ML20209G750
| ML20209G750 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 07/08/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20209G748 | List: |
| References | |
| GL-88-20, NUDOCS 9907190239 | |
| Download: ML20209G750 (6) | |
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NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20655-0001 l
l STAFF EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION INDIVIDUAL PLANT EXAMINATION OF EXTERNAL EVENTS TEXAS UTILITIES ELECTRIC COMPANY COMANCHE PEAK STEAM ELECTRIC STATION. UNITS 1 AND 2 DOCKET NOS. 50-445 AND 50-446
1.0 INTRODUCTION
On June 28,1991, the NRC issued Generic Letter (GL) 88-20, Supplement 4 (with NUREG-1407, Procedural and Submittal Guidance), requesting all licensees to perform individual plant examinations of external events (IPEEE) to identify plant-specific vulnerabilities to severe accidents and to report the results to the Commission together with any licensee-determined improvements and corrective actions. In a letter dated June 27,1995, Texas Utilities Electric Company (the licensee), submitted its response to NRC for Comanche Peak Steam Electric Station (CPSES), Units 1 and 2.
The staff contracted with Energy Research, Inc. (ERI), to conduct a completeness and reasonableness " Step 1" review of the licensee's IPEEE submittal in May 1995 and sent a request for additional information (RAl) to the licensee on December 12,1995. The licensee responded to the RAI on February 1,1996, and ERI completed its technical evaluation report (TER) in December 1996. Based on the results of its review, the staff concluded that the aspects of seismic; fires; and high winds, floods, transportation, and other external events were adequately addressed. The review findings are summarized in the evaluation section below.
Details of the contractor's findings are presented in the TER attached to this staff evaluation (SE).
In accordance with Supplement 4 to GL 88-20, the licensee provided information to address the resolution of Fire Risk Scoping Study issues, Generic Safety Issue (GSI)-57," Effects of Fire Protection System Actuation on Safety-Related Equipment," GSI-131, " Potential Seismic Interaction involving the Movable in-Core Flux Mapping System Used in Westinghouse Plants,"
GSI-103," Design for Probable Maximum Precipitation (PMP)," and Unresolved Safety Issue (USI) A-45, " Shutdown Decay Heat Removal Requirements." The issues were explicitly requested in Supplement 4 to GL 88-20 and its associated guidance in NUREG-1407. The licensee did not propose to resolve any additional USIs or GSis as part of the CPSES IPEEE.
2.0 EVALUATION The CPSES plant is a two-unit, Westinghouse 4-loop pressurized-water reactor with a large dry containment. The plant was designed to a seismic acceleration level of 0.12g peak ground acceleration anchored to a Regulatory Guide 1.60 spectrum and was categorized in 9907190239 990708 PDR ADOCK 05000445 P PDR
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2 the Electric Power Research Institute (EPRI) seismic margin methodology for a reduced-scope plant. In the fire area, the licensee performed a Level 1 fire probabilistic risk assessment (PRA), used the fire-induced-vulnerability evaluation (FIVE) methodology for screening, and conducted a qualitative containment performance assessment for the CPSES IPEEE. In the high winds, floods, transportaticn and other external events (HFO) area, the licensee performed a tornado analysis using PRA techniques; other external events were assessed using a screening approach as described in NUREG-1407.
Core Damaae Freauency Estimates Since CPSES is a reduced-scope plant, the licensee's seismic analysis did not require quantitative estimates of seismic core damage frequency (CDF) or high confidence low probability of failuro; the licensee conducted a seismic walkdown in order to identify potential vulnerabilities. The licensee estimated a fire CDF of 2.1E-5/ reactor-year (RY). (The licensee estimated a CDF due to internal events of about 6E-5/RY, including internal flooding.)
The licensee estimated a CDF of 3.7E-6/RY due to tornadoes. Other external events (e.g., external floods and others) are considered insignificant contributors to severe accidents at CPSES.
Dominant Contributors The licensee conducted seismic walkdowns using the EPRI seismic margin assessment procedures. A safe shutdown equipment list was developed based on preferred and alternate success paths that assume loss-of-offsite power (LOSP) and equivalent small break loss-of-coolant accident (LOCA) conditions. The preferred success path relies on secondary decay heat removal (DHR) via the auxiliary feedwater system, whereas the alternate success path relies on bleed-and-feed cooling. The licensee did not identify any seismic vulnerabilities.
The fire CDF is dominated by a fire in the control room (37 percent of the total external event CDF). The important system / equipment contributors to the estimated fire CDFs that appear in the top sequences are associated mostly with operator failure to control the plant from the remote shutdown panel. These scenarios are modeled by LOSP and failure to align a diesel generator, with a resulting station blackout.
The licensee estimated that tornadoes contribute about 15 percent to the total CDF from external events.
The licensee's IPEEE assessment appears to have examined the significant initiating events and dominant accident sequences.
Containment Performance The licensee has assessed containment performance under seismic conditions at CPSES by reviewing the capability of various containment systems. The licensee has performed seismic containment walkdowns, including the assessment of the capability of the containment spray system and the containment isolation system to withstand seismic events.
