ML20153G884

Final Rule 10CFR50, Station Blackout. Amended Regulations Require That Light Water Cooled Nuclear Power Plants Be Capable of Withstanding Total Loss of Ac Electric Power, Station Blackout, for Specified Duration

ML20153G884
Person / Time
Issue date:	06/13/1988
From:	Chilk S NRC OFFICE OF THE SECRETARY (SECY)
To:
References
FRN-51FR9829, RULE-PR-50 NUDOCS 8809090019
Download: ML20153G884 (59)

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{u'c $ c b l $ 0 Federal Register Notice of Final Rulemaking BhN NUCLEAR REGULATORY COMISSION f

10 CFR Part 50 Station Blackout ,

AGENCY: Nuclear Regulatory Comission.

ACTION: Final rule.

SUMMARY

The Nuclear Regulatory Comission is amending its regulations to require that light-water-cooled nuclear power plants be capable of witnstanding a total loss of alternating current (ac) electric power (called "station black-out") for a specified duration and ir.2intaining reactor core cooling during that period. This requirement is based on information developed under the Comission's study of Unresolved Safety Issue A-44, "Station Blackcut." The amindment 1.4 intended to provide further assurance that a loss of both offsite pvwer and onsite emergency ac power systems will not adversely affect the public health and safety.

EFFECTIVE DATE: July 18, 1988 FOR FURTHER INFORMATION CONTACT: Aleck Serkiz, Division of Reactor and Plant Systems, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Comission, Washington, DC 20555, Telephone: (301)492-3555.

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SUPPLEMENTARY INFORMATION:

Background

I The alternating current (ac) electric power for essential and nonessential service in a nuclear power plant is supplied primarily by offsite powe'r.

Redundant onsite emergency ac power systems are also provided in the event that all offsite power sources are lost. These systems provide power for various safety functions, including reactor core decay heat removal and containment heat removal, which are essential for preserving the integrity of the reactor core and the containment building, respectively. The reactor core decay heat can also be removed for a limited time period by safety systems that are independent of ac power.

The term "station blackout" means the loss of offsite ac power to the essential and nonessential electrical buses concurrent with turbine trip and the unavailability cf the redundant onsite emergency ac power systems I,e.g., as a result of units out of service for maintenance or repair, failure to start on demand, or failura to continue to run after start). If a station blackout persists for a time beyond the capability of the ac-independent systus to remove decay heat, core melt and containment failuro could result.

The Comission's existing regulations establish requirements for the design and testing of onsite and offsite electric power systems that are intended to reduce the probability of losing all ac power to an acceptable level. (See l General Design Criteria 17 and 16,10 CFR Part 50, Appendix A.) The existing regulations do not require explicitly that nuclear power plants be designed to assure that core coolint: can be ruintained for any specified period of loss of all ac power.

l As operating experience has accumulated, the concern has arisen that the I reliability of both the onsite and offsite emergency ac power systems might be less than originally anticipated, even for designs that meet the requirements of General Design Criteria 17 and 18. Many operating plants have experienced a total loss of offsite power, and more occurrences can be expected in the l

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[7590-01) future. Also, operating experience with onsite emergency oower systems has included many instances when diesel generators failed to stet. In a few cases, there has been a complete loss of both the offsite and the onsite ac power systems. During these events, ac power was restored in a short time without any serious consequences.

In 1975, the results of the Reactor Safety Study (WASH-1400)I showed that station blackout could be an important contributor to the total risk from nuclear power plant accidents. Although this total risk was found to be small and not undue, the relative importance of the station blackout accident was established. Subsequently, the Comission designated the issue of station blackout as an Unresolved Safety Issue (USI); a Task Action Plan (TAP A-44) was issued in July 1980, and studies were initiated to determine whether additional safety requirements were needed. Factors considered in the analysis of risk from station blackout included: (1) the likelihood and duration of the loss of offsite power; (2) the reliability of the onsite ac power system; and (3) the potential for severe accident sequences after a loss of all ac pcuer, including consideration of the capaoility to remove core decay heat without ac power for a limited time period. .

The technical findings of the staff's studies of the station blackout issue are preseated in NUREG-1032, devaluation of Station Blac hut Accidents at Nuclear i Pwer Plants Technical Findings Related to Unresolved Sefety Issue A-44.

Additional information is provided in supporting contractor reports:

NUREG/CR-3226, "Statien Blackout A:cident Aaalyses," published in May 1983; NUREG/CR-2989 "Reliability of Er.ergency AC Power Systems at Nuclear Power Plants " published in July 1933; h0 REG /CR 3992. "Collection and Evaluation of l

A Copies of all NRC documents are available for public inspection and copying l

for a fee at the NRC Public Document Room at 1717 H Street, NW, Washington, DC 20555. Copies of published documents ma U.S. Government Printing Office by calling (y 275-2060 202) also be purchased or by writingthrough to the l

l the Superintendent of Documents, U.S. Government Printing Office, P. O. Box i

37082, Washington, DC 20013-7082.

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Complete and Partial Losses of Offsite Power at Nuclear Power Plants "

published in February 1985; and NUREG/CR-4347, "Emergency Diesel Generator Operating Experience, 1981-1983," published in December 1985. The major results of these studies are given below.

' Losses of offsite power can be characterized as those resulting ,from plant-centered faults, utility grid blackout, and severe-weather-induced failures of offsite pcwer sources. Based on operating experience, the frequency of total losses of offsite power in operating nuclear power plants was found to be about one per 10 site-years. The median restoration time was about one-half hour, and 90 percent of the offsite power losses were restored within approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> (NUREG/CR-3992).

'The review of a number of representative designs of onsite emergei.y ac power systems has indicated a variety of potentially important failure causes. However, no single improvement was identified that could result in a significant improvement in overall diesel generator reliability.

Data obtained from operating experience in the period from 1976 to 1980 showed that the typical individual emergency diesel generator failure r.ste was about 2.5 x 10-2 per demand (i.e., one chance of failure in 40 demands), and thet the emergency ac power system unavailability for a plant which has two emrgency ritesel generaturs, onc of which was l required 'or decay heat removal, wss about 2 x 10-3 per demand (NUREG/CR-2989).

' Compared to the data in NUREG/CR-2989, updated estimates of emergency diesel generator failure rates indicated that diesel generator reliability has improved somewhat from 1976 to 1983. For the period 1981 to 1983, the mean failure rate for all demands was about 2.0 x 10-2 per demand (i.e., one chance of failure in 50 demands). However, the data also indicate that the probability of diesel generator failures during actual demands (i.e., during losws of offsite power) is greater than i that during surveillance tests (NUREG/CR-4347).

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'Given the occurrence of a station blackout, the likelihood of resultant core damage or core melt is dependent on the reliability and capability of decay heat removal systems that are not dependent on ac powei, If sufficient ac-independent capability exists, additicnal time will be available to restore ac power needed for long-term cooling (NUREG/CR-3226). .

'It was determined by reviewing design, operational, and site-dependent factors that the expected frequency of core damage resulting from station blackout events could be maintained near 10-5 per reactor-year with readily achievable diesel generator reliabilities, provided that plants are designed to cope with station blackout for a specified duration. The duration for a specific plant is based on a comparison of the plant's characteristics to those factors that have been identified as the main contributors to risk from station blac' tout (NUREG-1032).

The staff's technical findings show that station blackout does not pose an undue risk to public health and safety. The findings sumarized above show that recovery from loss of offsite power occurs for the most part in less than 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, emergency diesel generator reliability is high (i.e., 2 0.95), end that given a station blackout the likelihood of core damage is more dependent on decay heat removal systems that are non-ac-dapandent. However, plant design and operational characteristics, plus site-dependent factors (such as anticipated weather conditions) introduce a level of variability which warrants a need for plant-specific coping analyses to provide greater assurance that core cooling can be maintained until ac power is restored. Thus the Comission believes that 650.63 of 10 CFR Part 50 will bring about a significant increase '

in protection to the public health and safety. As a result of station blackout coping analyses, improved guidance will be provided to licensees regarding maintaining minimum emergency diesel generator reliability to minimize the probability of losing all ac power. In addition, the Comission is amending its regulations by adding a new 550.63 to require that all nuclear power plants be capable of coping with a station blackout for some specified period of time.

The period of time for a specific plant will be determined based on a comparison

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of the individual plant's design with factors that have been identified as the main contributors to risk of core damage resulting from station blackout.

These factors, which vary significantly from plant to plant because of considerable differences in design of plant electric power systems as well as site-specific considerations, include: (1) redundancy of onsite emergency ac power sources (i.e., number of sources minus the number needed for decay heat removal), (2) reliability of onsite emergency ac power sources (usually diesel ,

generators), (3) frequency of loss of offsite power, and (4) probable time to restore offsite power. The frequency of loss of, and time to restore, offsite power are related to grid and switchyard reliabilities, historical weather data for severe storms, and the availability of nearby alternate power sources (e.g., gas turbines). Experience has shown that long duration offsite power outages are caused primarily by severe stonns (hurricanes, ice, snow, etc.).

The objective of the rule is to reduce the risk of severe accidents resulting from station blackout by maintaining highly reliabla ac electric power systems and, as additional defense-in-depth, assuring that plarts can cope with a station blackout for some period of tine. The rule requires all plants to be able to cope with a station blackout for a specified acceptable duration selected on a plant-specific basis. All licensees and applicants are required to assess the capability of their plants to cope with a station blackout (i.e.,

deter:nine that the plant can maintain core cooling witn ac power unavailable for an acceptable period of time), and to have procedures and +.rainirig to cope

with such an event. Licensees may use an alternate ac pcwer source if that source meets specific criteria for independence and capacity and can be shown to be available within one hour to cope with a station blackout. A coping analysis is not required for those plants that choose this alternate ac approach if the alternate ac can be demonstrated by test to be available to i

power the shutdown buses within 10 minutes of the onset of station blackout.

Use of an alternative ac source, one that minimizes comon mode failure, is a preferred option since this approach will also benefit other safety concerns.

On the basis of station blackout studies conducted for USI A-44 and presented in the reports referenced above, the NRC staff has developed Regulatory Guide

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1.155 entitled"StationBlackout,"whichpresentsguidanceon(1) maintaining a high level of reliability for emergency diesel generators, (2) developing procedures and training to restore offsite and onsite emergency ac power should either one or both become unavailable, and (3) selecting a plant-specific acceptable station blackout duration which the plant would be capable of sur-viving without core damage. Application of the methods in this guide would result in selection of an acceptable station blackout duration (e.g., 2, 4, 8, or 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br />) which depended on the specific plant design and site-related ,

characteristics acceptable to the staff. However, applicants and licensees could propose alternative methods to those specified in the regulatory guide in order to justify other acceptable durations for station blackout capability.

Additionally, the regulatory guide on station blackout presents guidance on quality assurance and specifications for alternate ac source (s) and non-safety-related equipment required for coping with station blackout. The equipment installed to meet the station blackout rule must be implemented so that it does not degrade the existing safety-related systems. This is to be accomplished by making the non-safety-related equipment independent to the extent practicable from existing safety-related systems. The guidance provided in the regulatory guide illustrates the specifications that the staff would find acceptable for non-safety systems and equipment. The quality assurance guidance for the non-safety-related equipnent for which there are no existing NRC quality assurance requirements (e.g., Appendix B, Appendix R) errbody the following elements: (1)designcontrolandprocurementdocument control, (2) instructions, procedures and drawings, (3) control of purchased material,equipmentandservices,(4) inspection,(5)testandtestcontrol, (6) inspection, test and operating status (7) non-confonning items, (8) correctiveaction,(9) records,(10) audits. NRC inspections will focus on the implementation and the effectiveness of these quality controls as described in the regulatory guide.

