ML20248C413
| ML20248C413 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 05/26/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20248C400 | List: |
| References | |
| REF-GTECI-***, REF-GTECI-057, REF-GTECI-103, REF-GTECI-131, REF-GTECI-147, REF-GTECI-148, REF-GTECI-156, REF-GTECI-A-45, REF-GTECI-DC, REF-GTECI-NI, TASK-*****, TASK-057, TASK-103, TASK-131, TASK-147, TASK-148, TASK-156, TASK-57, TASK-A-45, TASK-OR GL-88-20, NUDOCS 9806020144 | |
| Download: ML20248C413 (8) | |
Text
. . _ .
3 -
l STAFF EVALUATION REPORT OF INDIVIDUAL Pl. ANT EXAMINATION OF EXTERNAL EVENTS (IPEEE) SUBMITTAL ON MILLSTONE NUCLEAR POWER STATION, UNIT 3
- l. INTRODUCTION On June 28,1991, the NRC issued Generic Letter 88-20, Supplement 4 (with NUREG-1407, Procedural and Submittal Guidance) requesting all licensees to perform individual plant examinations of extemal 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 December 23,1991, the licensee, Northeast Nuclear Energy Company (NNECO), submitted its response to the NRC. In its letter, the licensee stated that the evaluation of extemal events at Millstone Unit 3 was submitted as part of the IPE submittal, entitled " Millstone Unit 3 (MP3) Individual Plant Examination (IPE) for Severe Accident Vulnerabilities." (This was an option allowed in Section 2.5 of NUREG-1335, " Individual Plant Examination: Submittal Guidance.") The licensee provided a " road map" that cross referenced the detailed supporting information in the " Millstone Unit 3 Probabilistic Safety Study (PSS)."
(The PSS was submitted to the NRC in August 1983.)
The PSS was the comerstone on which the IPE/IPEEE was conducted. The PSS is based on a Level 111 probabilistic risk assessment (PRA) and includes intemal fires, intemal flooding, ;
l seismic, and other extemal events. The original PSS and two of its revisions (April and
! November 1984) were extensively reviewed by the staff (documented in NUREG-1152, i "Millstone 3 Risk Evaluation Report") and contractors (NUREG/CR 4142, "A Review of the l Millstone 3 Probabilistic Safety Study" and NUREG/CR-4143, " Review and Evaluation of the Millstone 3 Probabilistic Safety Study: Containment Failure Modes, Radiological Source Terms, and Offsite Consequences").
The staff initiated a Step 1 review of the licensee's IPEEE submittal and its associated documentation in August 1993 and sent a request for additional information (RAl) to the !
licensee in April 1994. The licensee responded to the RAI in June 1994. Based on the results of the Step 1 review, the staff concludes that the aspects of seismic; fires; and high wind, floods, and transportation and other extemal events were adequately addressed. Staff review ,
findings are summarized in the evaluation section below. Details of the specific staff review '
findings of MP3 IPEEE submittal are provided in Appendix 1 for seismic, high winds, floods, and other external events and Appendix 2 for intemal fires.
In accordance with Supplement 4 to GL 88 20, the licensee proposed to resolve Unresolved Safety issue (USI) A-45, " Shutdown Decay Heat Removal Requirements," and Generic issue
. (GI) 131, " Potential Seismic Interaction involving Movable In-Core Flux Mapping System Used in Westinghouse Plants." No other specific USIs or G!s were proposed by the licensee for resolution as part of the MP3 IPEEE.
~
9806020144 990526 $
4 . . J
- )
II. EVALUATION The MP3 plant is a Westinghouse 4-Ioop pressurized-water reactor (PWR) with a subatomospheric containment. The plant was designed to a seismic acceleration level of 0.17g PGA (peak ground acceleration) and was categorized in NUREG-1407 as a 0.3g focused-scope plant. The licensee used the PSS (a Level 111 probabilistic risk assessment (PRA) including an assessment of containment performance), performed in 1983 for MP3 following the guidance described in NUREG/CR-2300. The licensee provided a cross-referencing " road map" of its PSS to the detailed information requested in NUREG-1407. However, the " road
- map" lacks specific cross-references on certain aspects related to some GSis which were
' specifically requested in NUREG-1407 (to be discussed further in the GSI section of this evaluation).
r Core Damaae F'eauency Estimates 1
The licensee estimated a mean core damage frequency (CDF) of g.1E-6 per reactor-year (/RY) I from seismically initiated events using site specific seismic hazards and a CDF of 4.9E-6/RY from internal fires. The licensee estimated that other external events (e.g., external floods, high winds) aie itssignificatit contributors to severe accident risk at MP3. The licensee estimated that the CDF due to internal event is about SE-5/RY, including intemal flooding.