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The licensee has also reviewed containment pedormance as part of the evaluation of
- l. firo-initiated accident sequences. The licensee concludes that, other than for fire-caused interfacing systems LOCA events, containment performance is the same as that analyzed in the individual plant examination.
The licensee's containment performance analyses for seismic and intemal fire events appeared to have considered important containment performance issues and are consistent with the intent of Supplement 4 to GL 88-20.
Generic Safety issues As a part of th' e IPEEE, a set of generic and unresolved safety issues (USI A-45, GSI-131, GSI 103, GSI-57, and the Sandia Fire Risk Scoping Study issues) were identified in Supplement 4 to GL 88-20 and its associated guidance in NUREG-1407. as needing to be addressed in the IPEEE. The staff's evaluation of these issues is provided below.
- 1. ' USl A-45, " Shutdown Decay Heat Removal (DHR) Requirements" The licensee's process of addressing USI A-45 external events was similar to that used for internal events quantification. The fire PRA event trees and plant system fault trees j were based on the internal event / fault trees and modified for fire events. The staff finds ;
that the licensee's USl A-45 evaluation is consistent with the guidance provided in Section 6.3.3.1 of NUREG-1407, and therefore the staff considers this issue resolved. l
- 2. GSI-131, " Potential Seismic Interaction involving the Movable In-Core Flux Mapping i System Used in Westinghouse Plants" l 1
The licensee's process of addressing GSI-131 involved an evaluation of the seismic adequacy of the flux-mapping system. The licensee found that the flux-mapping system i
, is capable of withstanding safe shutdown earthquake (SSE)ioads, The staff finds that the licensee's GSI-131 evaluation is consistent with the guidance provided in Section 6.3.3.1 ;
of NUREG-1407, and therefore the staff considers this issue resolved.
- 3. GSI-103," Design for Probable Maximum Precipitation" The licensee has assessed the effects of flooding and roof ponding as a result of probable maximum precipitation (information provided in the licensee's response dated Februar/1, 1996, to the staff's RAl). The staff finds that the licensee's GSI-103 evaluation is consistent with the guidance provided in Section 6.2.2.3 of NUREG-1407, and therefore the staff considers this issue resolved.
4.: ' GSI-57," Effects of Fire Protection System Actuation on Safety Related Equipment" i
The licensee addressed GSI-57 as part of plant walkdowns (seismic and fire walkdowns).
The licensee completed a survey of various historical programs and design documents that address the GSI-57 issues and concluded that the effects of fire protection system actuation on safe shutdown equipment have been addressed in the design of CPSES.
The staff finds that the licensee's GSI-57 evaluation is consistent with the guidance
4' provided in EPRI's FIVE which was accepted by the NRC staff, and therefore the staff considers this issue resolved.
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- 5. Fire Risk Scoping Study issues j l
l The licensee has addressed the fire risk scoping study issues (fire-;nduced alternate shutdown / control room panel interaction (Section 4.8 of IPEEE); seismic-fire interactions (Section 4.8 of IPEEE); effects of fire protection system actuation on safety-related equipment (Section 4.8 of IPEEE); smoke control and manual fire fighting effectiveness (Section 4.8 of IPEEE); and adequacy of fire barriers (Section 4.8 of IPEEE)). Based on the results of the IPEEE submittal review, the staff considers that the licensee's process is consistent with the guidance provided in NUREG-1407, and therefore the staff considers these issues resolved.
In addition to those safety issues previously discussed that were explicitly requested in Supplement 4 to GL 88-20, four GSis were not specifically identified as issues to be resolved i under the IPEEE program; thus, they were not explicitly discussed in Supplement 4 to GL 88-20 l or NUREG-1407. However, subsequent to the issuance of the Supplement 4 to GL 88-20, the l NRC staff evaluated the scope and the specific information requested in Supplement 4 to GL 88-20 and the associated IPEEE guidance, and concluded that the plant-specific analyses being requested in the IPEEE program could also be used, through a satisfactory IPEEE submittal review, to resolve the external event aspects of these four safety issues. The :
following discussions summarize the staff's evaluation of these safety issues at CPSES: 1
- 1. GSI-147," Fire Induced Alternate Shutdown / Control Room PanelInteractions" The licensee has examined fire-induced alternate shutdown / control room ' interactions for CPSES (Section 4.8 of IPEEE). Each of CPSES, Units 1 and 2, is cesigned with an independent remote shutdown panel that can be isolated from the control room for a fire event that causes loss of control from the control room. Based on the results of the IPEEE submittal review, the NRC staff considers that the licensee's process is capable of identifying potential vulnerabilities associated with this issue. On the basis that no vulnerability associated with this issue was identified in the IPEEE submittal, the staff considers this issue resolved.
- 2. ' GSI-148," Smoke Control and Manual Fire-Fighting Effectiveness" The licensee's IPEEE submittal contains information addressing this issue (i.e., the effect of smoke and misdirected fire suppression was considered (Section 4.8 of IPEEE)).