Based on the rule and regulatory guide, those plants with an already low risk from station blackout would be required to withstand a station blackout for a relatively short period of time and probably would need few, if any, modifi-cations as a result of the rule. Plants with currently higher risk from

[7590-01) station blackout would be required to withstand somewhat longer duration blackouts. Depending on their existing capability, these plants might need to make hardware modifications (such as increasing station battery capacity or condensate storage tank capacity) in order to cope with the longer station blackout duration. The rule requires that each light-water-c:uled nuclear power plant licensed to operate must be able to withstand fer a specified duration and recover from a station blackout. The rule reagires each plant to perform a coping analysis and identify the coping duration, along with the basis therefor and a description of procedures established for coping and recovery. If modifications to equipment or plant procedures are necessary, these are to be identified and a schedule provided for implementing such changes.

It should be noted, based on all evidence that staff has on hand, that no undue risk exists with, or without, the promulgation of the station blackout rule.

However, station blackout may still remain an important contributor to residual risk. This station blackout rule will enhance safety by accident prevention and thereby reduce the likelihood of a core damage accident being caused by a station blackout occurrence. This does not mean however, that further enhancements in reducing the overall residual risk are not achievable by additional improvements in severe accident management, given the assumption that core damage occurs, whether from station blackout sequences or other causes (such u small or large loss-of-coolant accident sequences).

Initiatives that provide such safety enhancements (through improvements of core damage management procedures) are currently being pursued apart from the

! station blackout rule. Therefore, this rule should be viewed as being in the same accident prevention context as the ATWS rule (650.62) and the fire protection rule (550.48) in that it recognizes, as the other two rules recognize, multiple failure possibilities resulting from comon cause effects that should be addressed. This concern has been recognized in the Introduction i to Appendix A of 10 CFR Part 50.

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Proposed Rule On March 21, 1986, the Comission published a proposed rule in the Federal Register (51 FR 9829) that would require (1) light-water-cooled nuclear power plants to be capable of coping with a station blackout for a specified duration, and (2) licensees to detemine the maximum duration for which their plants as currently designed are able to cope with a station blackout. A 90-day connent period expired on June 19, 1986.

On April 3, 1986 (13 days after the proposed rule was published), the NRC published in the Federal Register (51 FR 11494) a notice of availability and request for coments on a draft regulatory guide entitled "Station Blackout" (Task SI 501-4). This draft guide provided guidance for licensees to comply with the proposed station blackout rule. Many letter.s comenting on the proposed rule also included coments on the draft regulatory guide. Responses to these coments provided below address the public coments on the draft guide as well as on the proposed rule.

Coments. on the Proposed Rule The Comission received 53 letters comenting on the proposed rule.2 Forty-five of these were from the nuclear industry, comprised of electric utilities, consortiums of electric utilitics, vendors, a trade association, and an architect / engineering fim. Other letters were submitted by the Union of Concerned Scientists, the Department of Nuclear Safety of the State of Illinois, a representative of the Professional Reactor Operator Society, a citizens group, a consultant, and three individuals. Largely, the industry connents were opposed to generic rulemaking to resolve the station blackout issue. The Nuclear Management and Resources Council (hUMARC), fomerly the Nuclear Utilities Management and Resources Comittee, submitted, along with its l

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2 Copies are available for public inspection and copying for a fee at the NRC Public Document Room at 1717 H Street, NW, Washington, DC.

[7590-01) coments on the proposed rule, a set of four industry initiatives that it believes would resolve this issue without rulemaking. Thirty-nine of the industry letters supported NUMARC's submittal. NUMARC proposed a fifth initia-tive (see item 21) by letter dated October 5, 1987. On the other hand, the Union of Concerned Scientists, the Illinois Department of Nuclear Safety, and the citizens group supported the Comission's objective in the proposed rula, but did not believe the rule and guidance associated with the rule went far enough to reduce the possibility of a serious accident that could be initiated by a total loss of ac power.

Every letter was reviewed and considered by the staff in fomultting the final resolution of USI A-44. Because of the large number of coments, it was not practical to prepare formal responses to each one separately. However, since many coments were on similar subjects, the discussion and response to the comments have been grouped into the following subjects:3

1. Quality classification of modifications

2. Whether the backfit analysis adequately implements the Backfit Rule

3. Cost-benefit and whether $50.63 meets "substantial increase in the overall protection of the public health and safety"

4. Whether NRC should require substantial improvements in safety that go beyono those proposed in this rulersking

5. The need for generic rulemaking

6. Applicability of the proposed $50.63 to specific plants

7. Plant-specific features and capabilities 3 ones on which the Comissioners specifically The first four subjects ..e requested public coments when the proposed rule was published.

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8. The source term used to estimate consequences

9. Specificity on the extent of required coping studies

10. Acceptable duration for coping with a station blackout

11. Credit for alternate or diverse ac power sources

12. Trends on the reliability of ac power sources

13. Sharing of emergency diesel generators between units at multi-unit sites

14. Clarification of the definitions of station blackout and diesel generator failure

15. Specificity and clarification of requirements

16. Technical comments on NUREG-1032

17. Relationship of USI A.44 to other NRC Generic Issues

18. An alternative of plant-specific probabilistic assessments

19. Procedures and operator actions during station blackout

20. Schedule provisions in the proposed $50.63.

21. Industry initiatives The comments and responses to each of these subjects are presented on the i following pages.

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1. Quality Classification of Modifications The Comission requested corcents on whether the staff should give further consideration to upgrading to safet/ grade the plant modifications needed (if any) te meet the proposed rule. Upgrading to safety grade would further ensure appropriate licensee attention is paid to maintaining equipment in a high state of operability and reliability.

Coments - The prevailing view by industry on this subject is represented by the following coments submitted by NUMARC:

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Quality classification is ba.necessary - Equipment used to prevent or respond to a station blackout should be sufficiently a ailable and operable to meet its required function. To this exten , the Comission's desire that appropriate attention be paid to maintaining a sufficiently high state of operability and reliability is appropriate. The point of departure begins with the method for achievii.9 this objective.

Specifically, by itself, a "safety grade" classification scheme does not solelv %uate with high states of equipment operability and reliability.

Such M asW fication systems too often can become a documentation exercise rnore tnan a process for providing the requisite level of system functionality.

Duquesne Light agreed with this view and expressed the following coments:

Any plant modifications or additional equipnent required to meet the proposed rule should nct be specified safety grade. For equipment which is to be manutlly started and placed in service for testing or in the 6 event of a loss of power condition there is no necessity for specifying safety grade since adequate reliability can be obtained through nonnal surveillance testing and the proper maintenance of comercid power plant equipment. The cost difference in safety grade vs. comercial grade modifications is significant and must be empharized.

The opposite point of view was taken by the Illinois Department of Nuclear Safety.

No credit should be given for the capability of equipment to respond to a station blackout unless that equipment was originally designed, dmi19 constructed, inspected, performance tested, qualified, certified for the intended safety-related purposi , and the equipment is maintained to the highest industry safety standards.

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Gulf States Utilities connented, The proposed rule does not provide sufficient direction on the quality classification of plant modifications that may be required to meet the rule.

...the quality classification of piant modifications implemented to meet the proposed rule should be connensurate with classification of the system they support. ,

Response - The proposed $50.63 does not specifically address the topic of safety classification of plant modifications; however, detailed guidance is ,

provided in Regulatory Guide 1.155 dealing with quality assurance and equipment specifications for non-safety-related equipment. Any safety-related equipment used either presently, or in modifications resulting from this rule, should meet the criteria currently applied to such equipment.

The technical analyses performed for USI A-44 (NUREG-1032) show that plant-ce,'.ered events (i.e., there events in which design and operational characteristics of the plant itself play a role in the likelihood of loss of offsite perr), and area- or weather-related events (e.g., grid reliability or external influences on the grid) are the dominant causes of loss of offsite power. Neither seismic events nor events related to single failure causes were found to be ma.for contributors to loss of offsite power. Therefore, both the staff's findings and public coments received do not support an explicit need for plant modifications for coping with station blackout to be seismically qualified.

The st.bstantial increase in protection sought by this rule can be achieved by modifications which meet criteria somewhat less stringent than generally required by safety grade criteria. Safety-related equipment modifications to meet all safety-grade-related criteria would be more burdensome and expensive

! and would likely achieve only a very small further reduction in risk. The major contributors to the residual risk of loss of offsite power are adequately dealt with by modifications which confonn to the quality assurance and equipnent specification guidance provided in Regulatory Guide 1.155.

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2. Whether the Backfit Analysis Adequately Implements the Backfit Rule In addition to corrrnents on the merits of the proposed rule, the Comission specifically requested coments on whether the backfit analysis for this rule adequately implements the Backfit Rule, 550.109 of 10 CFR Part 50.

Coments - The Commission received two differing views in response to this request. On one hand, NUMARC expressed the view that the proposed rule does not meet the backfit rule standard because the analysis of the factors set forth in 150.109(c)werenotadequatelyconsideredbythestaff. Specifically, NUMARC stated:

1. Installation and continuing costs associated with the backfit have beon underestimated.

2. Potential impacts on radiological exposure of facility employees should be further addressed.

3. The relationship to proposed and existing regulatory requirements should be considered further,

4. Potential impacts of differences in facility, type, design, or age should be considered further.

5. The reduction in risk from offsite releases to the public has been overestimated.

I On the other hand, the Ohio Citizens for Responsible Energy (OCRE) and the i

Union of Concerned Scientists comented that the backfit rule should not apply to the proposed rule. OCRE took the position that "application of the backfit rule to (NRC] rulemakings ... is plainly illegal," and the Comission is not empowered to considar costs to licensees in deciding whether to impose new requirements. The Union of Concerned Scientists comented that the cost-benefit analysis should not be applied in this cese because safety improvements

(7590-01) are needed to secure compliance with existing NRC regulations, specifically General Design Criterion 17. Electric Power Systems (Appendix A to 10 CFR Part 50).

Response - NUMARC's coments on the backfit analysis were taken into account by the staff in revising the draft version of NUREG-1109, "Regulatory Backfit Analysis for the Resolution of Unresolved Safety Issue A-44 Station Blackout," and a separate appendix that addresses the factors in 150.109(c)was added to that report. All but Item 2 above are on the same subjects as letters from other com;nenters and are discussed in more detail under subjects 3 (Item 1),6(Item 4),8(Item 5),and17(Item 3)inthissection. NUMARC's Item 2, the potential impact on radiological exposure of facility employees, would need ,

to be assessed in detail only if it were a major factor in the value-impact analysis. The effect of radiological exposure on facility employees, if any, would be extremely small in comparison to the reduction in radiological exposure to the public from accident avoidance. Therefore, this factor would have no impact on the overall value-impact analysis.

Contrary to OCRE's and the Union of Concerned Scientists' coments, the Comission may subject, the rulem;. king process to internal controls. Moreover, the Comission is empowered to consider the costs of incremental safety improvements which go beyond the level of safety necessary to ensure no undue j

risk to the public health and safety. See UCS, et al., v. NRC, D.C. Cir. Nos.  ;

85-1757 and 86-1219 (August 4, 1987). The improvements embodied in 550.63 go beyond the level of safety necessary to ensure no undue risk. Finally, I

contrary to the Union of Concerned Scientists' coment on GDC 17 new station blackout measures cannot be imposed on licensees as a matter of compliance with -

l GDC 17, under the compliance exception in the backfit rule, paragraph

, 50.109(a)(4)(1). GDC 17 does not explicitly require that each plant be able to withstand station blackout for a specified time, or that each licensee perfonn a coping assessment and make whatever modifications may be necessary in the light of that assessment. Nor are any of these highly specific requirements logically compelled by any part of GDC 17. Moreover, GDC 17 has never been

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interpreted by the staff or the Comission to contain these specific requirements. Thus, to impose them under GDC 17 would amount to a backfit which resulted from a new staff and Comission interpretation of GDC 17.