The MP3 CDF estimates compare reasonably with those of other plants. 1 1
Dominant Contribuigtg The licensee found that the major contributors to mean seismic CDF and latent fatality risk at MP3 are related to seismically induced station blackout sequences which contribute about 63%
of the seismic CDF (e.g., sequences caused by loss of effsite power (LOOP] in combination with loss of onsite AC power) and seismically induced small-break LOCA sequences which contribute about 21% of the seismic CDF (e.g., seismically induced reactor coolant pump (RCP) seal LOCAs with no containment cooling).
The fire CDF is dominated by fires in the control room, cable spreading room, and charging and component cooling pump zone, which contribute more than half of the total fire-induced CDF.
However, there are no fire zones in which a fire can directly lead to core melt at MP3 (i.e.,
without additional non-fire-related failures).
The licensee's IPEEE Level lli PRA analysis appears to have examined the significant initiating events and dominant accident sequences.
Containment Performance in its containment performance evaluation, the licensee evaluated major contributors to early fatality risk due to seismic and intemal fire events in addition to evaluating the seismic capability of the containment.
2
1 .
The licensee stated in its response to the staffs request for additional information that both the containment structure and the systems that impact post core-melt containment performance (e.g., quench spray pumps, header and piping, containment recirculation system pl ping, pumps, and heat exchangers) were investigated in the MP3 seismic PRA. These investigations determined that the containment systems and structures possess adequate strength, and no seismically unique vulnerabilities exist. From the seismic point of view, the weakest components of the containment recirculation system are the heat exchangers, which have a high confidence of low probability of failure (HCLPF) value of 0.21g. However, the weakest i component of the quench spray system (a diverse system for containment heat removal) is the i quench spray piping, which has a high confidence of low probability of failure (HCLPF) value of 0.46g. Therefore, there is adequate ruggedness for the containment heat removal function.
. i The major contributors to early fatalities due to seismic events are dominated by two release classes: one attributed to a plant damage state U i.)S) in which a LOCA occurs together with a loss of containment isolation due to the collapse of the containment crane wall for seismic events far beyond the SSE, and the other attributed to the plant damage state which results from a large break LOCA with early core melt. The seismic PRA did not uncover any other seismically unique containment performance vulnerabilities.
The contribution to latent fatality risk due to fire with late containment failure and no sprays is estimated at 1.5E-6/RY, which is about 25% of the contribution from intemal events (6.1E-6/RY). However, the contribution to early fatality risk due to fire events tied to containment I bypass sequences is estimated by the licensee to be minimal.
l The kcensee's containment performance analyses for seismic and intemal fire events appear to have considered important severe phenomena and are consistent with the intent of Supplement 4 to Generic Letter 88-20.
Generic Safety lasues i
As a part of the IPEEE, a set of generic and unresolved safety issues (e.g., USl A-45, GSI-131, 1 GSI-103, and the Sandia Fire Risk Scoping Study [FRSS) issues) were specifically identified during the initial planning of the IPEEE program (and explicitly discussed in Supplement 4 to GL 1 88-20 and its associated guidance in NUREG-1407) as needing to be addressed in the IPEEE. l The staffs evaluation of these issues is provided below.
- 1. USl A-45," Shutdown Decay Heat Removal Requirements" The licensee's process of addressing USI A-45 extemal events was to identify specific methods that can be used for decay heat removal at MP3, and discuts the seismic capacities of components associated with those decay heat removal methods. On the i'
basis of the seismic capacities of those components, the licensee concluded that MP3 has no vulnerabilities related to the seismic aspects of USl A-45. Although, the licensee did not specifically provide a USl A 45 discussion in its fire IPEEE, the fire PRA performed at MP3 does cover this aspect. Therefore, the staff finds thit the licensee's USl A-45 l evaluation is consistent with the guidance provided in Section 6.3.3.1 of NUREG-1407.
