modeled. In the RAW calculation, this common-cause term remains unaffected. If calculation is performed considering the changes in the common-cause failure probability term due to the inspection finding for a AFW pump, then the results are expected to match.
For the LPI train with SDC HX, notebook result is conservative by one color. This is because of the conservative approach used in approximating the impact in situations where the system redundancy is not affected by the inspection finding. Here, the delta change is approximated by counting the basecase rating of the affected sequences. In addition, sequences with ratings up to 9 was counted. Counting up to 8 would have resulted in a match for this case.
Conservative result for the operator failure to close the block valve is attributable to the difference in SORV frequency and HPSI unavailability data. SORV frequency and the common-cause failure probability of the HPSI pumps in the licensee PRA are lower compared to the credits given in the notebook.
One difference in plant modeling compared to the modeling in the SDP notebook was noted during the benchmarking. This resulted in different colors between plant PRA results and SDP notebook.
But, same color would have been obtained if the same modeling assumption would have been used. We considered those cases to have provided a match, since the same color is obtained when the same assumption is used. This applies to cases involving PORVs (both for findings on PORVs and operator failure to conduct feed and bleed), MSIVs, and operator failure to isolate the feed to the SG. In the plant PRA, it is assumed that following failure of both MSIVs fail to close, core damage is avoided if feed and bleed, i.e., once-through cooling, is successful. In the SDP notebook, it is assumed that core damage will ensue when both the MSIVs fail to close. This difference in assumption results in lower importance for the MSIVs and the operator failure to isolate the feed to the SG in the plant PRA. Also, higher importance is obtained for the PORVs and the operator failure to conduct feed and bleed, i.e., once-through cooling. The assumption in the notebook is consistently made for all the Combustion Engineering (CE) and is not changed.
Justification of this assumption is expected to be taken into consideration in the detailed analysis.
Table 1. Comparison Table for St. Lucie, Unit 2 Benchmarking CDF = 1.46E-05/yr, W = 1.07 (RAW), Y = 1.69 (RAW), R = 7.85 (RAW)
Basic Event Name SDP RAW Plant CDF SDP Comments Before Color After MD AFW Pump train Y 1.19 W Y Conservative by one order of magnitude.
TD AFW Pump train Y 1.47 W Y Conservative by one order of magnitude.
1 HPSI Pump R 5.89 Y Y Match 1 CSS train W 1.0 G G Match 1 Fan Cooler Unit W 1.0 G G Match 1 CCW train R 8.8 R R Match CCW Swing Pump W 1.0 G G Match 1 Charging Pump W 1.0 G G Match 1 Condensate Pump G 1.0 G G Match 1 Feedwater Pump G 1.0 G G Match Basic Event Name Before RAW Plant CDF After Comments Color 1 EDG Y 1.42 W W Match Both Dedicated EDGs R 8.59 R R Match 1 IA Compressor W 1.0 G G Match 1 ICW train R 8.8 R R Match 1 PORV W 1.59 W W Match 2 PORVs R 11.94 R Y Plant credits once-through cooling when both MSIVs fail to close. This results in higher risk-significance for the PORVs in the plant model. Similar assumption in the notebook will result in a match.
1 TCW Train G 1.0 G G Match 1 MSIV Y 1.0 G Y Plant credits once-through cooling when both MSIVs fail to close. This results in lower risk-significance for the MSIV in the plant model. Similar assumption in the notebook will result in a match.
1 SIT Y 4.01 Y Y Match 1 LPSI train with HX W 1.05 W Y Conservative by one order-of-magnitude 1 LPSI pump W 1.15 W W Match Basic Event Name Before RAW Plant CDF After Comments Color 1 AC Bus R 55.55 R R Match 1 DC Bus R 30.1 R R Match. Battery charger can not carry the SI loads Basic Event Name Before RAW Plant CDF After Comments Color Operator Actions Operator fails to FB R 11.94 R Y Plant credits once-through cooling when both MSIVs fail to close. This results in higher risk-significance for the PORVs in the plant model. Similar assumption in the notebook will result in a match.
Operator fails to initiate SDC Y 1.27 W W Match Operator fails to trip RCP to prevent seal LOCA R 611.64 R R Match Operator fails to start MFW following trip G 1.07 W W Match Operator fails to close block valve W 1.04 G W Conservative by one order-of-magnitude.