The issue in this rulemaking is whether some additional protection is warranted beyond that already provided. The Comission is entitled to inquire. 'and seek public coment on, whether additional safety measures should be imposed where there is a substantial increase in the overall protection of public health and safety and the co:t of implementation is justified in view of this increased protection.

3. Cost-Benefit Analysis and Whether 150.63 Meets the "Substantial Increase in the Overall Protection of the Public Health and Safety" Chef rman Zech and Comissioner Roberts requested comments on the analysis of cost benefit, value impact, and safety improvements and the station blackout standing on the overall risk (e.g., is the reduction of risk only a small percentage of the overall risk, or is it a major component of an already small risk?). Chairman Zech and Comissioner Roberts were particularly interested in specific coments assessing whether or not this proposal meets the "substantial increase in the overall protection of the public health and safety..."

threshold now required by the backfit rule.

Coments - (A) One of the major coments by industry on the cost-benefit analysis was that the costs of implementing the proposed requirements have been underestimated. NUMARC and the Atomic Industrial Forum (AIF) comented that the cost estimates for hardwarc modifications reported in NUREG/CR-3840, "Cost Analysis for Potential Modifications To Enhance the Ability of a Nuclear Plant To Endure Station Blackout," were too low. Comonwealth Edison and other utilities felt that performanes of an analysis to determine the maximum duration a nuclear plant could rope with a station blackout would be substantially costlier than what is estimated in NUREG-1109. Industry also expressed concern that the interpretations associated with the proposed rule could lead to substantial costs above those addressed by the NRC staff in its

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backfit analysis. AIF connented that "The estimate of 120 NRC man-hours per plant [for NRC review] ... appears inadequate to account for technical review and evaluation of the dete mination of maximum coping capability and of the description of station blackout procedures which the rule would require each licensee to submit."

4 (3) Several connenters expressed the view that the NRC failed to consider all the risks associated with a station blackout in its value-impact assessment.

The Union of Concerned Scientists thought independent failures, in addition to

failures that lead to a station blackout, should be included. One individual stated that "both NRC reports [NUREG-1109 and NUREG.1032] are completely 4

deficient in that neither look at sabotage." OCRE connented that seismic events should also be considered.

1 (C) With respect to safety improvements and overall risk, di/ferent points of view were expressed. On one hand, NUMARC comented that, while the risk l

i reduction might be large for a limited number of plants, the risk reduction l

associated with the majority of plants will be small. Thus, ss a general l

matter, the reductions in risk offered by the proposed rule constitute a small percentage of the overall risk, a risk which is already small (and acceptable).

AIF stated that there is no standard by which to concludo that "substantial l additional prctection will be realized."

A different view was expressed by the Union of Concerned Scientists who stated

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l that "station blackout is clearly a major component of the total risk posed by f operating nuclear plants. The magnitude of the total risk is largely unknowable due to the enonnous uncertainty which surrounds probabilistic l

assessments."

Response - (A) In order to adequately respond to industry's connents above, the staff and NRC contractors reviewed the cost estimates associated with imple-menting the station blackout rule. Based on this review, the estimated costs for hardware modifications were reviewed and are in the range of from 20 parcent to almost 140 percent greater than the estimates in NUREG/CR-M40,

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depending on the specific modification considered. On average, the cost estimetes for hardware backfit were found to be approximately 80 percent greater than estimated in NUREG/CR-3840. However, the cost estimates in NUREG/CR-3840 were not used by the staff in the value-impact analysis in the draft version of NUREG-1109 where estimates approximately 100 percent greater than the NUREG/CR-3840 estimates were used. Therefore, the revised cost estimates used in the final value-impact analysis are not significantly different from the estimates used in the draft version.

9 Industry's comments on the costs to assess a plant's capability to cope with a station blackout were based on the proposed rule that required an assessment of the maximum coping capability and the potentially unbounded nature of such an assessment. Based on public consnents, the Consnission has revised the final rule to modify the requirement for licensees to determine the maximum coping capability. (See response to public consnents in subject number 9.) Instead, a coping assessment is required only for a specific duration. The cost for such a study is estimated to be from 70 to 100 percent higher than the original estimates by the staff, and these revised costs are used in the final value-impact analysis.

The staff revised its estimate of the resource burden on NRC for review from 120 to 175 person-hours per reactor. This revision was based on technical review required for other comparable NRC activities.

(B) The technical analyses perfonned for USI A-44 indicated that the contribution to core damage frequency from independe.t failures, in addition to failures that must occur to get to a station blackout, is low. Likewise, results of US! A-44 studies anc other probabilistic risk assessments have shown that, for station blackout sequences, the contribution to core damage frequency from seismic events is low.

Not all events can be analyzed on a probabilistic basis. Sabotage is an example. Even though sabotage was not explicitly considered in the staff's value-impact analysis, it is discussed in NUREG-1109 under other

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considerations. Tilese considerations support the conclusion that a station blackout rule will provide a substantial safety benefit.

(C) The revised value-impact analysis perfonned for the resolution of USI A-44 indicates that there are sebstantial benefits in terms of reduced core damage frequency and reduced risk to the public that result from the station blackout rule, and the costs are warranted in light of these benefits. The best estimate for the overall value-impact ratio is 2,400 person-rem per million ,

dollars. Even if those plants with the highest risk (and therefore the greatest risk reduction) were not considered, the value-impact ratio for the remaining plants is still favorable (i.e., about 1,500 person-rem per million dollars).

Analyses reported in NUREG-1150, "Reactor Risk Reference Dcnument" (draft issued for coment in February 1987),4 indicate that station blackout is a dominant risk contributor to overall residual risk for most of the Six plants analyzed. Tnese results support the coment by the Union of Concerned Scientists in response to the Comissioner's request for corments on this subject.

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4. Whether NRC Should Require Substantial Improvements in Safety that Go Beyond Those Proposed in this Rulemaking.

Comissioner Asselstine requested coments on whether the NRC should require substantial improvements in safety with respect to station blackout, like those being accomplished in some other countries, which can be achieveo at reasonable cost and which go beyond those proposed in this rulemaking.

Coments - NRC received eight letters that included coments on this subject.

Five of these were from the nuclear industry, none of which felt that the 4

Free single copies may be obtained from the Division of Information Support Services, U.S. Nuclear Regulatory Cocynissiun, Washington, DC 20555.

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approach to station blackout taken in European countries should be used to justify safety improvements that go beyond the proposed $59.63. The main justification for industry's argument is that foreign countries may have reasons for requiring activities that differ from, or exceed, those in the U.S.

For example, Washington Public Power Supply Systems (WPPSS) connented, "It is not apparent that the details of U.S. grid stabilities and onsite power reliabilities are substantially similar enough to those found abroad to warrant a simple adoption of these [ European] measures."

In another comment from industry on this subject, NUMARC stated that there are several reasons why many of the features for coping with a station blackout in new French nuclear power plants may already exist at most U.S. plants. In fact, they said, "The French approach to station blackout does nnt appear to depart significantly from current regulatory approaches in the U.S." Similarly, AIF stated, "The assertions of extensive station blackout coping capability at foreign (notably European) nuclear power plants are not sufficiently substantiated to serve as even part of the basis for the proposed requirements."

Three other letters (Union of Concerned Scientists. OCRE, and Illinois Department of Nuclear Safety) supported the NRC rulemaking to require all plants to be able to cope with a station blackout, but urged the Comission to go beycnd the proposed rule. The Illinois Department of Nuclear Safety stated that:

Thegoalofholgingtheexpectedfrequencyofcoredamagefromstation blackout to 10' per reactor-year is not sufficiently stringent. With relatively modest modifications to the proposed rule, a frequency of 10,7 appears achievable at reasonable cost. Specifically, the rule should require no less than 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> decay heat removal capacity instead of only four or eight hours in the proposed rule, in the event of a blackout.

Reponse - The staff agrees with industry's coments that foreign countries may have valid reasons for imposing requirements that differ from or exceed those in the U.S. For example, it appears that there is a higher frequency of losses of offsite power in France than in the U.S. This experience, along with French safety objectives, led the French to design their new standard nuclear power

_ _ _ _ _ _ . 1

[7590-01) plants to be able to cope with a very long duration station blackout (i.e., up to three days). The French safety approach and their station blackout design features are documented in NUREG-1206, "Analysis of French (Paluel) Pressurized Water Reactor Design Differences Compared to Current U.S. PWR Designs " June 1986.

The Commission believes that the staff has adequately considered foreign approaches in preventing core melt from station blackout in developing the resolution of US! A-44. Although the rule requires plants to be able to cope with station blackout for a specific duration, that duration is not specified is the rule. Guidance to detennine an acceptable duration is included in Regulatory Guide 1.155. This guidance should apply to most plants, but if there were adequate justification, different requirements (either more or less stringent than the regulatory guide) could be applied to specific plants. The use of alternate ac sources provides a means to achieve further incremental decreases in core melt frequency.

5. The Need for Generic Rulemaking Comments - Five letters from the nuclear industry comented that generic rulemaking is not necessary to resolve the station blackout issue. Their reasons for this issue were as follows:

A generic rulemaking is inappropriate since the historic number of sites

experiencing a loss of all offsite power is small. (TexasUtilities)

The station blackout issue should be handled on a plant-specific basis and l does not need to be resolved by generic rulemaking. Each plant has unique probability for a loss-of-power event based on transmission system, location of plant, and onsite power systems. (Duquesne Light)

The Comission need not pursue generic rulemaking in order to resolve a non-generic issue. In the proposed station blackout rule, the number of j plants of concern is acknowledged to be limited. (NUMARC)

Station blackout has been found not to be a generic issue. Station blackout risk is plant specific and, according t*d the staff's cwn analyses, the proposal requirements are expected to result in modifications at no more than a few facilities, if at any. Requiring all i

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licencees to undertake extensive analyses under the provisions of the proposed rules when only a small group of plants may have a need for remedial action is not apprcpriate. (AIF)

Response - The Comission believes that a rule is appropriate to ensure that station blackout is addressed at all nuclear power plants. The plant-specific features that contribute to risk for station blackout (e.g., diesel gene'rator configuration, probability of loss of offsite power) are considered by the staff in the station blackout regulatory guide to detennine an acceptable coping duration for each plant. Even though not all sites have experienced a loss of offsite power, there is not sufficient assurance that such events would not occur in the future. Since historic experience has shown that a total loss of offsite power occurs about once every 10 site-years, and many nuclear plants have operated for less than 10 years, it is not surprising that some plants have experienced a loss of offsite power while others have not.

Even though it is likely that many plants will not need hardware modifications to comply with the rule, the assessment of station blackout coping capability for a specific duration and implementation of associated procedures will affect a safety benefit for all plants. The "limited number of plants of concern" in NUMARC's letter refers to those plants having the highest risk from station blackout (i.e., those that would need hardware modifications). Without a plant-specific assessment, these plants can not be identified. Even excluding these plants from consideration, the staff's analysis has shown that the improvements in safety associated with the rule are consistent with backfit considerations set forth in 650.109.

6. Applicability of the Proposed $50.63 to Specific Plants Coments - Four letters included conenents or questions regarding the appifcability of the rule to specific plants. For example, does the rule apply to high-temperature gas-cooled reactors (HTGR) (i.e., Fort St. Vrain)? What about THI-2 or plants that are near completion but will not have an operating

! license prior to the amendment's effective date? Houston Power and Lighting l

Company wrote:

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Proposed Section 50.63 provides scheoular guidance for implementing station blackout-related modifications on plants that already hold oper-ating licensees or will be licensed to operate prior to the effective date of the amendment. Plants who may be NT0L's (near-tenn operating license but will not be licensed prior to the amendment's effective date should be accorded the same compliance period under parts (c) and (d) of this section. Otherwise this proposed rule could be interpreted to imply that plants not licensed prior to the effective amendinent date must comply with the rule and make all necessary modifications prior to receiving 'an 0.L.

[operatinglicense]. The rule should be a anded to address plants which are scheduled to receive an 0.L. within a short time following imple- F mentation of this rule.

Response - Rather than identifying specific plants for which the rule does not

apply, 650.63(a) specifies when it does apply (i.e., "each light-water-cooled nuclear power plant licensed to operate"). Since Fort St. Vrain is an HTGR, the generic rule would not apply. Station blackout will be considered individually for that plant based on its unique design. Since THI-2 is not licensed to operate, likewise the rule would not apply to that plant. Any plant licensed to operate after the date the rule becomes effective will comply with the same 270-day schedule for information submittal applied to plants previously licensed. This affords NT0Ls the same compliance features as plants already licensed to operate.

7. Plant-Specific Features and Capabilities Consnents - A number of utilities described plant-specific features and

(

i capabilities that reduced the risk posed by a station blackout event compared to the staff's analysis. Examples of such features are given below.

I

' Availability of alternate, independent ac power sources such as diesel

]

j generators, gas turbines, or nearby "black start" ac power sources.

l

• Extremely reliable offsite power supplies because of multiple l right-of-ways or underground feeders to back up above ground transmission I lines.

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! ' Dedicated shutdown systems and associated diesel generators to meet the fire protection requirements of Appendix R to 10 CFR Part 50.

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' Common or shared systems between two units at multi-unit sites such as direct current (de) power, auxiliary feedwater, or diesel generators.

Response - The analyses perfonned for USI A-44 clearly show that plant-specific features do affect the risk from station blackout, and the station blackout regulatory guide takes this into account in providing guidance on different acceptable coping durations depending on the most significant of these features. Those plants with extremely reliable offsite and onsite ac power .

supplies need only have a very short (e.g., 2-hour) coping duration to be acceptable. Plants that have a dedicated shutdown system with its own inde-pendent power supply could take credit for this system to cope with a station blackout. The final rule and Regulatory Guide 1.155 have been clarified to give credit for alternate ac oower supplies (see response to subject 11).

Therefore, the Comminion believes that for almost all sites, plant-specific differences have been adequately accounted for in the resolution of USI A-44, but the door is open to licensees who believe their plants have additional capability that should be considered by the staff in demonstrating compliance with the rule.

8. The Source Term Used To Estimate Consequences Corments - NUMARC and others in the industry commented that the consequences of offsite releases that would result from a station blackout event are overestimated, and new source term infonnation would lead to the prediction of much lower consequences for this event. Several comenters felt that the approach taken by the staff to estimate consequences of a station blackout event was improper -- decreasing by a factor of three the estimated consequences of the siting source term (SST1) from NUREG/CR-2723 "Estimates of the FinancialConsequencesofNuclearPowerReactorAccidents"(September 1982).

AIF felt that "implementation of any requirements resulting from the resolution of USI A-44 should be deferred until the results of the Jource term research

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can be taken into account." They based this statement on the premise that if the consequences used in the staff's value-impact analysis were reduced by a factor of 10 none of the alternatives would be feasible.

The Union of Concerned Scientists expressed a different point of view in their letter which said "... available evidence indicates that the consequences of an accident involving station blackout may be even worse than those estimated either in WASH-1400 or the NRC's more recent studies." ,

Response - NRC has had an extensive research effort underway since about 1981 to evaluate severe accident source terms. The staff has reviewed the results of this research to take into account the public coments received on this subject. Since there is still a great deal of uncertainty regarding source terms and associated consequences, the staff revised its value-impact analysis for USI A-44 considering a range of estimates for consequences of a station blackout.

The NRC research on severe accident source terms has resulted in the develop-ment of significant new analytical tools by NRC contractors, as discussed in NUREG-0956, "Reassessment of the Technical Bases for Estimating Source Terms,"

July 1986. The analytical methods developed, generally referred to as the Source Term Code Package (STCP), have been used to analyze a number of severe accident sequences for five reference plants, namely: Peach Bottom, a BWR Mark I design; Sequoyah, a PWR ice condenser; Surry, a PWR with a s 2b-atmospheric containment; Grand Gulf, a BWR with a Mark III containment; and Zion, a PWR with a large dry containmr (NUREG-1150, "Reactor Risk Reference Document "

Draf t for Coment Februarj 1987).

The results of these analyses show that releases from station blackout sequences can be expected to vary significantly depending upon tne plant and the specific sequence. Although generalizations are difficult, it appears that calculations using the STCP yield release fractions for most of the sequences range from about one third of an SST1 release (for the case of Surry, without

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condensation) to roughly one order of magnitude less than this. However, the uncertainties in our present understanding also do not preclude the possibility of a large release, approaching that of the SST1 estimate.

To detemine the consequences in tems of person-rem, given the above range of release fractions, data taken from NUREG/CR-2723 indicate that the variations in person-rem associated with releases of magnitude SST1, SST2 and SST3 are virtually identical to the variations in latent cancer fatalities for the same three releases. Hence, the estimated change in latent cancer fatalities with release fractions provides a reliable indication of change in person-rem as well.

Table 10 in NUREG/CR-2723 presents variations in estimated latent cancer fatalities associated with changes in SST1 release fractions (for all elements exceptnoblegases). This table shows that a release fraction of one~ third of an SST1 release would yield a value of about 50 percent of the latent cancer fatalities (and person-rem) of an SST1 release. Similarly, a release fraction of one-third of an SST1 release would yield an estimated person-rem of about 15 percent of that associated with an SST1 release. Consaquently, for value-impact calculations, the staff estimated the range of consequences of station blackout, in tems of person-rem, to be from 0.15 to 0.5 of the estimated person-rem of an SST1 release. As noted, the original value-impact analysis was based on 0.3 times the estimated person-rem of an SST1 release.

l With regard to a possible delay in the resolution of USI A-44 until "better" source tems become available, key considerations appear to be when better source tems are likely to become available and to what degree uncertainties in phenomenology as well as differences between investigators will be resolved.

Although research on source tems is expected to continue well into the future, improvements in our knowledge are expected to be largely evolutionary beyond this point, in that the major phenomena appear to have been accounted for, at least in a first-order fashion, both in NRC as well as industry models.

Resolution and narrowing of the remaining uncertainties would also benefit from improved experiments and analytical enodels that are likely to become available y-- - - - - -- - -- -- - -

[7590-01) gradually. For these reasons, significantly better source terms than those presently available are likely to be forthcoming only after a number of years.

Since the range of severe accident source terms and consequences suggested above from estimating station blackout sequences is sufficiently broad to cover likely improvements in source term knowledge, the resolution of USI A-44 should not be delayed. *

9. Specificity on the Extent of Required Coping Studies Comments - Several letters by industry expressed concern that the studies necessary to demonstrate that a plant can cope with a station blackout are not well defined end could potentially be unbounded. These corrnents focused on two main points. First, the proposed rule required plants to determine the maximum duration the plant could cope with a station blackout, yet the draft regulatory guide included specific guidance on acceptable coping durations (e.g., 4 or 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />). Determining the maximum duration, rather than assessing the plant's capability for a specific acceptable duration, could be an open-ended requi rement. Along these lines, NUMARC stated:

Unless the required coping demonstration is specifically bounded by clearly stated definitions, assumptions, and criteria, there could conceivably be hundreds of supporting special effects analyses which licensees may have to consider as a result of the exercise of discretion by individual staff reviewers. Under the rule as proposed, licensees cannot ascertain the ultimate requirements they will be expected to meet (including the potential plant modifications they will need to make) to demonstrate compliance.

Second, industry also commented on the potential open-endedness of analyses to determine the operability of equipment in environmental conditions resulting from a station blackout (e.g., without heating, ventilation, and air condition-ing). Unless these analyses were well defined, industry felt the analyses could be much more costly than estimated by the staff. However, NUMARC made the following statement relating to the need for detailed prescriptive require-ments by NRC that appears to contradict their earlier statement.

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The point .... is not that regulations must be prescriptive by their very nature. Prescriptive regulations, which outline in detail exactly what steps are required by licensees to satisfy a proposed regulation, are, in many instances, unnecessary and counterproductive.

Response - With regard to the proposed requirement that each plant detemine  ;

its maximum duration for coping with station blackout, the staff agrees with the industry comments. First of all, it would be difficult to adequately define "maximum duration" in this sense. Second, if licensees detemine that their plants can cope with a station hiackout for a specified duration and restore ac power through an acceptable coping analysis, the additional safety benefit gained from simply the knowledge that a longer, or "maximum duration,"

coping duration exists is small. Third, the costs for assessing "maximum l duration" will be higher since more extensive analyses will be required to analyze a transient which would go beyond the coping analysis for a specified duration and recovery from station blackout. Therefore, the rule and regulatory guide have been revised accordingly to delete the requirement for licensees to determine a plant's maximum coping capability.

With regard to the comments on assessments to determine equipment operability i during a station blackout, the staff feels strongly that such assessments are necessary to determine a plant's response to station blackout. By deleting the requirement to detemine a plant's "maximum" coping capability, the assessment of equipment operability would not be as costly as assumed by industry.

Guidance on acceptable coping assessments is provided in the station blackout regulatory guide. Also, guidelines to evaluate the effects of loss of ventilation under station blackout conditions are provided in Appendix E of NUMARC-8700, "Guidelines and Technical Bases for NUMARC Initiatives Addressing ,

Station Blackout at Light Water Reactors." These efforts provide additional definitions, criteria, and standards for licensees' assessments of equipment operebility without the need for "prescriptive regulations" by NRC.

! In order to further evaluate industry's coments on this subject NRC requested l Sandia National Laboratories to identify specific tasks necessary to detemine operability of equipment during a station blackout and to estimate the cost to t

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perform these tasks. Results of this study were used in the revised value-impact analy:is perfonned for this issue ("Equipment Operability During Station BlackoutEvents."NUREG/CR-4942).

10. Acceptable Duration for Coping with a Station Blackout Coments - Several coments with differing views were directed at guidance in the draf t regulatory guide on acceptable station blackout coping durations in order for plants to comply with the proposed rule.

Washington Public Power Supply comented that "it should be possible for certain utilities to demonstrate [an acceptable) zero hour blackout."

One individual recomended "that a 30 minute period be a margin, and that no duration under 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> be accepted by the staff." NucleDyne Engineering comented that "advanced reactors should require the capability to safely withstand a station blackout of at least 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />," and the Illinois Department of Nuclear Safety wrote that "the rule should require .io less than 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> decay heat removal capability instead of only 4 or 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />."

Response - Although diverse coments were received on this subjec:, none provided supporting analysis or infomation to back up the opin4ns expressed.

However, the staff did reanalyze the estimated risk from station blackout events for different plant- and site-related characteristics and revised its guidance on acceptable coping durations accordingly based on a goal of limiting the average contribution to core damage from station blackout to about 10-5 per reactor-year. Most plants would still need a 4- or 8-hour coping capability.

Those few plants with the most redundant onsite emergency ac power system, coincident with significantly lower than everage expected frequency of loss of offsite power, would need only a 2-hour capability to be acceptable. Any plant with minimum redundancy in the onsite emergency ac power system coincident with low reliability and a significant*,y higher than average expected frequency of loss of offsite power would need to substantially improve its ac power j reliability or be able to cope >ith a station blackout for 16 hours1.851852e-4 days <br />0.00444 hours <br />2.645503e-5 weeks <br />6.088e-6 months <br />.

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11. Credit for Alternate or Diverse AC Power Sources Consnents - Ten letters from the utility industry comented that more credit should be allowed for the availability of alternate power sources such as onsite gas turbines. The coments below represent the utilities' viewpoint.

The station blackout rule should be clarified to allow credit for diverse and very reliable offsite power sources or diverse and very reliable onsite electrical generation. (Public Service Company of Colorado)

• The option of 3roviding an additional alternate source of ac power is eliminated by :the proposed resolution). The inconsistency in this app roach can best be understood by considering an example at a generic noc. lear power station. (Toledo Edison)

!! tr.a licensee were to provide an additional independent diesel generator capable of providing the necessary ac power to prevent station blackout, the licensee ... would still be required to withstand at least 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> l without ac power. They would receive no credit for the additional diesel generator in the coping analysis. If the licensee were to use that same l diesel engine to power a charging pump, even though it would be of less significance to mitigation of reactor core damage than the diesel generator, the licensee could take credit for it in coping with the ,

blackout. (Toledo Edison)

Since a diesel-powered charging pump will not provide for equipment loading flexibility, lighting, ventilation, instrumentation, etc., it is obviously of lower value than an additional source of ac power. The fixed category approach taken in [the proposed resolution), however, will not

, permit taking credit for the same diesel engine when used as a generator though the actual reliability for the machine is the same. (Toledo Edison)

Response - The proposed regulation did not intend to ignore the alternative of adding additional power sources or taking credit for such sources if they already exist. For er. ample, as specified in the regulatory guide, if a licensee added an emergency diesel generator to one of its plants that had minimum redundancy in the onsite emergency ac power sytem, the acceptable station blackout coping duration could be reduced. For some plants, however, adding a diesel generator would not result in a reduction in the acceptable coping duration, and the point made by Toledo Edison is a valid one. The rule and regulatory guide have been revised to clarify that alternate ac power I sources are given credit to cope with a station blackout piovided that certain 1

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criteria are met (e.g., independence, redundancy, high reliability, maintenance, and testing).

12. Trends on the Reliability of AC Power Sources Consnents - Five letters included coments on the reliability of ac power sources. Four letters from industry felt that improved ac power reliability should be factored into the staff's technical analysis. Examples of these ,

comments include the following:

. .. the f requenc decreasing..." (yWashington of loss ofPublic offsitePower power activities Supply has been System);

... offsite power availability in the absence of regulation has significantly improved over the past decade." (Southern California Edison Company);

"[NUREG/CR-4347]...showsanimprovementindieselgeneratorreliability over that shown in the earlier document [NUREG/CR-2989)." (General Electric);and "Typically the reliability of onsite power systems increases during the ,

first few years following startup." (Gulf States Utilities)

The Illinois Department of huclear Safety, on the other hand, felt that potential vulnerabilities still exist in onsite emergency ac power systems, and licensees should demonstrate that they have taken step: to reduce the probability of loss of ac power.

Response - The staff and its contractors have extensively analyzed the industry experience and trends in ac power reliability as documented in NUREG-1032, NUREG/CR-2989, NUREG/CR-3992, and NUREG/CR-4347. Trends have shown that two aspects of ac power reliability have improved somewhat -- the reduced frequency of losses of offsite power due to plant-centered events, and a slight improvement in average diesel generator reliability from 1976 through 1983.

These factors have been taken into account in the staff's analyses and the resolution of US! A-44. However, data also demonstrate that there are practical limits on ac power reliability, and the defense-in-depth approach of being able to cope with a station blackout is warranted.

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13. Sharing of Emergency Diesel Generators Between Units at Multi-Unit Sites Connents - Several letters from industry stated that some plants with two units on a site have the capability to crosstie electrical buses between units and therefore have improved flexibility in providing ac power. Since the magnitude of the electrical loads necessary to provide core cooling during a sta, tion blackout is significantly less than that required for a design basis accident, it could be possible to provide ac power to both units at the site using only a single diesel generator.

Response - The proposed rule and draft regulatory guide de not prohibit the approach discussed above. If licensees can demonstrate that such crosstie capability exists, procedures are in place to accomplish the crosstie and shed nonessential loads (if necessary), and no NRC regulations are violated (such as separation,minimumredundancy,andindependence),thencreditwouldbegiven for this capability as shown in Regulatory Guide 1.155 (i.e., reduced acceptable station blackout coping durations for greater diesel generator redundancy).

14. Clarification of the Definitions of Station Blackout and Diesel Generator Failures Comments - (4) Three connenters from the utility industry reconnended that the definition of station blackout in 650.2 should be clarified to exclude ac power from the station batteries through inverters. This source of ac power from the station batteries would be available in the event of a loss of both the offsite and onsite emergency ac power sources (i.e., diesel generators).

(B) Several from industry connented that the definition of diesel generator failure should be clarified, particularly with respect to the treatment of short-term failures that can be recovered quickly. Sargent and Lundy Engineers corienented that:

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A definition of failure on demand for emergency diesel generators needs to be provided. Under the context of a station blackout, a diesel generator which fails to start automatically upon detection of an offsite power loss, but is successfully started manually from the main control room or from the local control panel, should not be considered a tailure on demand.

Response - (A) The staff agrees with com,ent A and revised the definit, ion of station blackout accordingly.

(B) Based on actual experience, failures of diesel generators to start due to failures in the auto-start system make up less than 20 percent of all diesel generator failures. Therefore, discounting these failures would not have a significant impact on overall diesel generator reliability statistics. However, the staff agrees in principle with coment B and has clarified the station blackout regulatory guide so that auto-start failures of diesel generators need not be counted in determining the failure rate if the diesel generator is capable of being started manually imediately after it does not start automatically.

15. Specificity and Clarification of Requirements Coments - Public coments were received regarding the specificity and clarifi-cation of the proposed rule and draft regulatory guide. These ranged fron general to specific comments as the following two excerpts indicate:

We are concerned that, if the proposed rule is adopted, the staff will promulgate regulatory guidance criteria which will be unrealistic and excessive, i.e., compounding the event with other accidents. Imposing passive failure criteria, applying seismic, environmental qualification and other qualifications to equipment that could otherwise be used in response to such an event, etc. (Maine Yankee Atomic Power Company)

DefinitionsofP1andP2[inTable3ofthedraftRegulatoryGuide]use frequency of extremely severe weather and severe weather interchangeably, thus creating confusion in the definition. (Washington Public Supply System)

Response - Some of the coments on this subject relate to other subjects discussed elsewhere in this section. Some coments were quite specific while others were general in nature or expressed views that were not substantiated

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With backup material. The staff has taken these coments into consideration and revised and clarified the rule and regulatory guide accordingly. Additional guidance is provided in NUMARC-8700 which has been reviewed by the staff and referenced in the regulatory guide as providing a method the staff finds i acceptable for meeting the rule.

I

16. Technical Coments on NUREG-1032 Coments - In addition to coments on the proposed rule and draft regulatory guide, several letters contained coments on the staff's draft technical report, NUREG-1032, "Evaluation of Station Blackout Accidents at Nuclear Power  !

Plants."

Response - NUREG-1032 was issued in draft form for public coment in May 1985 (50FR24332). The coments received were reviewed and considered by the staff and resulted in a re-evaluation of the technical analysis. Details of the l specific comments and responses are not presented here. Rather, NUREG-1032 was revised extensively over the past year to address the public coments. In general, the overall conclusions on the risk from station blackout events cid not change significantly as a result of the reanalysis. One of the major changes resulting from the reanalysis was a revision to the definitions of plant characteristics, especially the clustering of plants into site and weather-related groups (Appendix A in NUREG-1032). These changes are reflected l in revisions to the guidance in the station blackout regulatory guide to f determine plant-specific acceptable station blackout coping durations.

17. Relationship of USI A-44 to Other NRC Generic Issues Coments - The major public coment regarding the relationship of USl A-44 to other NRC generic safety issues was that the proposed rule may not be necessary or should be postponed because of ongoing work to resolve related generic t issues. Some coments were general in nature such as the following one from '

Southern California Edison Company:

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Promulgation of a final station blackout rulemaking at this time will unnecessarily complicate the final resolution of related generic technical issues.... The NRC must develop and implement a program to coordinate the resolution of all power-related generic issues prior to finalizing any individual proposed rule.

AIF suggested that the implementation of any requirements for station blackout be deferred until the requirements from USI A-45, Shutdown Decay Heat Semoval Requirements, are known and until the effect of source tenn changes can be evaluated. <

NUMARC mentionew pecific proposed and existing regulatory requirements that should be considered becane they could reduce the need for a station blackout rule (e.g., B-53 Diesel Generator Reliability, and GI 23. Reactor Coolant Pump Seal Failures). 9thee related issues mentioned in the public connents were A-30 Adequacy of Safety-Related DC Power Supplies, and implementation of safe shutdown facilities to meet the fire protection requirements of Appendix R.

Response - The question that needs to be addressed is "should a requirement be imposed now to reduce risk, or should it be postponed until related issues are resolved sometime in the future?* Potentially, this could result in sub-stantial delays, thereby not resolving generic safety issues in a timely manner. The staff has considered the resolution of USI A-44 in light of *ae related issues mentioned in the connents. Although these issues are identified as separate tasks within NRC, they are all managed in a well established program that coordinates all related issues. A brief discussion of the most relevant issues is presented below. (Additional infonnation is provided in NUREG-1109, "Regulatory Analysis for the Resolution of Unresolved Safety Issue A-44, Station Blackout ")

Resolution of USI A-45 will occur at some time following issuance of the station blackout rule (650.63) and after plant-specific station blackout coping evaluations have been perfonned by licensees per NUMARC/NUGSB0 Initiative 5.

utilizing guidelines provided in NUMARC-8700. Further, the resolution of USI A-45 is expected to be highly plant-specific and focused on loss of decay heat removal considerations from other causes beyond station blackout. Utilization will be made of A-44 evaluations (as applicable) and any plant equipment

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modification needs identified from A-45 will be carefully evaluated to maximize effective use of previously identified A-44 equipment needs.

Maintaining emergency diesel generator reliability, the purpose of B-56, is an integral part of the resolution of USI A-44. However, the Comission believes that additional defense-in-depth will achieve a substantial increase in protec-tion to public health and safety.

The resolution of GI 23 (reactor coolcnt pump seal leakage) deals with loss of reactor coolant system inventory and associated degraded core conditions. US!

A-44 deals with station blackout induced effects, which result in loss of ac power, thereby impacting a broader spectrum of plant equipment and safety-related functions. Although the resolution of GI 23 will contribute to establishing a higher level of assurance that seni leakage will be minimized (thereby minimizing the need for power to replace water inventory losses over the station blackout duration and recovery phase), resolution of GI 23 by itself will not address the broader scope of US! A-44 safety concerns.

Some licensees h6ve implemented dedicated shutdown systems that are independent of normal and emergency ac power to meet Appendix R requirements. If appli-cable, these features would be credited in the resolution of USI A-44 by providing the capability to cope with a station blackout.

Thus, the resolution of USI A-44 is coordinated with related generic issues, and implementation of a final resolution should not be delayed further.

(Response to coments on the effect of source tem changes is included in subjectnumber8.)

18. An Alternative of Plant-Specific Probabilistic Assessments Comments - Several utilities suggested that, in lieu of the requirements in the rule, licensees should be pemitted to submit plant-specific evaluations to demonstrate that the frequency of core damage from station blackout events is 10-5 per reactor-ycar or less. In a similar vein, the suggestion was made that

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t NRC should specify a target level of reliability for ac power systems in order to satisfy NRC's criteria for core damage frequency. A few licensees submitted limited probabilistic assessments to show that for some plants station blukout 1 could have a very small probability of severe consequences.

Response - The Comission does not preclude licensees from submitting plant-specific probabilistic assessments to support a detennination that station blackout would have a very small probability for causing core damage. However, ,

the requirements of the rule must be met. The Comission would observe that the use of probabilistic assessments was important as input to the regulatory decisionmaking that culminated in the station blackout rule and related I

guidance. As expressed in the Comission's Safety Goal Policy statement of

, August 1986 (51 FR 28044), the Comission has acquired a reasonable degree of

confidence about the usefulness and value of probabilistic assessments in assisting regulatory decisionmaking on complex safety issues. In short, such assessments are of value in complementing and focusing the more traditional and deterministic defense-in-depth approaches. On the other hand, any licensee must decide whether or not its plant-specific ac power configuration and other related equipment are sufficiently unique to merit the conduct and submittal of

) a probabilistic assessment as part of achieving compliance to 150.63. The I Comission's experience also indicates that probabilistic assessments are

resource intensive and can be of marginal utility if their only and result is to delay rule compliance.

19. Procedures and Operator Actions During Station Blackout i

I Coments - (A) Several letters from industry comented that, in response to Generic Letter 81-04, "Emergency Procedures and Training for Station Blackout Events," dated February 21, 1981, utilities already have procedures in place to prepare plant operations for station blackout events. (kners' groups have established generic guidance for station blackout operating procedures for licensees to use in developing plant-specific procedures. A representative of the Professional Reactor Operator Society comented that:

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Generic procedures are used by cost operating facilities. These procedures are not carried into adequate depth of specific power plant operations. The industry has relied too heavily on generic procedures and i has not given a real look at what specific steps must be taken.

Extrapo16 tion of these procedures must be required. Specific maintenance procedures must be established and followed.

(B) Other coments on procedures related to the timeliness of operator, actions, both inside and outside the control room. Houston Lighting and power suggested that:

In Section 3.1 (part 6) [of the regulatory guide), the first sentence should be revised to read ' Consideration should be given to timely operator actions both inside and outside of the control room that ....' so I

t1at credit can be taken for existing equipment that may not have actuation and control from the control room.

Illinois Power Company recommended that:

... Section C.3.3 Item 3.a. of the proposed regulatory guide should be modified to read:

a. The system should be capable of being actuated and controlled from the control room, or if other means of control are required (e.

manual jumping of control logics or manual operation of valves)g.,, it should be demonstrated that these steps can be carried out in a timely fashion.

Response - (A) Licensees may take credit for station blackout procedures already in place to comply with the station blackout rule. However, for the most part, these procedures were developed without having the benefit of a

{ plant-specific assessment to determine whether a plant could withstand a

{

station blackout for a specific duration. Therefore, these procedures may need to be modified after licensees have detemined an acceptable station blackout coping duration and evaluated their plant's response to a station blackout of this duration.

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(B) The staff agrees with the coments related to operator actions outside the I

control room, and the regulatory guide was revised accordingly, 1

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20. Schedule Provisions in the Proposed $50.63 Conrants - Two letters contained cocinents on the proposed schedule in 150.63.

OCRE felt the scheduling provisions in the proposed rule were far too generous.

One individual recorrrnended that the schedule be modified to require licensees to submit, within 9 months of the date of the amendment, a list of modifications along with a proposed schedule to implement those modifications.

(According to the proposed rule, licensees would not have to submit a schedule for implementing equipment modifications until after the staff received and reviewed licensees' submittals on their plant's acceptable station blackout duration.)

Response - The staff agreed in part with these toments, and the schedule was revised accordingly. Section 50.63(c)(1)(iii) now requires that licensees

submit within 9 months after the rule is issued a list of equipment modifications and a proposed schedule for implementing them. A final schedule would be developed after NRC has reviewed the licensees' submittal of their plant's acceptable station blackout duration.

21. Industry Initiatives 1

Corrrnents - In addition to cocrnents on the proposed rule. NUMRC endorsed the following five initiatives tob address the more importart contributors to station blackout:

1. Each utility will review tnetr 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 4 time thereaf ter.

]

5 NUMARC initially proposed a set of four initiatives. The fifth initiative regarding the performance of a coping assessment was provided in NUMARC 8700, which was submitted by letter from J. Opeka (huMARC) to T. Speis (RES) dated November 23, 1987 A copy is available for public inspection and copying for a fte at the NRC Public Document Room at 1717 H Street NW., Washington, DC.

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

a. copi' "'i a station blackout event

b. > 3 of ac power i ; awing a station blackout event, and

c. p., tie plant for :evere weather conditions (e.g.,

and tornados) to reduce the likelihood and

^

I ,

ct. . .as of a loss of cffsite power and to reduce the overo. risk of a station blackout event.

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

4 Each utility will monitor emergency ac power unavailability utilizing data utilities provide to INP0 on a regular basis.

5. Each utility will assess the ability of its plant (s) to cope with a station blackout. Plar.ts utilizing alternate AC power for station blackout response which can be shown by test to be available to sower the shutdown busses within 10 minutes of the onset of station

>1ackout do not need to perform any coping assessment. Remaining alternate AC plants will assess their ability to cope for 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />.

Plants not utilizing an alternate AC source will assess their ability to cope for 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. Factors identified which prevent demonstrating the capability to cope for the appropriate duration will be addressed through hardware and/or procedural changes so that successful demonstration is possible.

NUMARC previously opposed generic rulemaking and felt that the first four initiatives would resolve the station blackout issue.

Respon:e - These five initictives now include many of the elements that are includsd in the NRC resolution of USI A 44. The staff has followed up on the NUMARC initiatives through a series of meetings in 1986 throve 1987. The resuit has been the development of NUMARC-8700 which provide, juidelines and criteria acceptable to the staff. The procedures in NUMARC _ 00 have been referenced in Regulatory Guide 1.155 as providing guidance acceptable to the staff for reeting the requirements of the rule. Table 1 in Regulatory Guide 1.155 provides a cross-reference to NUMARC-8700 and notes where the regulatory guide takes precedence. NUMARC's previous concerns have been addressed in the development of Regulatory Guide 1.155 and NUMARC-8700.

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Finding of No Significant Environmental Impact: Availability The Comission has detennined under the National Environmental Policy Act of 1969, as amended, and the Comission's rules in Subpart A of CFR Part 51, that this rule is not a major Federal action significantly affecting the quality of the human environnent, and therefore, an environmental impact statenent is not required. There are not any adverse environmental impacts as a result of the rule because there is no additional radiological exposure to the general public or plant employees, and plant shutdown is not required so there are no additional environmental impacts as a result of the need for replacement power.

The environmental assessment and finding of no significant impact on which this determination is based are available for inspection and copying for a fee at the NP.C Public Document Room 1717 H Street, NW, Washington, DC. Single copies of the environmental assessment and the finding of no significant impact are available from Mr. Warren Minners, Office of Nuclear hegulatory Research, U. S.

Nuclear Regulatory Comission Washington, DC 20555 Telephone:

(301)492-7827.

Paperwork Reduction Act Statement This final rule amends infonnation collection requirements that are subject to the Papeniork Reduction Act of 1980 (44 U.S.C. 3501 et seq.). These requirements were approved by the Office of Management and Budget approval number 3150-0011.

Regulatory Analysis The Comission has prepared a regulatory analysis on this final regulation. The analysis examines the costs and benefits of the alternatives considered by the Comission. A copy of the regulatory analysis, NUREG-1109, "Regulatory /

Backfit Analysis for the Resolution of Unresolved Safety Issue A-44, Station Blackout,' is available for inspection and copying for a fee at the HRC Public Document Room, 1717 H Street, NW, Washington, DC 20555.

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Regulatory Flexibility Certification As required by the Regulatory Flexibility Act (5 U.S.C. 605(b)), the Comission certifies that this rule does not have a significant economic impact on a substantial number of small entities. The rule requires that nuclear power plants be able to withstand a total loss of ac power for a specified t'ime duration and maintain reactor core cooling during th6t period. These facilities are licensed under the provisions of il50.21(b) and 50.22 of 10 CFR -

Part 50. The companies that own these facilities do not 411 within the scope of "small entities" as set forth in the Regulatory Flexibility Act or the small business size standards set forth in regulations issued by the Small Business Administration in 13 CFR Part 121, List of Subjects in 10 CFR Part 50 Antitrust, Classified infonnation, Fire prevention, Incorporation by reference, Intergovernmental relations, Nuclear power plants and reactors, Penalty, Radiation protection, Reactor siting criteria, Reporting and recordkeeping requirements.

For the reasons set out in the preamble and under the authority of the Atomic Energy Act of 1954, as amenced, the i'nergy Reorganization Act of 1974, as amended, and 5 U.S.C. 553, the NRC is adopting the following amendments to 10 CFR Part 50.

Part 50 - Domestic Licensing of Production and Utilization Facilities

1. The authority citation for Part 50 is revised to read as follows:

AUTHORITY: Secs. 102, 103, 104, 105, 161, 182, 183, 186, 189, 68 Stat. 936, 937, 938, 948, 953, 954, 955, 956, as anended, sec. 234, 83 Stat. 1244, as amended (42 U.S.C. 2132, 2133, 2134 E135, 2201, 2232, 2233, 2236, 2239, 228'. ) .

secs. 201, as amended. 202, 206, 88 Stat. 1242, as amended. 1244, 1246 (42 U.S.C. 5841, 5842, 5846).,

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Section 50.7 also issued under Pub. L.95-601, sec. 10, 92 Stat. 2951 (42 U.S.C. 5851). Sections 50.10 also issued under secs. 101, 185, 68 Stat.

936, 955, as amended (42 U.S.C. 2131, 2235); sec.102, Pub. L.91-190, 83 Stat.

853 (42 U.S.C. 4332). Sections 50.23, 50.35, 50.55, and 50.56 also issued under sec.185, 68 Stat. 955 (42 U.S.C. 2235). Sections 50.33a, 50.55a and Appendix Q also issuea under sec.102, Pub. L.91-190, 83 Stat. 853 (42 U.S.C.

4332). Sections 50.34 and 50.54 also issued under sec. 204, 88 Stat.1245 (42 U.S.C. 5844). Sections 50.58, 50.91, and 50.92 also issued under Pub. L.97-415, 96 Stat. 2073 (42 U.S.C. 2239). Section 50.78 also issued under sec. 122, 68 Stat. 939 (42 U.S.C. 2152). Sections 50.80-50.81 also issued under sec. 184, 68 Stat. 954, as amended (42 U.S.C. 2234). Section 50.103 also .

issued under sec.108, 68 Stat. 939, as amended (42 U.S.C. 2138). Appendix F also issued under sec. 187, 68 Stat. 955 (42 U.S.C. 2237).

For the pur il50.10(a), (b) poses of :ec. 223, 68 Stat. 958, as amended (42 U.S.C. 2273);, and under sec.161b, 68 Stat. 948, as amended (42 U.S.C. 2201(b)); il50.10(b) and (c), and 50.54 are issued under sec. 1611, 68 Stat. 949, as amended (42 U.S.C.

2201(i)); ana 1950.9, 50.55(e), 50.59(b), 50.70, 50.71, 50.72. 50.73, and 50.78 are issued under sec. 1610, 68 Stat. 950, as amended (42 U.S.C. 2201(o)).

2. In 150.2, definitions of "alternate ac source" and "station blackout" are added in the alphabetical sequence to read as follows:

150.2 Definitions "Alternate ac source" means an alternating current (cc) power source that is available to and located at or nearby a nuclear power plant and meets the following requirements:

(1) Is connectable to but not normally connected to the offsite or onsite emergency ac power systems; (2) Has minimum potential for comon mode failure with offsite power or the onsite emergency ac power sources; (3) Is available in a timely manner after the onset of station blackout; and (4) Has sufficient capacity and reliability for operation of all systems required for coping with station blackout and for the time required to bring and maintain the plant in safe shutdown (non-design basis accident).

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"Safe shutdown (non-design basis accident (non-DBA))" for station blackout means bringing the plant to those shutdown conditions specified in plant technical specifications as Hot Standby or Hot Shutdown, as appropriaN (plants have the option of maintaining the RCS at nonnal operating temperatures or at reduced temperatures).

"Station blackout" means the complete loss of alternating current (ac) electric power to the essential and nonessential switchgear buses in a nuclear power plant (i.e., loss of offsite electric power system concurrent with turbine trip and unavailability of the onsite emergency ac power system). Station blackout does not include the loss of available ac power to buses fed by station t,atteries through inverters or by alternate ac sources as defined in this section, nor does it assume a concurrent single failure or design basis accident. At single unit sites, any emergency ac power source (s) in excess of the number required to meet minimum redundancy requirements (i.e., single failure) for safe shutdown (non-DBA) is assumed to be available and may be designatedasanalternatepowersource(s)providedtheapplicablerequirements are met. At multi-unit sites, where the combination nf emergency ac power sources exceeds the minimum redundancy requirements for safe shutdown (non-DBA) of all units, the remaining er.ergency ac power sources may be used as alternate ac power sources provided they meet the applicable requirements. If these criteria are not met, station blackout must be assumed on all the units.

3. A new $50.63 is added to read as follows:

650.63 Loss of all alternating current power.

(a) Requirements. (1) Each light-water-cooled nuclear power plant licensed to operate must be able to withstand for a specified duration and recover from a station blackout as defined in 650.2. The specified station blackout duration shall be based on the following factors:

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! (i) The redundancy of the onsite emergency ac power sources; (ii) The reliability of the onsite emergency ac power sources; (iii) The expected frequency of loss of offsite power; and (iv) The probable time needed to restore offsite power.

(2) The reactor core and associated coolant, control, and protection systems, including station batteries and any other necessary support systems, must ,wovide sufficient capacity and capability to ensure that the core is coo'.ed and appropriate containment integrity is maintained in the event of a station blackout for the specified duration. The capability for coping with a station blackout of specified duration shall be determined by an appropriate coping analysis. Utilities are expected to have the baseline assumptions, analyses, and related information used in their coping evaluations available for NRC review.

(b) 1.%itation of Scope. Paragraph (c) of this section does not apply to those plants licensed to operate prior to July 18, 1988 , if the capability to withstand station blackout was specifically addressed in the operating license proceeding and was explicitly approved by the NRC.

(c) Implementation. (1) Information Submittal: For each light-water-cooled nuclear power plant licensed to operate on or before July 18, 1988 . the licensee shall submit the information defined below to the Director of the Office of Nuclear Reactor Regulation by April 14, 1989 . For each light-water-cooled nuclear power plant licensed to operate after the effective date of this amendment, the licensee shall submit the infonnation defined below to the Director by 270 days after the date of license issuance.

(1) A proposed station blackout duration to be used in determining compliance with paragraph (a) of this section, including a justification for the selection based on the four factors identified in paragraph (a) of this section;

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(ii) A description of the procedures that will be implemented for station blackout events for the duration determined in paragraph (c)(1)(1) of this section and for recovery therefrom; and (iii) A list of modifications to equipment and associated procedures, if any, necessary to meet the requirements of .

paragraph (a) of this section, for the specified station blackout curation determined in paragraph (c)(1)(1) of this section, and a proposed schedule for implementing the stated modifications.

(2) Alternate ac cource: The alternate ac power source (s), as defined in 550.2, will constitute acceptable capability to withstand station blackout provided an analysis is performed which demonstrates that the plant has this capability from onset of the station blackout until the alternate ac source (s) ar.d required shutdown equipment are started and lined up to operate. The time required for startup and alignment of the alternate ac power source (s) and this equipment shall be demonstrated by test. Alternateacsource(s)servinga l multiple unit site where onsite emergency ac sources are not shared between units nust have, as a minimum, the capacity and capability for coping with a station blackout in any of the units. At sites where onsite emergency ac sourcesaresharedbetweenunits,thealternateacsource(s)musthavethe l

capacity and capability as required to ensure that all units can be brought to

! and maintained in safe shutdown (non-DBA) as defined in 650.2. If the alternate ac source (s) meets the above requirements and can be demonstrated by test to be available to power the shutdown buses within 10 minutes of the onset of station blackout, then no coping analysis is required.

l (3) Regulatory Assessment: After consideration of the information submitted in accordance with paragraph (c)(1) of this section, the Director, Office of Nuclear Reactor Regulation, will notify the licensee of the Director's conclusions regarding the adequacy of the proposed specified station blackout duration, the proposed equipment modifications and procedures, and the proposed

(

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schedule for implementing the procedures and modifications for compliance with paragraph (a) this section.

(4) Implementation Schedule: For each light-water-cooled nuclear power plant licensed to operate on or before July 18, 1988 , the licensee shall, withir. 30 days of the notification provided in accordance with paragraph (c)(3) of this section, submit to the Director of the Office of Nuclear Reactor Regulation a schedule comitment for implementing any equipment and associated procedure modifications necessary to meet the requirements of paragraph (a) of this section. This submittal must include an explanation of the schedule and a justification if the schedule does not provide for completion of the modifications within two years of th6 notification provided in accordance with paragraph (c)(3)ofthissection. A final schedule for implementing modifications necessary to comply with the requirements of paragraph (a) of this section will be established by the NRC staff in consultation and coordination with the affected licensee.

Dated at Rockville, Maryland, this ! b day of Mt 1988.

Fo the Nuclear Regulatory Comission.

O, a $

(Samuel J. Chilk N T Secretary of the, Comission. )

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BACKFIT ANALYSIS Analysis and Detennination That the Rulemaking To Amend 10 CFR 50 Concerning Station Blackout Complies With the Backfit Rule 10 CFR 50.109 The Comission's existing regulations establish requirements for the design and testing of onsite and offsite electrical power systems (10 CFR Part 50, Appendix A, General Design Criteria 17 and 18). However, as operating experi- .

ence has accumulated, the concern has arisen regarding the reliability of both the offsite and onsite emergency ac power systems. These systems provide power for various safety systems, including reactor core decay heat removal and con-tainment heat removal, which are essential for preserving the integrity of the reactor core and the containment building, respectively. In numerous instances emergency diesel generators have failed to start and run during tests conducted at operating plants. In addition, a number of operating plants have experienced a total loss of offsite electric power, and more such occurrences are expected. Existing regulations do not require explicitly that nuclear power plants be designea to withstand the loss of all ac power for any

specified period.

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This issue has been studied by the staff as part of Uncesolved Safety Issue (USI) A-44, "Station Blackout." Bcth deterministic and probabilistic analyses I were performed to deterenine the timing and consequences of various accident sequences and to identify the dominant factors affecting the likelihood of core melt eccidents from station blackout. Although operational experience shows that the risk to public health and safe,ty is not undue, these studies, which have evaluated plant design features add site-dependent factors in detail, show l that blackout can be a significant contributor to the overall residual risk.

Consequently, the Comissien is amending its regulations to require that plants be capable of withstanding a total loss of ac power for a specified duration .

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and to maintain reactor core cooling during that period.

An analysis of the benefits and costs of implementing the station blackout rule l 1s presented NUREG-1109, "Regulatory /Backfit Analysis for the Resolution l

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of Unresolved Safety Issue A-44, Station Blackout. The estimated benefit from implementing the station blackout rule is a reduction in the frequency of core damage per reactor-year due to station blackout and the associated risk of '

offsite radioactive releases. The risk reduction for 100 operating reactors is estimated to be 145,000 person-rem and supports the Comission's conclusion that $50.63 provides a substantial improvement in the level of public' health and safety protection.

The cost for licensees to comply with the rule would vary depending on the existing capability of each plant to cope with a station blackout, as well as the specified station blackout duration for that plant. The costs would be primarily for licensees (1) to assess the plant's capability to cope with a station blackout (2) to develop procedures, (3) to improve diesel generator reliability if the reliability falls below certain levels, and (4) to retrofit plants with additional components or systems, as necessary, to meet the requirements.

The estimated total cost for 100 operating reactors to comply with the resolu-tion of USI A-44 is about $60 million. The average cost per reactor would be around $600,000, ranging from $350,000, if only a station blackout assessment and procedures and training are necessary, to a maximum of about $4 million if substantial modifications are needed, including requalification of a diesel generator.

I The overall value-irract ratio, not including accident avot_ dance costs, is about 2,400 person-rem averted per million dollars. If the ' et cost, which includes the cost savings from accident avoidance (i.e., cleanup and repair of onsite damages and replacement powe following an accid a t), were used, the overall value-impact ratic would improve significantly to about 6,100 person-rem averted per million dollar.i. These valcas, which exceed the $1,000/ person-rem interim guidance provided by the Comn lssion, support proceeding with the implementation of 150.63.

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The preceding quantitative value-impact analysis was one of the factors considered in evaluating the rule, but other factors also played a part in the decision-making process. Probabilistic risk assessment (PRA) studies performed for this USI, as well as some plant-specific PRAs, have shown that station blackout can be a significant contributor to core melt frequency, and, with consideration of containment failure, station blackout events can rephsent an important contributor to reactor risk. In general, active systems required for containment heat removal are unavailable during station blackout. Therefore, the offsite risk is higher from a core melt resulting from a station blackout than it is from many other accident scenarios.

Although there are licensing requirements and guidance directed at providing reliable offsite and onsite ac power, experience has shown that there are prac-tical limitations in ensuring the reliability of offsite ar.d onsite emergency ac power systems. Potential vulnerabilities to conson cause failures associated with drsign, operational, and environmental factors can affect ac power system reliability. For example, if potential comon cause failures of emergency diesel generators exist (3.g., in service-water or de power support systemu , then the estimated core damage frequency from station blackout events can increase significantly. Also, even though recent data indicate that the i average emergency rifesel generator reliability has improved slightly since l 1976, these data also show that diesel generator failure rates during unplanned demand (e.g., following a loss of offsite power) were higher than that during l surveillance tests.

The estimated frequency of core damage from station blackout events is directly proportional to the frequency of the initiating event. Estimates of station blackout frequencies for this USI were based on actual operational experience with credit given for trends showing a reduction in the frequency of losses of offsite power resulting from plant-centered events. This is assumed to be a realistic indicator of future perfonnance. An argument can be made that the future perfonnance will be better than the past. For example, when problems l with the offsite power grid arise, they are fixed and, therefore, grid reli-l l

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ability should improve. On the other hand, grid power failures may become more frequent because fewer plants are being built, and more power is being trans-mitted among regions, thus placing greater stress on transmission lines.