3
l
- 2. GSI-131, " Potential Seismic interaction involving the Movable in-Core Flux Mapping System used in Westinghouse Plants" The licensee's process of addressing GSI-131 involved an evaluation of the seismic adequacy of flux-mapping cart supports. As a result of that evaluation, a modification was made in 1985 to limit the relative displacement between the flux mapping equipment and the seal table to an acceptable level. Based on that action, the licensee considered GSI-131 closed for MP-3. The staff finds that the licensee's GSl-131 evaluation and improvements are consistent with the guidance provided in Section 6.2.2.1 of NUREG-1407.
- 3. GSI-103, " Design for Probable Maximum Precipitation" The licensee did not specifically mention GSI-103 in its submittal nor in its road map.
However, the licensee stated in its submittal (Section 3.5.3) that "In view of the maximum precipitation intensities, the associated frequencies, and the plant features designed to withstand such conditions, intense precipitation is an insignificant (r'sk) contributor."
Based on the fact that the Probable Maximum Precipitation (PMP) criterion was used in the flood protection design at MP3, even though the licensee did not specifically assess GSI-103 in sceordance with the guidance provided in Section 6.2.2.3 of NUREG 1407, the staff agrees with the licensee's conclusion that intense precipitation is an insignificant 1 contributor to severe accident risk at MP3. I i
- 4. Fire Risk Scoping Study issues The licensee has explicitly addressed some of the Sandia Fire Risk Scoping Study issues, (i.e., effects of fire suppressant on safety-related equipment, attemate shutdown / control l panel interactions, and effectiveness of manual fire-fighting). However, the licensee did !
not provide specific information to demonstrate that a supplementary walkdown (e.g., to assess the effects of potential seismic / fire interactions) as suggested in NUREG-1407 was l
performed. Instead, the licensee stated that a walkdown was performed in 1984 by a l contractor and reviewed independently. Since the contractor and the independent i reviewer are considered experts in these fields, the staff considers that the original walkdown is adequate for the IPEEE purpose. Therefore, the staff finds that the licensee's i evaluation is consistent with the guidance provided in NUREG-1407.
- 5. GSI-57," Effects of Fire Protection System Actuation on Safety-Related Equipment" Although, the licensee's IPEEE submittal did not explicitly discuss GSI 57, information provided in their response to an RAI addressing fire suppression and the treatment of i suppression-induced damage to equipment is related to this issim. The staff finds that the licensee's evaluation is consistent with the guidance provided in NUREG-1407.
In adGtion to those safety issues discussed above that were explicitly requested in Supplement 4 to GL 88-20, four generic safety issues were not specifically identified as issues that should be resolved under the IPEEE program; thus, they were not explicitly discussed in Supplement 4 to GL 88-20 and NUREG-1407. However, subsequent to the issuance of the 4
m
generic letter, the NRC evaluated the scope and the specific information as requested in the letter and its associated 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 extemal event aspects of these four safety issues. The following )
discussions summarize the staff's evaluations and resolutions of these safety issues at MP3. )
- 1. GSI-147,
- Fire-Induced Altemate Shutdown / Control Room Panel Interactions"
- . The licensee's responses to RAls related to the fire-initiated events database used in the l licensee's analysis, and their treatment of fire suppression and suppression-induced l damage to equipment, contain some information that can be used to address this issue.
l Based on the results of the IPEEE submittal review, the staff considers that the licensee's process is capable of identifying potential vulnerabilities associated with this issue. On the ;
l basis that no vulnerability associated with this issue was identified in the (PEEE submittal, the staff considers this issue resolved for MP3.
- 2. GSI-148, " Smoke Control and Manual Fire-Fighting Effectiveness" The licensee's response to an RAI addressing their treatment of fire suppression and suppression-induced damage to equipment contains some information addressing this issue. Also, the licensee performed a review of the MP3's fire protection program and its associated fire brigade training program islated to this issue. Based on the results of the IPEEE submittal review, the staff considers that the licensee's process is reasonable and 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 for MP3.
