ML20235P116

Summary of ACRS Ac/Dc Power Sys Reliability Subcommittee 870506 Meeting in Washington,Dc Re Review of Proposed Station Blackout Rule 10CFR50.63

ML20235P116
Person / Time
Issue date:	05/13/1987
From:	Advisory Committee on Reactor Safeguards
To:	Advisory Committee on Reactor Safeguards
References
ACRS-2500, NUDOCS 8707200345
Download: ML20235P116 (13)

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g g = 7//yjff l h! DATE ISSUED: 5/13/87 4r017 PROPOSED MEETING MINUTES /

SUMMARY

FOR THE AC/DC POWER SYSTEMS RELIABILITY SUBCOMMITTEE MEETING MAY 6.-1987 i WASHINGTON, D.C.  !

Purpose l The AC/DC Power Systems Reliability Subcommittee met on May 6,1987 in l Washington, D.C., to review the proposed station blackout rule 10 CFR l 50.63.

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I Atteridees j ACRS NRC Staff I C. Wylie. Chairman T. Speis, RES J. Ebersole, Member A. Rubin, RES C. Michelson, Member P. Baranowsky, NRR G. Reed, Member R. Hernan, NRR M, El-Zeftawy, Staff J. Flack, RES M. De, Fellow P. Norian, RES J. Jackson, RES Others A. Serkiz, RES J. Trotter, NUS Corp.

M. McGarry, BCP&R R. Helfrich, BCP8R C. S. Ondash, Devonrue S. Maloney, Devonrue J. J. Linehan, TVA R. R. Reeves, TVA W. T. McBride, VA Power R. S. Berger, VA Power l A. Meekins, VA Power 1

S. Floyd, CP&L i l R. Gritz, FL Power & Light j A. Wyche, Serch Lic. - Bechtel ,

! R. B. Borsun, B&W H. L. Wyckoff, EPRI 9707200345 G70"J13 0.'lIGIHAL 2YO PDit Certified By &

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. i AC/DC Power Systems Reliability Minutes May 6, 1987 Meeting Highlights, Agreements, and Requests.  ;

1 I. Mr. C. Wylie Subcommittee Chairman, stated the purpose of the Subcommittee meeting and introduced the other present ACRS members. )

He also stated that the Subcommittee has received a request from the Nuclear Utilities Management and Resources Comittee (NUMARC) i to make a brief statement, which the Subcommittee will accomodate at the conclusion of the meeting.

Mr. Wylie indictted that the NRC Staff is. proposing to the Comis-sion to proceed with the final resolution of USI A-44 " Station Blackout" by amending its regulation to require that all nuclear power. plants be capable of coping with station blackout (SBO) for a specified duration. The NRC Staff has prepared a regulatory guide which presents a method acceptable to the Staff.for selecting the specified duration on a plant specific basis. The ACRS has previ-ously reviewed the proposed resolution of (SBO) on two occasions and wrote two letters to the EDO, generally supportive, with comments. In the first letter to EDO dated July 13, 1983, the ACRS l

i emphasized that the resolution cf USI A-44 be coordinated with the j resolution of USI A-45 " Shutdown Decay Heat Removal Requirements." j The ACRS also urged the resolution of USI A-44 to be coordinated l

with B-56 " Diesel Reliability" and Safety adequacy of DC related power supplies. In the second letter to EDO dated March 12, 1985,  ;

the ACRS recommended that efforts should be made to seek a method of implementing the Staff's requirements with a minimum of Staff and licensee resources. Mr. Wylie solicited coments from the

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other ACRS members regarding this issue. .

I II. Mr. T. Speis Deputy Director for Generic and Regulatory issues (RES), sumarized the Staff's current approach to resolve the USI A-44 as follows:

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j. AC/DC Power Systems Reliability Minutes; May 6, 1987-

• Determine current estimated frequency.of core damage frequency

_ (CDF) due to station blackout for a spectrum of plant designs.

Identify dominant factors affecting CDF and cost effective improvements, and specifically the:

(a)~ likel'ihood.of frequency and duraticn of loss of AC power, 1

(b) ability to cope with extended loss of AC power, q

Propose new or revised requirements consistent with level of risk, and cost effectiveness.