Based on the results of the IPEEE submittal review, the NRC staff considers that the licensee's process is capable of identifying potential vulnerabilities associated with this issue. On the basis that no vulnerability associated with this issue was identified in the IPEEE submittal, the staff considers this issue resolved.
- 3. GSI-156," Systematic Evaluation Program (SEP)"
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- 4. GSI 172," Multiple System Responses Program (MSRP)"
The licensee's IPEEE submittal contains information directly addressing the following external event-related MSRP issues: (1) effects of fire protection system actuation on nonsafety related and safety related equipment (Section 4.8 of IPEEE); (2) seismically induced spatial and functional interactions (Section 4.8 of IPEEE); (3) seismically induced fires (Section 4.8 of IPEEE); (4) nonsafety-related control system / safety-related system dependencies (Section 4.8 of IPEEE); (5) effects of flooding and/or moisture intrusion on l nonsafety-related and safety-related equipment (Section 4.8 of IPEEE); (6) seismically induced fire suppression system actuations (Section 4.8 of IPEEE); and (7) seismically l induced flooding (Section 4.8 of IPEEE). Based on the overall results of the NRC staff's IPEEE submittal review, the NRC staff considers that the licensee's process is capable of identifying potential vulnerabilities associated with these issues. On the basis that no potential vulnerability associated with these issues was identified in the IPEEE submittal, the NRC staff considers the IPEEE-related aspects of these issues resolved.
In addition, the NRC staff considers that the following MSRP issues are resolved for CPSES for the reasons given below:
- (i) - CPSES is a reduced-scope plant and is not a USI A-46 plant. In accordance with Table 7.17.2 of NUREG-1407, for plants in this category (i.e., sites where the seismic hazard is low), the NRC staff recommendations for relay chatter review are the same as the Nuclear Energy Institute recommendations, namely, no relay evaluation is necessary.
(ii) In accordance with Section 3.2.4.5 of NUREG-1407, reduced-scope plants should use the SSE ground response spectra in their seismic margins methodology. Therefore, evaluation of an earthquake greater than the SSE did not need to be addressed for CPSES, which is a reduced-scope plant.
(iii) Regarding the effects of hydrogen line ruptures, even though the licensee did not explicitly discuss these effects, the licensee used the FIVE methodology. This methodology includes a walkdown verification checklist to identify any flammable liquid or gas storage vessels or piping and determine whether they are subject to leakage under seismic conditions.
(iv) Regarding the IPEEE-related aspects of common cause failures associated with human errors, human errors occurring as part of recovery actions during certain fire scenarios were addressed in Section 4.6.8 of IPEEE. With respect to the seismic events, the licensee reviewed the procedures and the human actions including recovery actions needed to respond to a seismically induced event. In response to the staff's December 12,1995, RAI, the licensee provided an enclosure," Individual Plant Examination of External Events Comanche Peak Steam Electric Station, ER-EA-001, Revision 0, August 1994," which addressed the human errors during seismic scenarios.
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. , , , . 1 6 I Plant Safety Features. Potential Vulnerabilities. and imorovements The licensee reported venting features for the buildings that are designed to operate in high j
' winds 9nd tornadoes. These include blowout doors that release at various pressure !
differentials, and dampers that are designed to open in either direction when reaching a certain ;
differential pressure. .These features provide substantial protection from the effects of high winds and tornadoes.
' The licensee did not provide a definition of a severe accident vulnerability. However, a number l of minor equipment enhancements have been implemented in the seismic area to resolve the l following anomalies that were found during the IPEEE review:
(1) Unanchored, ancillary equipment close to safety-related equipment, in the control room.
(2) Insufficient clearance between a motor control center and adjacent cable-tray supports.
With respect to fire and HFO events, the licensee did not identify any plant improvements in l these areas. !
3.0 CONCLUSION
i On the basis of its findings, the staff notes that (1) the licensee's IPEEE is complete with regard to the information requested by Supplement 4 to GL 88-20 (and associated guidance in NUREG-1407), and (2) the iPEEE results are reasonable given the design, operation, and ;
history of CPSES. Therefore, the staff concludes that the licensee's IPEEE process is capable of identifying the most likely severe accidents and severe accident vulnerabilities, and therefore, that the CPSES IPEEE has met the intent of Supplement 4 to GL 88-20 and the resolution of specific GSis discussed in this SE.
t It should be noted that the NRC staff focused its review primarily on the licensee's ability to examine CPSES for severe accident vulnerabilities. Although certain aspects of the IPEEE ,
were explored in more detail than others, the review was not intended to validate the accuracy of the licensee's detailed findings (or quantification estimates) that underlie or stemmed from the examination. Therefore, this SE does not constitute NRC staff approval or endorsement of any IPEEE material for purposes other than those associated with meeting the intent of Supplement 4 to GL 88-20 and the resolution of specific GSis discussed in this SE.
Attachment:
.TER Principal Contributor: E. Chow Date: July 8, 1999 b