Operator fails to borate during ATWS W 1.42 W W Match Operator fails to Plant credit once-through cooling when isolate SG during W W R both MSIVs fail to close. Similar steamline break assumption in the notebook will result in a match.
Table 2: Comparative Summary of the Benchmarking Results Comparisons Rev. 0 SDP Notebook Revised SDP Notebook Considering the Changes identified in Benchmarking Total Number of Cases Compared = 30 Number of Cases Percentage Number of Cases Percentage SDP: Less 2 7 0 0 Conservative SDP: More 11 37 4 13 Conservative SDP: Matched 17 57 26 87
- 3. Proposed Modifications to Rev. 0 SDP Notebook A set of modifications were proposed for the Rev. 0 SDP notebook as a result of the site visit.
These proposed modifications are driven by the licensees comments on the Rev. 0 SDP notebook, better understanding of the current plant design features, allowance for additional recovery actions, revised Human Error Probabilities (HEPs), modified initiator frequencies, and the results of benchmarking.
3.1 Specific Changes to the Rev. 0 SDP Notebook for St. Lucie, Unit 2 The licensee provided comments on the Rev. 0 SDP notebook, most of these comments clarify the detail design, procedure, and operational features in the plant and they will be incorporated in the next revision of the SDP. The following comments were considered to be important for the Benchmarking exercise and were considered for color determination.
- 1. Loss of a DC Bus frequency was changed to 5E-04/yr from 8E-04; its placement in Table 1 remained unaffected.
- 2. The full creditable mitigation capability for Containment Heat Removal (CHR) is revised to 2/4 fan coolers or 1/2 containment spray trains with SDC heat exchangers in all applicable worksheets.
- 3. Recovery of PCS is credited in the SLOCA worksheet. An operator action credit of 1 is assigned. The associated HEP is 0.35.
- 4. The credit for the operator action for SDC in SLOCA and SORV is changed to 2.
- 5. Capability to cross-tie other unit EDG is credited with an operator action credit of 1. The associated HEP is 1.1E-2 and the equipment unavailability is 5E-02.
- 6. In the SGTR worksheet, SDC is redefined to exclude the need for HPSI trains. The event tree is modified and the previously defined HPSI function is used. Also, 1/2 charging pump is included in the description for long term RCS makeup from BAM tanks.
- 7. In the ATWS worksheet, the success criteria for SRVO is changed to 3/3 SRVs and 1/1 PORV open from 3/3 SRVs and 1/2 PORVs open; the success criteria for SRVR is changed to 3/3 SRVs and 1/1 PORV reclose from 3/3 SRVS and 1/1 PORV reclose.
- 8. In the MSLB worksheet, the isolation of feed to the faulted SG (ISOF) is redefined to exclude closure of 2/2 MSIVs (1 train). A correction is made to the worksheet and the event tree which eliminates the sequence MSLB - HPSI.
- 9. In the LDCBUS worksheet, the function Bus Transfer (TRANSF) is eliminated.
- 10. In the LDCBUS worksheet, the full mitigation capability for CHR function is revised to 2/2 fan coolers or 1/1 containment spray train (1 multi-train system).
3.2 Generic Change in 0609 for Inspectors Three issues can be identified for consideration in the Inspection Manual 0609
- 1. In evaluating inspection finding for both the unit EDGs where only crosstie capability to the other unit EDG remains, all LOOP sequences may need to be evaluated. It is assumed that the crosstie would enable use of 1 bus, thus the non-SBO sequences will have reduced mitigation capability contributing to the delta CDF. (Same as identified in St. Lucie, Unit 1)
- 2. As noted, for LPSI train including the HX, the notebook gives a conservative estimate. Counting sequences up to the rating of 8 will result in a match. For situations where system redundancy is not lost, i.e., credits are not affected, counting up to 8 should suffice. (Same as identified in St. Lucie, Unit 1)
- 3. Some guidance for inspectors to check with additional calculations where the impact on common-cause failures is included may be useful, if comparisons are being obtained with the licensee PRA. In some situations, not including the impact on common-cause terms may make a difference in the color obtained and calculations considering the impact on common-cause terms will be appropriate to assess the impact.
3.3 Generic Change to the SDP Notebook None identified.
- 4. Discussion on External Events Integrated external event PRA model was not available for the St. Lucie, Unit 2 plant. No evaluation was conducted for the external event risk during the benchmarking exercise.