Mr. Speis indicated that the above mentioned approach has been the same'since the proposed resolution. Mr. Speis emphasized that the' main ein is to reduce the frequency of occurrence of core damage from SB0. He also indicated that as operating experience has accumulated, the concern has arisen that the reliability of offsite and onsite AC power systems might be less than originally antic-ipated. There are potentially severe consequences from the loss of AC power e.g.: limited decay heat removal capability and no con-tainment heat removal.

The Commission's existing regulations establish requirements for j the design and testing of onsite and offsite electric power systems  !

i that are judged to minimize the probability of losing.all AC power. l (GDC-17 and 18). However, the existing regulations do not require l explicitly that nuclear power plants be designed to assure that the . j core can be cooled and the integrity.of the reactor coolant pres- -

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sure boundary can be maintained for any specified period of loss of AC power.

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AC/DC Power-Systems Reliability Minutes May 6, 1987 The technical findings cf the NRC Staff are presented in NUREG-1032. The main findings indicate the following important charac-teristics of SB0 accidents:

The likelihood of estimated station blackout ranges from 10-5 to 10-3 per reactor year. A typical estimated frequency is on the order of 10~4/RY.

The redundancy of onsite AC power systems can have a large influence on the likelihood of station blackout.

1 Restoration of offsi.a power in a timely manner can reduce the accident consequences.

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• The capability of the decay heat removal system to cope rith l long duration blackouts (> 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />) can be a dominant factor influencing the likelihood of core damage.

Re estimated frequency of station blackout events that result in core damage rarges from 10-6 to greater than 10~4/RY. A typical estimate is on the order of 10-5jpy,

" Containment failure as a result of overpressure may follow a station blackout-snduced core melt. Smaller, low design pressure containments are most susceptible to early failure, possibly in less tran 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />.

As a result of design reviews, the NRC Staff concluded that the ,

estimated core melt frequency from station blackout could be maintained around 10-5/RY, provided that a plant would have to be able to cope with station niackout on the order of 2 to 4 and perhaps 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> long and have emergency diesel generator l

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. . AC/DC Power Systems Reliability Minutes May 6, 1987 reliabilities of 0.95 per demand or better, with relatively low susceptibility to common cause failures.

Mr. Speis indicated that, there .is no single fix' applied uniformly to all plants will resolve this issue in a cost-effective manner.

The Staff expects the ACR5/CRGR review to be completed by the end

! of June 1987 and the final SB0 rule to be submitted to the Commis-sion by August 1987.

III. Mr. P. Baranowsky, NRR/ Events assessment Branch, presented a brief sumary of operating experience relevant to SBO. From 1968 to.

1985: there were 64 total loss of offsite' power events of a few minutes duration, From 1976-1985: over 600 EDG failures during l testing and actual demands (with'22 instances involving multiple-l failures due to common cause), and From 1968-1985: several SB0 j precursors involving total loss of offsite and unavailability'or l 1 failure of one or more emergency AC power supplies. Several of j these incidents involved loss of all AC power (EBO).

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IV. Mr. J. Flack, RES/ Advanced Reactor Group, presented the loss of i offs"te power (LOOP) events. The LOOP events can be categorized as I those resulting from: (a) plant-centered faults, (b) utility grid bleebmts, and (c) failures of offsite power sources inducec! by l severe weather. The industry average frequency of total losses of l offsite power was determined to be about 0.1 per site / year, and the median restoration time was about one-half hour.

The Staff's original analyses (since March 1985) was revised to .

include updated LOOP events (e.g.: added salt spray weather hazard i for coastal sites) and emergency AC power (EAC) reliability analy-  !

ses. The factors identified as affecting the frequency and l duration of offsite power losses are: ,

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• The design of preferred power distribution system, particular-ly the number and independence of offsite power circuits from the point where they.' enter the site up to the safety buses. -J l

• Operations.that can comprom9se redundancy or' independence of-multiple offsite power sources, including human error.

The reliability and security of the power-grid, and the ability to restore power to a nuclese plant site with a grid' blackout.

The hazard from, and susceptibility to, severe weather con-ditions that can cause loss of offsite power for extended periods.

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The NRC Staff developed a test of LOOP correlation. These corre-l lations were compared with plants specific results from PRAs. A l reasonable agreement was achieved, for~ example, for Indian Point, j Shoreham, and Limerick.

V. Mr. P. Baranowsky, NRR, stated that a typical unavailability of a two-divisien emergency AC pcwer systems is about 10-3 per demand, and it is 2 x 10-2 per demand for EDG. The Staff integrated the LOOP analysis with the emergency AC power reliability models to determine plant's ability to cope with SB0.

