Regulatory Guide 1.93

Availability of Electric Power Sources

ML003740292
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Issue date:	12/31/1974
From:	Office of Nuclear Regulatory Research
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RG-1.93
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December 1974 U.S. ATOMIC ENERGY COMMISSION

REGULATORY GUIDE

DIRECTORATE OF REGULATORY STANDARDS

REGULATORY GUIDE 1.93 AVAILABILITY OF ELECTRIC POWER SOURCES

A. INTRODUCTION

than are required by GDC-17 can tolerate the loss of one or more sources and still meet the LCO. During the Section 50.36(cX2), "Limiting Conditions for Opera normal course of operation, however, any nuclear power tion," of 10 CFR Part 50, "Licensing of Production and plant may lose power sources to the extent that the LCO

Utilization Facilities," requires the Technical Specifica is not met. This guide addresses such cases.

tions to include the limiting conditions for operation (LCO) and actions required to be taken by the licensee GDC-17 specifies design requirements, not operating when the LCO is not met. Power operation may be initi requirements; it therefore does not stipulate opera ated and continued without restriction only when the tional restrictions on the loss of power sources. Never theless, operational restrictions based on the intent of LCO is met.

This guide describes operating procedures and restric GDC-17 on the loss of power sources have been included tions acceptable to the Regulatory staff which should be in the Technical Specifications of recently licensed implemented if the available electric power sources are nuclear power plants. Such restrictions are based on the less than the LCO. This guide is applicable to single and -following assumptions:

multi-unit plants, including multi-unit plants that share

• The LCO of nuclear power plants is met when all the required electric power sources. the electric power sources required by GDC-17 are The LCO with respect to available electric power available.

sources is an electric power system that satisfies General

• Under certain conditions, it may be safer to Design Criterion 17 (GDC-l 7), "Electric Power Systems," continue operation at full or reduced power for a limited of Appendix A, "General Design Criteria for Nuclear time than to effect an immediate shutdown on the loss Power Plants," of 10 CFR Part 50 by including the follow of some of the required electric power sources. Such ing electric power sources: (1) two physically independent decisions should be based on an evaluation that balances circuits from the offsite transmission network, each of the risks associated with immediate shutdown against which is either continuously available or can be made those associated with continued operation. If, on bal available within a few seconds following a loss-of-coolant. ance, immediate shutdown is the safer course, the unit accident (LOCA), (2) redundant onsite a.c. power sup should be brought promptly to an orderly shutdown, plies, and (3) redundant onsite d.c. power supplies. and to a cold-shutdown state as soon as possible. For Nuclear power plw)ts wherein only one of the two example, the risks associated with an immediate shut required offsite circuits can be made available within a down on the loss of onsite a.c. power supply during a few seconds following a LOCA are outside the scope of period of light system load would tend to be less than this guide. However, the restrictions imposed on such those during a peak load period because the stability of plants on the loss of required sources would generally be the offsite power system would be relatively higher. If, more stringent than those recommended in this guide. on balance, continued power operation is the safer course, the period of continued operation should be used to restore the lost source and to prepare for an

B. DISCUSSION

orderly, shutdown, provided, of course, that these activities do not risk further degradation of the electric Electric loads important to safety of nuclear power plants are served by an electric power system that power system or in any way jeopardize plant safety.

conforms to GDC-17. Plants with more power sources

• If the LCO has not been achieved, the unit should be promptly brought to an orderly shutdown after the USAEC REGULATORY GUIDES Copies of publshed guides may be obtoelud by request Indicating the diison Regulatory Guides wm issued to describe amd malke available to the public desired to Director the US. Atomic Energy Commission. Washington, D.C. 20545, methods c*Wtebl to the AEC Regulatory staff of implementing specific parts of Attention: of Regulatory Stenderds.

the Commission's regulations, to delineets techniques "ndby Vih staff in improvenents in these guides we encouraged andComnents and suggtions for should be sewtto the Secretary of the Commission, US. Atomic Energy Commission, Washington. D.C. 20545, evisluating specific problems or postulated accidents, or to provide guidance to Attention. Docicating end Service Section.

appicants Regulatory Guides ma not substitutes for regulations and compllance with them is not required. Methods and solutions different from those setout in The guides ae issued In the following ton broad divisions:

the guldas will be accepteble if they provide a besis for the findings requisite to the buanm or continuance of a permit or licens by the Commission.

1. PoWAN" Reactors

6. Products

2. Rlesseri arnd Test Reactors 7. Transportation Published guides will be revised periodically, asappropriate, to accommodate 3. Fuels end Maerials Facilities S. Occupational Health comments and to reflect new informntion or esperience. 4. EnvironmentalandSiring

9. Antitrust Review

5. Materials and Plant Protection 1

0. Genral

allowed time for continued power operation has elapsed 2. The Avatble Offalte A.C. Power Sources Are Two and to a cold, shutdown state as soon as possible Lou Than the LCO.

thereafter. The premise here is that the time allowed for continued operation could have been used to enhance This degradation level means that the offsite power Ll the safety of the imminent shutdown. For example, the dispatcher could take such system-wide actions as system does not have the capability to effect a safe increasing generation at other plants or dropping se shutdown and to mitigate the effects of an accident;

lected loads to ensure that the shutdown does not cause however, the onsite a.c. system has not been degraded.

grid instability. In addition, if the loss of sources beyond This degradation level generally corresponds to total loss the LCO were to occur during a peak load period, the of-the immediately accessible offsite power sources.

allowed operating time could be used to defer the Because of the normally high availability of the shutdown to an off-peak period when the stablility of offsite sources, this level of degradation may appear to the grid should be higher. be more severe than the next two degradation levels To develop bases for specific guidance, five levels of listed. However, two factors tend to decrease the degradation of the power systems are described below in severity of this degradation level as compared to the order of increasing severity: next two degradation levels: (a) the configuration of the redundant a.c. power system that remains available for this degradation level is not susceptible to a single bus or switching failure, whereas the next degradation level

1. The Available A.C. Power Sources Are One Less listed (No. 3) may be so susceptible, and (b) the time Than the LCO. required to detect and restore -an unavailable offsite This degradation level means that one of the required power source is generally much less than that required to offsite or onsite ac. sources is not available. Thus, either detect and restore an unavailable onsite a.c. power the offsite or the onsite a.c. power system has no source.

redundancy; however, each system retains full capability This degradation can be caused by any of several (one system with redundancy) to effect a safe shutdown events, including the loss of two offsite circuits, an and to mitigate the effects of a design basis accident. unstable offsite power system, or any condition that Operation could therefore safely continue if the avail renders offsite power unavailable for safe shutdown and ability of the remaining sources is verified; however, emergency purposes. Since the onsite power system has since the system is degraded below the LCO, a time limit on continued operation is warranted. Operating experi not been degraded and since simultaneous loss of offsite power and a LOCA were postulated as a design basis, a brief interval of continued operation is justified. (Note:

11 ence indicates that the availability of a typical offsite source is higher than that of'a typical onsite a.c. supply. Some nuclear power plants are designed to cause an Thus, if risk is evaluated in terms of availability, the risk automatic shutdown or to initiate load rejection at this associated with the loss of an offsite power source (the level of degradation. Plants designed to cause an auto source with the higher availability) would appear to be matic shutdown at this level of degradation need no more severe than the risk associated with the loss of an further discussion; however, those that reject. load are onsite a.c. supply (the source with the lower availabil considered to remain operating within the context of ity). this guide.)

However, this apparent difference in severity is usually offset by maintainability considerations; that is, the time required to detect and restore an unavailable 3. The Available Offsite and Onsite A.C. Power Sources offsite source is generally much less than that required Are Each One Less Than the LCO.

to detect and restore an unavailable onsite a.c. supply.

Based on these considerations, a general distinction This degradation level results in the loss of individual does not appear to be warranted for operating restric redundancy in both the offsite power system and the tions associated with the loss of an offsite source and onsite a.x. power system. However, since power system those restrictions associated with the loss of an onsite redundancy is provided by two diverse sources of power, a.c. supply. However, the loss of an offsite source due to the reliability, and hence the safety, of this degradation a cause associated with extensive consequences such as a level appears to be higher than that of the previous severe ice storm or a forest fire would have implications degradation level. This apparent improvement could, more severe than the loss of an onsite a.c. supply. The however, be offset by the susceptibility of this power risks associated with such an offsite loss would be system configuration to a single bus or switching failure.

compounded by three effects: (a) the maintainability For example, the failure of an emergency power advantage of the offsite sources would be lost, (b) the distribution bus that is energized by either the single remaining offsite circuit could be susceptible to the same available offsite circuit or the single available onsite a.c.

cause, and (c) the stability of the offsite power system supply could render all emergency a.c. power ineffective.

might be affected. Thus, the loss of an offsite source by Based on these considerations, the operating restric such a cause should be treated as equivalent to the loss tions imposed on this level of degradation should be of both required offsite sources. similar to those of the previous degradation level.

1.93-2

However, the allowed operating time should be short

C. REGULATORY POSITION

ened because the onsite a.c. power system has been degraded, and the simultaneous loss of the offsite power The intent of each regulatory position1 is to imple and a LOCA (or any event that causes generator trip) is a ment the safest operating mode whenever the available design basis event. electric power sources are less than the LCO. According ly, various levels of degradation of the electric power system are listed below in order of increasing degrada

4. The Available Onsite A.C. Electric Power Supplies tion; the regulatory position given for each degraded Are Two Less Than the LCO. level should be incorporated in the Technical Specifica tions. Whenever the Technical Specifications allow unre This degradation level means that the onsite a.c. stricted operation to be resumed, such resumption power system does not have the capability to effect a should be contingent on the verification of the integrity safe shutdown and to mitigate the effects of an accident. and capability of the restored sources. Whenever the In a single unit plant, this condition usually means the Technical Specifications allow power operation to con unavailability of two diesel generators. In multi-unit tinue during a specific degradation level, such continued plants that share onsite a.c. supplies, this degradation power operation should be contingent on (a) an imme level means that the available onsite supplies, if any, do diate verification of the availability and integrity of the not have the capacity to mitigate the effects of an remaining sources, (b) reevaluation of the availability of accident in one unit and to safely shut down the other the remaining diesel-generator(s) at time intervals not to unit(s). exceed eight hours, (c) verification that the required Since the offsite power system is the only source of maintenance activities do not further degrade the power a.c. power for this level of degradation, the risk system or in any way jeopardize plant safety, and (d)

associated with continued operation for a very short compliance with the additional conditions stipulated for time could be less than that associated with immediate each specific degradation level.

shutdown (the immediate shutdown could cause grid instability which could result in total loss of a.c. power). 1. If the available a.c. power sources are one less than However, since any inadvertent generator trip could also the LCO, power operation may continue for a period result in total loss of a.c. power, the time allowed for that should not exceed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> if the system stability continued operation should be severely restricted. The and reserves are such that a subsequent single failure (including a trip of the unit's generator, but excluding an intent here is twofold: (a) to avoid the risk associated unrelated failure of the remaining offsite circuit if this with immediate shutdown and (b) to subsequently degraded state was caused by the loss of an offsite minimize the risk associated with this level of degrada source) would not cause total loss of offsite power.

tion by severely limiting its exposure time.

If these conditions for continued power operation are A concerted effort should be made to restore at least one onsite a.c. supply during this restricted time period, met and the affected source is restored within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, and system-wide actions should be initiated to ensure unrestricted operation may resume. If the conditions for that the offsite power system can accommodate the continued power operation are met but the source is not restored within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, the unit should be brought to imminent shutdown.

a cold shutdown state within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.

If the conditions for continued power operation cannot be met, the unit should be ramped down

5. The Available Onsite D.C. Supplies Are One Less immediately to the minimum power level required for Than the LCO. stable operation (preferably that required to accommo This degradation level means that the available d.c. date the unit's auxiliary loads only). The ramping rate power supplies do not have the required redundancy; should be at the maximum permitted by the Technical

• however, the d.c. power system has full functional Specifications without resorting to blowdown. While the capability to effect a safe shutdown and to mitigate the Unit is operating at this reduced power level, it should effects of an accident. Since a subsequent degradation provide minimum real power to the grid (preferably could jeopardize plant safety (e.g., a subsequent single zero); however, the generator may supply reactive power failure could render the entire power system ineffective to the grid within its rating, as required to enhance grid on a generator trip), the time allowed for continued stability. The unit may operate at this reduced power operation should be severely restricted. If the affected level for a period that should not exceed 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. If the affected source is restored within 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />, unrestricted d.c. supply is restored within this time period, unre operation could be resumed. If not, the unit should be stricted operation may be resumed. If not, the unit brought to a cold shutdown state within the next 36 should be brought promptly to an orderly shutdown and hours.

to a cold shutdown state as soon as possible. The required functions of the d.c. system should be critically monitored during the shutdown period and corrective ISee "Decision Flow Diagram for Availability of Electric Power actions taken, if required, to ensure the safety of the Sources" (5 sheets) appended to this guide. These sheets depict graphically the regulatory position for each of the five levels of shutdown. degradation covered by this guide.

1.93-3

2. If the available offsite a.c. power sources are two less for a period that should not exceed 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. If both than the LCO, power operation may continue for 24 sources are restored within this 12-hour period, unre hours if it appears likely that at least one of the offsite stricted operation may be resumed. If either an offsite or sources can be restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. an onsite a.c. source is restored within this 12-hour

. If these conditions for continued power operation are period, reduced power operation may continue for an met and both offsite sources are restored within 24 additional 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> subsequent to the restoration of one hours, unrestricted operation may be resumed. If only source. If the other source is restored within these one offsite source is restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, power additional 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, unrestricted operation may resume;

operation may continue for a total time that should not if not, the unit should be brought to a cold shutdown exceed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> in accordance with the conditions state within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. If neither an offsite nor described in Regulatory Position I (loss of one a.c. an onsite a.c. source is restored within the first 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> source). If no offsite source is restored within the first of reduced power operation, the unit should be brought

24-hour period of continued power operation, the unit to a cold shutdown state within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.

should be promptly shut down and brought to a cold 4. If the available onsite a.c. electric supplies are two shutdown state, or the lowest attainable pressure-tem less than the LCO, power operation may continue for a perature state, within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. period that should not exceed two hours. If both onsite If the conditions for continued power operation a.c. electric power supplies are restored within these two cannot be met, the unit should be ramped down to the hours, unrestricted operation may be resumed. If only minimum power level as described in Regulatory Posi one onsite a.c. supply is restored within these two hours, tion 1. The unit may operate at this reduced power level power operation may continue for a total time that for a period that should not exceed 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. If both should not exceed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> in accordance with the offsite sources are restored within the 24-hour period, conditions described in Regulatory Position 1 for the unrestricted operation could be resumed. If only one loss of one a.c. source. If no onsite a.c. supply is restored offsite source is restored within this 24-hour period, within the first two hours of continued power operation, reduced power operation may continue for an additional the unit should be brought to a cold shutdown state

24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> subsequent to the restoration of one source. If within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.

the other offsite source is restored within this additional 5. If the available onsite d.c. supplies are one less than time, unrestricted operation could be resumed; if not, the LCO, power operation may continue for a period the unit should be promptly shut down and brought to a that should not exceed two hours. If the affected d.c.

cold shutdown state, or to the lowest attainable pres supply is restored within these two hours, unrestricted sure-temperature state, within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. If no operation may be resumed. If not, the unit should be offsite source is restored within the first 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of brought promptly to a controlled shutdown and to a reduced power operation, the unit should be shut down cold shutdown state within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. The re as previously described. quired functions of the d.c. system should be critically

3. If the available offsite and onsite a.c. power sources monitored during the shutdown process and necessary are each one less than the LCO, power operation may actions taken, such as cross-connecting a supply to a continue for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> if (a) the reserves and system load, if required, to ensure a safe shutdown.

stability are such that a subsequent single failure The operating time limits delineated above are ex (including a trip of the unit's generator, but excluding an plicitly for corrective maintenance activities only. The unrelated failure of the remaining offsite circuit) would operating time limits should not be construed to include not cause total loss of offsite power and (b) it appears preventive maintenance activities which require the likely that at least one of the affected sources can be incapacitation of any required electric power source.

restored within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Such activities should be scheduled for performance If these conditions for continued power operation are during cold shutdown and/or refueling periods.

met and both sources are restored within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, unrestricted operation may be resumed. If either an

D. IMPLEMENTATION

offsite or an onsite a.c. source is restored within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, power operation may continue for a total time The purpose of this section is to provide information that should not exceed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> in accordance with the to applicants and licensees regarding the Regulatory condition described in Regulatory Position I for the loss staff's plans for utilizing this regulatory guide.

of one a.c. source. If neither an offsite source nor an This guide reflects current regulatory practice. There onsite source is restored within the first 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of fore, except in those cases in which the applicant continued power operation, the unit should be brought proposes an acceptable alternative method for comply to a cold shutdown state within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. ing with specified portions of the Commission's regula If the conditions for continued power operation tions, this guide will be used by the Regulatory staff in cannot be met, the unit should be ramped down to the evaluating all construction permit applications for which minimum power level as described in Regulatory Posi the issue date of the Safety Evaluation Report (SER) is tion 1. The unit may operate at this reduced power level July 1, 1974, or after.

1.934

DECISION FLOW DIAGRAM FOR AVAILABILITY

OF ELECTRIC POWER SOURCES

(REGULATORY POSITION 1 - THE AVAILABLE A.C.

SOURCES ARE ONE LESS THAN THE LCO)

(E N D) IE N D)

SHEET 1 OF 5

1.93-5

(FROM SHEET 1) (REGULATORY POSITION 2 - THE AVAILABLE OFFSITE

SOURCES ARE TWO LESS THAN THE LCO)

J

(TO SHEET 11 SHEET 2 OF 5 Il

1.93-6

(FROM SHEET 2) (REGULATORY POSITION 3 - THE AVAILABLE ONSITE AND

OFFSITE A.C. SOURCES ARE ONE LESS THAN THE LCO)

I

J

j (TO SHEET 1)

SHEET 3 OF 5

1.93-7

(REGULATORY POSITION 4 - THE AVAILABLE ONSITE A.C.

POWER SOURCES ARE TWO LESS THAN THE LCO)

11_

(NO)

(YES) ONE

SSUOURCE (NO)

J

(TO SHEET 1)

SHEET4 OF 5

1.93-8

(REGULATORY POSITION 5- THE AVAILABLE ONSITE

(FROM SHEET 1) D.C. SUPPLIES ARE ONE LESS THAN THE LCO)

POWER OPERATION

MAY CONTINUE

FOR 2 HOURS

(NO)

(YES)

(TO SHEET 1)

SHEET 5 OF 5

193-9