The factors discussed above support the determination that additional defense-in-depth provided by the ability of a plant to cope with station blackout for a specific duration would provide substantial increase in the overall protection of the public health and safety, and the direct and indirect costs of implemen-tation are justified in view of this increased protection. The Comission has considered how this backfit should be prioritized and scheduled in light of other regulatory activities ongoing at ope.ating nuclear power plants. Station blackout warrants a high priority ranking based on both its status as an "unresolved safety issue" and the results and conclusions reached in resolving this issue. As noted in the implementation section of the rule (150.63(c)(4)),

the :;chedule for equipment modification (if needed to meet the requirements of the rule) shall be established by the NRC staff in consultation and coordination with the licensee. Modifications that cannot be scheduled for completion within two years af ter NRC accepts the licensee's specified station blackout duration must be justified by the licensee. The NRC retains the l

authority to determine the schedules for modifications.

l In addition. some foreign countries, including France, Britain, Sweden, Germany l and Belgium, have taken steps to reduce the risk from station blackout events.

These steps include adding design features to enhance the capability of the plent to cope with a station blackout for a substantial period of time and/or l adding redundant and diverse emergency ac power soyrces.

Analysis of 50.109(c) Factors l

l 1. Statement of the specific objectives that the backfit is designed to achieve

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The NRC staff has completed a review and evaluation of information developed since 1980 on Unresolved Safety Issue (USI) A-44, Station Blackout. As a result of these efforts, the NRC is amending 10 CFR Part 50 by adding a new i 50.63, "Station Blackout."

The objective of the station blackout rule is to reduce the risk of severe accidents associated with station blackout. Specifically, the rule requires all light-water-cooled nuclear power plants to be able to cope with a station blackout for a specified duration and to have procedures and training for such an event. A regulatory guide, to be issued along with the rule, provides an acceptable method to determine the station blackout duration for each plant. The duration is to be determined for each plant based on a comparison of the individual plant design with factors that have been identified as the main contributors to risk of core melt resulting from station blockout. These factors are (1) the redundancy of onsite emergency ac power sources, (2) the reliability of onsite emergency ac power sources, (3) the frequency of loss of offsite power, and (4) the probable time needed to restore offsite power.

2. General description of the activity required by the licensee or applicant in order to complete the backfit In order to comply with the resolution of USI A-44, licensees will be required to --

Maintain the reliability of onsite emergency ac power sources at or above specified acceptable reliability levels.

t Develop procedures and training to restore ac power using nearby power sources if the emergency ac power system and the norwal offsite power sources are unavailable.

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Determine the duration that the plant should be able to withstand a station blackout based on the factors specified in 650.63, "Station Blackout," and Regulatory Guide 1.155, "Station Blackout."

If available, an alternate ac power source that meets specific criter.a for independence and capacity can be used to cope with a station blackout.

Evaluate the plant's actual capability to withstand and recover from a station blackout. This evaluation includes:

- Verifying the adequacy of station br.ttery power, condensate storage tank capacity, and plant / instrument air for the station blackout duration.

- Verifying the operability of equipment needed to operate during a station blackout and the recovery from the blackout for environmental conditions associated with total loss of ac power (i.e. , loss of heating, ventilation, and air conditioning).

l Depending on the plant's existing capability to cope with a station blackout, licensees may or may not need to backfit hardware modifica-tions (e.g., adding battery capacity) to comply with the rule. (See item 8 of this analysis for additional discussion.) Licensees will be required to have procedures ar.d training to cope with and recover from a station blackout.

3. Potential change in the risk to the public from the accidental offsite release of radioactive material l

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Implementation of the station blackout rule will result in an estimated total risk reduction to the public ranging from 65,000 to 215,000 person-rem with a best estimate of about 145,000 person-rem.

4. Potential impact on radiological exposure of facility employees For 100 operating reactors, the estimated total reduction in occupational exposure resulting from reduced core damage frequencies and associated .

postaccident cleanup and repair activities is 1,500 person-rem. No significant increase in occupational exposure is expected from operation and maintenance activities associated with the rule. Equipment additions and modifications contemplated do not require work in and around the reactor coolant system and therefore are not expected to result in significant radiation exposure.

5.

Installation and continuing costs associated with the backfit, including the cost of facility downtime or the cost of construction delay For 100 operating reactors, the total estimated cost associated with the l station blackout rule ranges from $42 to $94 million with a best estimate of $60 million. This estimate breaks down as follows:

Estimated rumber of Estimated total cost (million dollars)

Activity reactors Best High Low Assess plant's capability to 100 25 40 20 cope with station blackout Develop procedures and 100 10 15 5 training Improve diesel generator 10 2.5 4 1.5 reliability Requalify diesel generator 2 5.5 11 2.5 Install hardware to increase 27 17 24 13 plant capability to cope with station blackout Totals 60 94 42

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6. The potential safety impact of changes in plant or operational complexity, including the relationship to proposed and existing regulatory requirements The rule requiring plants to be able to cope with a station blackout should not add to plant or operational complexity. The station blackout rule is closely related to several NRC generic programs and proposed and fexisting regulatory requirements as the following discussion indicates.

Generic Issue B-56, Diesel Generator Reliability The resolution of USI A-44 includes a regulatory guide on station blackout that specifies the following guidance on diesel generator reliability (Regulatory Guide 1.155, Sections C1.1. and C.1.2):

The minimum emergency diesel generator (EDG) reliability should be targeted at 0.95 per demand for each EDG for plants in emergency ac Groups A, B, and C and at 0.975 per demand for each EDG for plants in emergency ac Group D (see Table 2). These reliability levels will be considered minimum target reliabilities and each plant should have an EDG reliability program containing the principal elements, or their equivalent, outlined in Regulatory Position 1.2.

Plants that select a target EDG reliability of 0.975 will use the higher level as the target in their EDG reliability programs.

The reliable operation of onsite emergency ac power sources should be ensured by a reliability program designed to maintain and monitor the reliability level of each power source over time for assurance that the selected reliability levels are being achieved. An EDG reliability program would typically be composed of the following elements or activities (or their equivalent):

1. Individual EDG reliability target levels consistent with the plant category and coping duration selected from Table 2.

2. Surveillance testing and reliability monitoring programs designed to track EDG performance and to support maintenance activities.

3. A maintenance program that e n m h that the target EDG reliability is being achieved and that provides a capability for failure analysis and root-cause investigations.

4. An information and data col' cti

• tem that services the elements of the reliability r' ' that monitors achieved EDG reliability levels e" -

values.

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5. Identified responsibilities for the major program elements and a management oversight program for reviewing reliability levels being achieved and ensuring that the program is functioning properly.

The resolution of B-56 will provide specific guidance for use by the staff or industry to review the adequacy of diesel generator reliability programs consistent with the resolution of USI A-44.

Generic Issue 23. Reactor Coolant Pump Seal Failures Reactor coolant pump (RCP) seal integrity is necessary for maintaining primary system inventory during station blackout conditions. The estimates of core damage frequency for station blackout events for USI A-44 assumed that RCP seals would leak at a rate of 20 gallons per minute. Results of analyses perfomed for GI 23 will provide the information necessary to estimate RCP seal behavior during a station blackout. The industry coping analysis guidelines (NUPARC-8700) recognize the possibility of leakages exceeding an assumed 25 gpm per pump and incorporate the need to reevaluate the plant-specific coping analysis if the resolution of GI 23 identifies higher levels.

l USI A-45, Shutdown Decay Heat Removal Requirements l

l The overall objective of USI A-45 is to evaluate tne adequacy of current

! licensing design requirements to ensure that the nuclear power plants do

, not pose an unacceptable risk as a result of failure to remove shutdown l decay heat. The study includes an assessment of alternative means of l shutdown decay heat removal and of diverse "dedicated" systems for this purpose. Results will include proposed reconnendations regarding the l desirability of, and possible design requirements for, improvements in existing systems or an alternative dedicated decay heat removal method.

The USI A-44 concern for maintaining adequate core cooling under station blackout conditions can be considered a subset of the overall A-45 issue.

However, there are significant differences in scope between these two

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issues. USI A-44 deals with the probability of loss of ac power, the capability to remove decay heat using systems that do not require ac power, and the ability to restore ac power in a timely manner. USI A-45 deals with the overall reliability of the decay heat removal function in terms of response to transients, small-break loss-of-coolant accidents, and special emergencies such as fires, floods, seismic events, and sabotage.

Although the recomendations that might result from the resolution of USI A-45 are not yet final, some could affect the station blackout capa-bility, while others would not. Recomendations that involve a new or improved decay heat removal system that is ac power dependent but that does not include its own dedicated ac power supply would have no effect on USI A-44. Recomendations that involve an additional ac-independent decay heat removal system would have a very modest effect on USI A-44.

Recomendations that involve an additional decay heat removal system with its own ac power supply would have a significant effect on USI A-44. Such a new additional system would receive the appropriate credit within the USI A-44 resolution by either changing the emergency ac power config-uration group or providing the ability to cope with a station blackout for an extended period of time. Well before plant modifications, if any, will l be implemented to comply with the station blackout rule, it is anticipated that the proposed technical resolution of USI A-45 will be published for i public coment. Those plants needing hardware modifications for station blackout could be reevaluated before any actual modifications are made so that any contemplated design changes resulting from the resolution of USI A-45 can be considered at the same time.

Generic Issue A-30, Adequacy of Safety-Ralated DC Power Supply l

The analysis performed for USI A-44 assumed that a high level of de power system reliability would be maintained so that (1) de power system failures would not be a significant contributor to losses of all ac power and (2) should a station blackout occur, the probability of imediate dc

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power system failure would be low. Whereas Generte Issue A-30 focuses on enhancing battery reliability, the resolution of USI A-44 is aimed at ensuring adequate station battery capacity in the event of a station blackout of a specified duration. Therefore, these two issues are ,

consistent and compatible.

l Fire Protection Program Section 50.48 of 10 CFR Part 50 states that each operating nuclear power plant must have a fire protection plan that satisfies GDC 3. The fire protection features required to satisfy GDC 3 are specified in Appendix R to 10 CFR Part 50. They include certain provisions regarding alternative and dedicated shutdown capability. To meet these provisions, some licensees have added, or plan to acid, improved capability to restore power from offsite souren or onsite diesels for the shutdown system. A few 4

plants have installed a safe shutdown facility for fire protection that includes a charging pump powered by its own independent ac power source, i In the event of a station blackout, this system can provide makeup  ;

capability to the primary coolant system as well as reactor coolant pump seal cooling. This could be a significant benefit in terms of enhancing the ability of a plant to cope with a station blackout. Plants that h&ve

! added equipment to achieve alternate safe shutdown in order to meet l Appendix R requirements could take credit for that equipment, if

! available, for coping with a station blackout event.

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7. The estimated resource burden on the NRC associated with the backfit and the availability of such resources The estimated total cost for NRC review of industry submittals required by the station blackout rule is $1.5 million based on submittals for 100 l

reactors and an estimated average of 175 person-hours per reactor.

8. The potential impact of differences in facility type, design, or age on the relevancy and practicalf ty of the backfit

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The station blackout rule applies to all pressurized water reactors and boiling water reactors. However, in detenntning an acceptable station blackout coping capability for each plant, differences in plant charac-teristics relating to ac power reliability (e.g., number of emergency diesel generators, the reliability of the offsite and onsite emergency ac power systems) could result in different acceptable coping capabilities.

For example, plants with an already low risk from station blackout because of multiple, nighly re!iable ac power sources are required to wittstand a station blackout for a relatively short period of time; and few, tf any, hardware backfits would be required as a result of the rule. Plants with currently higher risk from station blackout are required to withstand somewhat longer duration blackouts; and, depending on their existing capability, may need some modifications to achieve the longer station blackout capability.

9. Whether the backfit is interim or final and, if interim, the justification for imposing the backfit on an interim basis The station blackout rule is the final resolution of USI A-44; it is not an interim measure.

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