Mr. Baranowsky indicated that the likelihood of a core damage from j l SB0 is dependent on the reliability of decay heat removal systems , j l that are independent of AC power. .The important factors relating . I to DHR during a SB0 are: l I

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AC/DC Power Systems' Reliability Minutes -7 . May 6. 1987-The capacity and ability to function of DHR' systems and suppert systems (e.g., DC power, Condensate storage) during SB0 including effects of inoperable HVAC systems.

• For PWR and BWR (without reactor coolant makeup capability),,

the magnitude of reactor coolant pump seal leakage.

• For BWR, the ability to mainta'in suppression pool integrity.

VI. Mr. A. Pubin, NRC/RES, defined SB0 as the complete loss of AC electrical power to the essential and nonessential switchgear buses in a nuclear power plant. SB0 involves the loss of offsite power concurrent with the failure of the onsite emergency AC power sys te"1. It does not include the loss of available' AC power to buses fed by station batteries through inverters.

The new proposed SB0 rule (50.63) will require that all nuclear power plants be capable of coping with a SB0 for some specified period of time. The period of time for.a specific plant will'be determined based on a comparison of the individual. plant's design with the following factors:

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• Redundancy of onsite emergency AC power sources, i l

• Reliability of onsite emergency AC power sources, I

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Frequency of loss of offsite power, and l l

• Probable time to restore offsite power.

All' licensees and applicants are required (within 9 months after the rule is issued) to assess and submit to the NRC~the capability  !

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. AC/DC Power Systems Reliability Minutes May 6,.1987 of their plants to cope with a SB0 (for the' duration determined above).

If the specitic plant can cope with the SB0, then the next step.

Will be the implementation procedures and training to cope with SB0. On the other hand, if the specific plant could not cope with SB0 (for the specified duration), a list of modification to equip-ment and associated procedures to extend SB0 coping capability will be required.

l Within 6 months of NRC revie:w, the licensee should submit a sched-ule for ccmpleting modification (with a justification if schedule is longer than 2 years). The licensee and the NRC Staff should '

mutually agree on the final schedule for implementing modifica-tions.

The Staff has prepared a draft regulatory guide entitled, " Station i Blackout" which presents a method for selecting a plant-specific-minimum duration for station blackout capability to comply with the proposed amendment to GDC 17. Application of the method in'this-regulatory guide would result in selection of a 2, 4, 8 or 16-I hours station blackout curation capability depending'on the plant's specific design and site-related characteristics.

The estimated benefit from implementing the proposed rule is a l reuction in the frequency of core damage /RY due to SB0 (approxi-mately, mean SB0 core damage frequency of 4.2 x 10-5 before rule to l 1.6x 10-5/RY after rule) and the associated risk of offsite radio- .

active releases. The risk reduction for 100 operating reactors.is I estimated to be 145,000 persons-rems. .The estimated cost for 100 operating reactors to comply with the rule is about $60 million.

The average cost will be ranging from'$350,000 to'$600,000. The overall value-impact ratio is about 2,400 person-rems /$1 million.

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- . AC/DC Power. Systems' Reliability Minutes Ma;< 6,'1987 On March 21, 1986, the Commission published the proposed rule in the Federal Register. A 90-day coment period expired. on June 19, 1986. The Comission received 53 letters comenting on the pro-pcsed rule. 45 of these were from the auclear industry. The other r> , raining 8 letters were submitted by tte linion of Concerned l

Scientists,- Department of Nuclear. Safeh' of the State 'of Illinois, and other individuals. The large major ity of industry comments were opposed to generic rulemaking.

Mr. Rubin briefly discussed the Nuclear Utilities Management and Resources Comittee (NUMARC) initiatives that they believe would resolve the SB0 issue without rulemaking. These initiatives are:

• Each utility will review their site (s) against the criteria specified in NUREG-1109, and if the site (S) fall into the-category of an eight-hour site after utilizing all power sources available, the utility will take actions to reduce the site (s) contribution to the overall risk' of station blackout.

Non-hardware changes will be made within one year. Hardware changes will be made within.a reasonable time thereafter.

Each utility will implement procedures at each of its site (s) for:

a. coping with a station blackout event, I

l b. restoration of AC power following a station blackout event, and

c. preparing the plant for severe weather conditions, such as hurricanes and tornados to reduce the likelihood and-l l

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, AC/DC Power Systems Reliability Minutes May 6, 1987 consequences of a loss of offsite power and to reduce the overall risk of a station blackout event.

Each utility will, if applicable, reduce or eliminate cold fast-starts of emergency diesel generators for testing through changes to technical specifications or other appropriate means.

Each utility will monitor emergency AC power unavailability utilizing data utilities provided to INP0 on a regulatory basis.

VII. Mr. M. McGarry, Consultant to NUMARC, stated that in February of f

1985, NUGSB0 (Nuclear Utility Group on Station Blackout) made a presentation to the ACRS, indicating that it was considering alternatives to rulemaking. In June 1986, NUMARC commented on the proposed rule and in addition presented four initiatives.

Briefly, they are the reduction of risk, procedures dealing with SB0 and power restoration in severe weather, reduction in cold fast starts of EDG, and monitoring the availability of EDGs.

NUMARC anticipates that work will be completed on these initiatives in the late summer /early fall of 1987.

VIII. As a result of the Subcommittee's discussion, the Subcommittee members raised some concerns regarding the following:

' Mr. Ebersole commented that a sharper and more explicit definition of a loss of AC power is required. He cited an example (for BWP.), that the " core spray system" it may or may net be excluded frum the present definition of loss of AC power.

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• Mr. Reed expressed some concern in regard to the fragment'ed approach which is used by the NRC Staff to resolve SBO. He .j stated that it could be very possible that the resolution of

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1 USI A-45 through an appropriate decay heat removal backup ^

system would resolve some of the concerns for USI A yet the Staf f are.not applying the integrated approach. j

• Mr. Ebersole expressed some concern .regarding a particular  ;

scenario in which a loss of offsite power could occur as a' result of a plant trip and subsequent failure to automatic- -)

transfer. He cited the Westinghouse. design.

Mr. Reed questioned the size aspect of the diesel generators and the relationship to reliability. He commented that small size EDG (ev 1000 KW) probably will be more reliable than the bigger size ( a 10,000 KW). )

l Mr. Wylie indicated that the Staff did'not perform an assess-ment with regard to the design of the switchyard configura- j tion.

Mr. Michelson questioned the changes that had been made to EDG .

to qualify them for nuclear service which eventually led to the reduction in reliability as o consequences. The Staff replied that, that could be true due to the way they have been  !

utilized in nuclear power plants.

• Mr. Michelson- questioned the possibility that a loss of AC power might accompany the. loss of the control room. He added

• that there is no requirements for the ability to maintain power of the offsite plant when there is a loss of power 'in control room.

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• Mr. Michelson expressed ~some concern in regard to fire' causing loss of offsite power. He also commented that if you lose the control. room due to a fire, there is no requirements to automatically set up the dieselst

• Mr. Michelson questioned the case if'there is a mechanistic connection between a seismic event end the loss of offsite power. The Staff replied that probability of a seismic event causing loss of offsite power at the SSE level, combined by -

the nonmechanistic failure of EDG is low enough to be accept-

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" Mr. Michelson cuestioned the use of non-seismically qualified equipment during a seismic event.

Mr. Reed expressed some concern regarding patching up the-

! designs and old concepts from 20 to 30 years ago, rather than 1

trying to advance conceptually. He referred to this technique as the " piecemeal" activities. Mr. Ebersole agreed and commented that the petching technique will deny the NRC the privilege of doing the right thing.

• Nr. Michelson commented that the Staff did not really analyze the situation after the SB0 event has occurred to determine the behavior of the equipment through out the nuclear power plant.

Mr. Wylie commented that the regulatory guite prepared by the ,

Staff is nnt very clear and is complex in ntsture to be'used as ,

a guidance for the industry to comply with the proposed SB0 rule.

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Outcome The Subcommittee' Chairman will brief the ACRS on May 8,1987, regarding the Subcommittee activities related to the station blackout proposed.

-rule. The NRC Staff will give.a brief presentation to the full Commit -

tee. Also, NUMARC will-have e short presentation regarding their.

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initiatives. 1

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NOTE: Additional meetino details can be obtained from a transcript of this meeting available in the NRC Public Document Room,.

1717 !i Street, NW, Washington, DC', or can be purchased from ACE-Federal Re orters, 444 ' North Capitol Street, Washington, DC 20001, (202 347-3700.

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