- 3. GSI-172, " Multiple System Responses Program (MSRP)"
The licensee's IPEEE submittal contains information directly addressing the following extemal events-related MSRP issues: IPEEE-related aspects of common cause failures related to human errors (response to an RAl), effects of fire protection system actuation on safety-related equipment (response to an RAI), smoke control and manual fire-fighting effectiveness (response to an RAl), seismically induced fire suppression system actuations (response to an RAI), seismically induced relay chatter (response to an RAI), effects of flooding and/or moisture intrusion on non-safety related and safety-related equipment (response to an RAl), and evaluation of earthquake magnitude greater than safe shutdown earthquake. It should be noted that the seismically induced spatialinteractions, seismic-fire interactions, seismically induced flooding, and the effects of hydrogen line rupture were not explicitly discussed in the MP3 IPEEE submittal. Although these issues were not explicitly discussed, they are mainly walkdown-related and should have been routinely examined during a seismic walkdown. Therefore, based on the composition of the walkdown team and the expertise of the licensee's independent reviewer for the MP3 PSS, the staff believes that these seismic-related issues were addressed in the licensee's walkdown process. In addition, the staff and its contractor had performed detailed reviews of the MP3 PSS in 1984 (NUREG/CR-4142 and -4143) and did not raise any concems related to these issues. Thus, the staff believes that the licensee's process is 5
i j capable of identifying extemal events-related vulnerabilities associated with GSI-172.
, Therefore, on the basis that no potential vulnerability associated with extemal event I
aspects of this issue was identified in the IPEEE submittal, the staff considers the IPEEE-related aspects of this issue to be resolved for MP3.
- 4. GSI-156, " Systematic Evaluation Program (SEP)*
MP3 is not an SEP plant.
Uniaue Plant Features. Potential Vulnerabilities. and imorovemgits The licensee reported no unique safety features at the plant.
The licensee did not identify any seismic, fire, or other extemal events related severe accident vulnerabilities. However, some procedural changes and a number of minor improvements have been identified in the MP3 IPEEE submittal and the licensee's response to RAls. These licensee-identified improvements are listed below:
- 1. In NUREG-1152, " Millstone 3 Risk Evaluation Report" (June 1986), the staff stated that there were cost-effective improvements in prevention and mitigation of a potential station blackout caused by an earthquake beyond the SSE. The staff recommended that the licensee perform an engineering analysis of the costs, benefits, uncertainties, and competing risks of upgrading the diesel generator lube oil cooler anchorage system. In its submittal, the licensee stated that the anchor bolts on the diesel generator oil coolers were replaced with bolts of stronger material.
- 2. To prevent the possibility of a fire causing inadvertent operation of a fire suppression system affecting the safe shutdown of the plant, control function capability for the east and west switchgear rooms were deleted. Thus, MP3 is capable of proceeding with attemate shutdown without an inadvertent operation of a fire suppression system affecting this operation.
- 3. As a result of the evaluation of GSI-131, a modification was made in 1985 to limit the relative displacement between the flux mapping equipment and the seal table to an acceptable level.
til. CONCLUSION On the basis of the above findings, the staff notes that (1) the licensee's IPEEE is complete with regard to the information requested by Supplement 4 to Generic Letter 88-20 (and associated guidance in NUREG-1407), and (2) the IPEEE results are reasonable given the MP3 design, operation, and history. 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 MP3 IPEEE ha.s met the intent of Supplement 4 to Generic Letter 88-20 and the resolution of specific generic safety issues discussed in this SER.
1 6
l It should be noted that the staff focused its review primarily on the licensee's ability to examine MP3 for severe accident vulnerabilities. Although certain aspects of the IPEEE were explored l in more detail than others, the review is not intended to validate the accuracy of the licensee's l detailed findings (or quantification estimates) that stemmed from the examination. Therefore, l this SER does not constitute NRC approval or endorsement of any IPEEE material for purposes i other than those associated with meeting the intent of Supplement 4 to GL 88-20 and resolving l the generic safety issues discussed in Section 11.
I i
i 7
l .
1 APPENDIX 1 STAFF EVALUATION REPORT ON SEISMIC, HIGH WIND, FLOOD, AND OTHER PORTIONS OF INDIVIDUAL PLANT EXAMINATION OF EXTERNAL EVENTS (IPEEE) SUBMITTAL FOR
, MILLSTONE 3 NUCLEAR POWER PLANT l
- _ _ - - _ _ - - _ _ _ - _ _ _ _ _ - _ _ - _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _