ML19332E909
| ML19332E909 | |
| Person / Time | |
|---|---|
| Issue date: | 04/02/1989 |
| From: | Beckner W, Chen J NRC |
| To: | |
| References | |
| NUDOCS 8912130142 | |
| Download: ML19332E909 (4) | |
Text
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Confc.rence
Title:
PSA '89 -- International Meeting Prob. Relia. & Safety Assessment D,ata & *Placet April 2 - 7, 1989 -- Pittsburgh, PA SEVERE ACCIDENT POLICY IMPLEMENTATION EXTERNAL EVENTS John T. Chen William D. Beckner U.S. Nuc! car Regulatory Commission Washington, D.C.
20555 301 492 3919 ABSTRACT ne staff, as stated in SECY 86162 (Ref. 4), is proceeding with the evaluation of severe accidents initiated by external in 1985, the NRC issued a severe accident policy statement hazards in two phases. The first phase program, which has that calls for a systematic examination of existing plants for been mmpleted, consisted of a Lawrence Livermore National severe accident vulacrabilities. The staff intends to proceed Laboratory (LLNL) study to assess the margins that past with the implementation of that policy using Individual Piant design bases provide relative to external events and to identify Examinations for internal events at the present time. The areas where an examination for external vulnerabilities may Individual Plant Examinations for external events will proceed be needed.The second phase of the externalevents evaluation later.This delay will allow the NRC to assess and identify areas program will consist of developlag specific guidance and where examinations for external event vulnerabilities are criteria for each external hazard to be considered in the needed, to develop the guidance for the examination,and to in.
IPFFIh.
j tegrate related ongoing programs.
Based on the ter,ults of the LLNL study (Ref. 5) and informa-tion gained during an external events workshop held August 4-INTRODUCTION 5,1987,some external events such as earthquakes and internal fires may be significant risk contributors at some plants. Also, -
02 August 8,1985, the Commission issued a policy statement the design bases for some other external events such as on severe accidents (Ref.1), The policy statement does not tornadoes and external floods may be sufficiently consen atim differentiate between events initiated within the plant and that they do not pose a significant risk at most plant sites.
events caused by external initiators. Current risk assessments However, there may be some structures or f acilities at some indicate that the risk from external events could be a significant sites that were not designed io current criteria and may pose ccatributor in some instances, although a distinct possibility some risk at those plants.These studiics provide insights to exists that the risk from external events has been over esti-assist in developing review guidance for the IPEEEs.
mated because of conservatisms used in the analyses.
There are many ongoing NRC programs addressing the po.
The staffintends to proceed with the implementation of the tential threat of external events to the safety of nuclear power l
Severe Accident Policy outlined in SECY 88-205 (Ref. 2) and plants. It became evident that, to avoid any unnecessary l
GL88-20(Ref 3)usingindividualPlant Examinations (IPE) duplication of effort, an integrated approach is absolutely for internally initiated events. The assessment methods and essential for NRC to address these external events. On De.
plan for Indhidual Plant Examinations for External Events cember 21,1987, NRC established an External Events Steer.
(IPEEE) tequires more development.Hence the evaluation of ing Group (EESG) to make recommendations to senior external events will proceed on a schedule different from that management regarding the degree to which external events for internal events. Delaying severe accident reviews for need to be considered in the context of the Severe Accident external events will not result in unacceptable public risk Policy Implementation and to determine the scope and iden-because design provisions for protecting nuclear power plants tify the methods for such an evaluation. The EESG estab-from external events are known to be conservative.The procc-lished three subcommittees to carry out its charter, dures and criteria outlined in industry codes and standards and 1
specinea :n NRC Regulatory Guides and Standard Review Plans result in substantial inherent margins in the design.
EXTERNAL EVENTS PHASE I PROGRAM However, the staff believes that plant and site-specific condi-tions and design and construction errors can decrease these Under the auspices of the NRC, the Lawrence Livermore inherent margins, in addition, use of maximum expected National Laboratory (LLNL) made a study of the risk of core events has not always been developed using probabilistic damage and large release due to externally initiated events at bases. Therefare, it is appropriate that an IPEEE be con-nuclear power plants. These events included internal fires, ducted to identify and correct plant specific vulnerabilities that high winds / tornadoes, external floods, transportation, scis, could reduce the inherent margin.
mic events, and others.
8912130142 690402 4
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Programs are being conducted at LLNL and Sandia to auist
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Two figures-of merit were used in the LLNL study as evalu-the staff to (1) develop IPEEE guidance and procedures for ation criteria to discriminate between the significant and the operating plants, (2) develop review plans and acceptance cri-4 less significant levels of risk.These two figures-of merit are de-teria,(3) cxtend the Seismic Design Margins Program meth-fined as 1E 5 / reactor year for a core damage accident and odology,(4) integrat e ongoing safety programs, an d, (5) asse ss 1E 6 / reactor year for a large release of radioactive materials the impact of separating examinations for internal and exter.
ta the emironment, nal events.These programs are scheduled to be completed by nem fall. Since the industry has sponsored numerous safety The results of LLNL's study (Ref. 5) showed that those emer-programs, NRC will continue to encourage industry particip nal initiators (seismic, internal fires, emernal floods, and high tion to identify needs, scope, and methods for IPEEE and to winds / tornadoes) can bc important (exceed the figure-of-develop or review guidance and procedures. Ultimately, the j
merit) with respect to core damage frequency (Table 1). The industry needs to be fauiliar with the IPEEE in order to carry j
results also showed that the current design bases for some out the program to identify plant specific vulnerabilities at
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emernal events, such as high winds / tornadoes and esternal each plant.
floods, may be sufDciently conservative that they do not pose any significant risk at most plant sites. However, there may be some structures or facilities at some sites that were not de-CONCLUSION l
signed to current criteria and may pose some risk at those plants (Table 2). The scismic initiator was found to have Based on NRC and industry experience with plant. specific frequencies of large releases exceeding the figure-of merit PRAs, the Commission has tecognized that systematic exami-while the internal fires were found to have frequencies of large nations are beneGeialin identifying plant specific vulnerabili-releases less than the figure of merit. Internal fires and scis-ties to severe accidents that could be fixed uith low-cost im.
mic events may be of importance to all plants, while other provements.The Commission, therefore, calls for each exist-external events are site specific.
ing plant to perform such an examination to identify plant.
i specific vulneraMtics. At the present time, licensees are These results, along with other considerations discussed be-requested to proceed with examination for internal events low, will assist the staffin developing the scope and content of only. The emernal e vent examination will proceed separately
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the IPEEE program, on a later schedule. This delay allows the s'aff (1) to identify which external events need an examination, (2) to develop ex-j amination guidance and procedures, and (3) tointegrate other i
EXTERNAL EVENTS PHASE 11 PROGRAM ongoing programs that deal with external events to ensure that there is no duplication of efforts. Programs are being con-To address the many external events and to avoid any duplica-ducted at LLNL and Sandia to assist the staffin accomplishing tion, the NRC established the Emernal Events Stecting Group the above by next fall. The staff encourages the industry to (EESG) in December 1987 to coordinate the effort. The propose methodologies for examining external events that EESG established three subcommittees (1. Seismic, 2. Inter-meet the intent of the s: vere accident policy.
nal Fires, and 3. High Winds, Floods, and Others) to carry out its mission. Specifically, each subcommittee is to recommend a potentialregulatory framework with respect to the implem.
REFERENCES entation of the severe accident policy. This effort included consideration of the various issues and integration of other L " Policy Statement on Severe Reactor Accident", USNRC, existing programs (Table 3) into a consistent approach for an Federal Register, Vol.50, pg. 32138, August 8,1985.
lidhidual Plant Examination for External Events (IPEEE).
- 2. USNRC Policy Paper SECY 88-205," Generic Letter for The scope and the approach of the examination for each the implementation of the Severe Accident Policy State.
indhidual event may differ. More detailed guidance and crite.
ment", memorandum from V.Stello, July 15,19SS.
ria need to be developed for the IPEEE. This guidance should enable the licensees or the NRC, as appropriate to:
- 3. USNRC Generic letter 88 20,"Indhidual Plant Examina.
tion for Severe Accident Vulnerabilities - 10 CFR 50.54(f)",
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- 1. Determine which external hazards need to be included in Novernber 23,1988.
IPEEE programs based on specific site and plant conditions.
- 4. USNRC Policy Paper SECY 86162," Treatment of Exter.
2.
Systematically assess plant specific vulnerabilities to nal Events in the Implementation of the Severe Accident severe accidents initiated by those external hazards.
Policy Statement", memorandum from V. Stello, May 22, 1986.
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- 3. Integrate NRC's ongoing safety programs that are related to external events.
- 5. NUREG/CR 5042, " Evaluation of External Hazards to Nuclear Power Plarits in the United States", C.Y. Kimura and
- 4. Establish criteria to resolve external event issues.
R. J. Budnitz, LLNL, December,1987.
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4 TABLE 1 CORE DAMAGE FREQUENCIES FROM EXTERNAL EVENTS ALL EXTERNAL ElGB WINDS fjL6 INITIATORS IIRES SEISMIC FLOODS TORNADOES l
Zion 1 & 2 5.7E 5 1.8E 6 5.6E 6 lasign.
NA.
(
Indian Pt. 2 4.7E-4 1.4E-4 1.4E-4 Extre.
2 5E 5 (Median) 4.8E.5R Small 43E 5 (Mean)
Indian Pt.3 23E 4 9.6E5 3.1E-6 Extre.
4.9E 8 (Median) 2.5E 5R Small 13E-6 (Mean) l C!3 Rock Pt.
9.8E 4 23E 4 NA.
NA.
NA.
L limerick 4.4E.5 23E5 4.0E-6 Negl.
9.0E 9 1:
'Seabrook 23E 4 2.5E-5 2.9E5 NA.
2.1E 9
_ Ococee 3 2.5E-4 1.0E 5 63E5 23E 5
< 1.0E 9 Millcone 3 7.0E 5 4.8E 6 9.4E5 Insign.
<1.0E-7 8.8E-6R Point Beach
.3.1E-4 33E5 6.0E 5 1.9E4 4.0E-6 Turkey Pt.
2.4E 4 7.5E 5 7.8E-6 4.6E5 2.4E5 i
St. Lucie 1 7.5E 5 4.4E 5 13E 5 3.2E-6 L6E-8 l
Quad Cities 2.0E-4 13E5 83E5 1.0E 7 1.4E 7 i
ANO1 1.8E 4 5.SE-6 73E5 7.2E 6 53E-6 Cooper 4.4E 4 1.1E 5 8.1E-5 5.0E 5 3.8E 6 1
TADLE 2 j
PRA INSIGHTS ON HIGH WINDS / TORNADOES RISK L
AFFECTED TOTAL HIGH WINDS /
VULNERABLE STRUCTURES /
l PLANT NAME TORNADOES CDF STRUCPURES COMPONENTS l
l.
Indian Polm 2 2 5E 5 (Median)
Unit 1 Unit 2 DG Bldg.
(66/71).
43E 5 (Mean)
Superheater Stack
& Control Bldg.
Arkansas Unit 1 2.6E-4 w/o Recovery DG Exhaust Fail Both DGs r
I (68/74) 53E-6w/ Recovery Stack Point Beach 1&2 6.0E 5 w/o Recovery DG Exhaust Fall Both DGs (67/70, 68/71) 4.0E-6 w/ Recovery Stack Turkey Point 3 3.6E 5 w/o Recovery Unit 2's DG Bldg., Fuel Oil Pumps (67/72) 2.4E 5 w/ Rtcovery 400' Concrete Switchgear Bldg. Unit 3 RWST L
Stack DG FuelTank, CST Intake Pumps
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TABLE 3 ONGOING NRC PROGRAMS AND RELATED UNRESOLVED j
SAFETY ISSUES / GENERIC ISSUES (USis/Gis) e (1) USI A-46,"Scismic Qualification of Equipment in Operating Plants", is developing an alternative method and acceptance critcria 13 verify the seismic adequacy of operatir.g plants equipment with construction permits before about 1972 against the current safe shutdown earthquake (SSE). Although not intended to examine seismic margins beyond the SSE, this effort is expected to provide some insights on equipment capacity beyond the SSE level.
(2) USI A 45," Shutdown Decay Heat Removal Requirement", has examined the ability of the decay heat removal systems of six operating plants to perform its function during and after seismic events up to and beyond the SSE, with internal flocding, int ernal fites and high winds. USI A-45 is now subsumed in the IPE program.
(3) USI A 17," System lateractions in Nuclear Power Hants", addresses ACRS concerns regarding the interactions of various systems with regard to whether actions or consequences could adversely affect the redundancy and independence of safety systems. The spatial system interaction during a seismic event is now included in USI A-46.
l (4) US! A 40,"Scismic Design Criteria, A Short Term Program", investigated selected areas of the seismic design process, and is proposing alternate approaches to part of the design sequences, as well as modifying the NRC criteria in the Standard Review Plan ta reflect the current state of the technology and industry practice. The concern of the seismic design of safety class tanks is now included in USI A 46.
(5)
- Seismic Hazard Characterization of the Eastern United States Project" has developed a method to assess the seismic hazard for the region cast of the Rocky Mountains.This will provide a better means of assessing the seismic hazards used in the plant design.
(6) " Seismic Design Margins Program" has developed a simplified method to estimate availabic seismic margins in operating plants at a predetermined scismic level above the current SSE. The method draws on the insights gained from completed seismic PRAs to focus on those plant functions that contribute most to seismic risk. Screening guidance has been established for earthquakes up to several times above the SSElevel.
(7) " Fire Risk Scoping Study" was performed to: (a) review and re quantify certain past fira risk scenarios in light of updated data bases and updated computer fire modeling capabilities, (b) identify potentially significant fire risk issues which have not previously been addressed and to quantify the potentialirupact of those identified fire risk issues, and (c) review current fire regulations and plant implementation practices for relevance to the identified unaddressed fire risk issues. it was found that application of a more extensive operational experience data base resulted in an increase to the estimated fite risk as compared to previous fire PRAs. Six fire risk issues were also identified which had not previously been addressed in the fire risk context.
(8) NUR2G 1150," Reactor Risk Reference Document", has performed external event analyses for the Surry and Peach Bottom tucle at power plants. Earthquakes, internal fires, and other specific external events were included in the analyses.The approach uses event trees and fault trees derived from internal event analyses. This allows some direct comparison between external and internal events.
(9) " Robust Techniques for Estimating the Probabilities of Extreme Floods" identified and reviewed various approaches to estimate extreme flood probabilities, it recomtnends two general approaches, a statistical technique and a runoff model technique to estimate catteme flood frequencies.
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