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DG-1153, Availability of Electric Power Sources, Redline/Strikeout Version (Enclosure 3)
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U.S. NUCLEAR REGULATORY COMMISSION December 1974October 2006 OFFICE OF NUCLEAR REGULATORY RESEARCH Division X1 DRAFT REGULATORY GUIDE

Contact:

AS.BK. AnyoneAggarwal (301) 415-xxxx 415-6005 DRAFT REGULATORY GUIDE 1.93 DG-1153 (Proposed Revision 1 of Regulatory Guide 1.93, dated December 1974)

AVAILABILITY OF ELECTRIC POWER SOURCES A. INTRODUCTION Section 50.36(c)(2), "Limiting Conditions for Operation," of 10 CFR Part 50, "In Title 10, Part 50, of the Code of Federal Regulations (10 CFR Part 50), Domestic Licensing of Production and Utilization Facilities," requires the Technical Specifications to Section 50.36(c)(2) [10 CFR 50.36(c)(2)] requires that the technical specifications (TSs)include the limiting conditions for operation (LCO) and actions required to be taken by the licensee when the LCO is not met. Power operation may be initiated and continued without restriction only when the LCO is met.

This, which are defined as the lowest functional capability or performance levels of equipment required for safe operation of the facility. Furthermore, when an LCO of a nuclear reactor is not met, the licensee shall shut down the reactor or follow any remedial action the TSs permit until the condition can be met.

This regulatory guide describes the operating procedures and restrictionsacceptable to the Regulatory staff which should be implementedthat the staff of the U.S. Nuclear Regulatory Commission (NRC) considers acceptable for implementationif the available electric power sources are less than the LCO. This regulatory guide is applicable tosingle and multi-unit plants, including multi-unit plants that share the required electric power sources.

The LCO withsingle- and multi-unit plants.

With respect to available electric power sources is an electric, the LCO establish the required capability of the electric power system that satisfies General Design Criterion17 (GDC-l 7)) 17, "ElectricElectric Power Systems," of as set forth in Appendix A, "GeneralGeneral Design Criteria for Nuclear Power Plants," of to 10 CFR Part 50, by including the following electric power sources: (1)

This regulatory guide is being issued in draft form to involve the public in the early stages of the development of a regulatory position in this area. It has not received staff review or approval and does not represent an official NRC staff position.

Public comments are being solicited on this draft guide (including any implementation schedule) and its associated regulatory analysis or value/impact statement. Comments should be accompanied by appropriate supporting data. Written comments may be submitted to the Rules and Directives Branch, Office of Administration, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001. Comments may be submitted electronically through the NRCs interactive rulemaking Web page at http://www.nrc.gov/what-we-do/regulatory/rulemaking.html. Copies of comments received may be examined at the NRCs Public Document Room, 11555 Rockville Pike, Rockville, MD. Comments will be most helpful if received by December 9, 2006.

Requests for single copies of draft or active regulatory guides (which may be reproduced) or placement on an automatic distribution list for single copies of future draft guides in specific divisions should be made to the U.S. Nuclear Regulatory Commission, Washington, DC 20555, Attention: Reproduction and Distribution Services Section, or by fax to (301)415-2289; or by email to Distribution@nrc.gov. Electronic copies of this draft regulatory guide are available through the NRCs interactive rulemaking Web page (see above); the NRCs public Web site under Draft Regulatory Guides in the Regulatory Guides document collection of the NRCs Electronic Reading Room at http://www.nrc.gov/reading-rm/doc-collections/; and the NRCs Agencywide Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html, under Accession No. ML062840370.

(1) two physically independent circuits from the offsite transmission network, each of which is either continuously available or can be made available within a few seconds following a loss-of-coolant.

accident (LOCA), (2)

(2) redundant onsite a.c.alternating current (ac) power supplies, and (3) sources (3) redundant onsite d.c. power supplies.

Nucleardirect current (dc) power sources to maintain core cooling, containment integrity, and other vital safety functions For nuclear power plants whereinthat are not licensed in accordance with GDC 17, the updated final safety analysis report provides the applicable design criteria. This report sets forth criteria similar to GDC 17, which requires that an offsite power system be provided to ensure the continued functioning of certain structures, systems, and components in the event of anticipated operational occurrences and postulated events.

Operating nuclear power plants for which only one of the two required offsite circuits can be made available within a few seconds following a LOCA are outside the scope of this guide.must justify the delayed offsite circuit availability to support the core cooling systems.

However, the restrictions imposed on such plants onregarding the loss of required sources would generally be more stringent than those recommended in thisguide.

B. DISCUSSION Backgroundregulatory guide.

The NRC issues regulatory guides to describe to the public methods that the staff considers acceptable for use in implementing specific parts of the agencys regulations, to explain techniques that the staff uses in evaluating specific problems or postulated accidents, and to provide guidance to applicants. Regulatory guides are not substitutes for regulations, and compliance with regulatory guides is not required. The NRC issues regulatory guides in draft form to solicit public comment and involve the public in developing the agencys regulatory positions. Draft regulatory guides have not received complete staff review and, therefore, they do not represent official NRC staff positions.

This regulatory guide contains information collections that are covered by the requirements of 10 CFR Part 50 which the Office of Management and Budget (OMB) approved under OMB control number 3150-0011. The NRC may neither conduct nor sponsor, and a person is not required to respond to, an information collection request or requirement unless the requesting document displays a currently valid OMB control number.

B. DISCUSSION Electric loads important to safetyofin a nuclear power plants are served by an electric power system that conformspursuant to GDC- 17. PNuclear power plants with more power sources thanare requiredGDC 17 requires mayby GDC-17 can toleratee able to withstand the loss of one or more sources and still meetsatisfy the LCO. DHowever, during the normal course of operation, however, any nuclear power plant may lose power sources to the extent that the LCO isare not met. ThisThis regulatory guide addresses such cases.

DG-1153, Page 3

GDC-17 specifies design requirements, not operating requirements; it therefore does not stipulate The TSs of operating nuclear power plants include the operational restrictions onresulting from the loss of power sources. Nevertheless,In general, plant TSs require the operability of the offsite power system as a part of the LCO and specify actions to be taken when the offsite power system is inoperable.

Plant operators should be aware of (1) the capability of the offsite power system to supply power during operation, and (2) situations that can result in a loss of offsite power or inadequate voltage following a trip of the plant or other transmission contingencies identified by the grid operator. If the offsite power system is not capable of providing the requisite power in either situation, the system should be declared inoperable and pertinent plant TS provisions should be followed.

The operational restrictions based on the intent of GDC-17 on the loss of power sources have been included in the Technical Specifications of recently licensed nuclear power plants. Such restrictionsin the TSs are based on the following three assumptions:

(1) The LCO of nuclear power plants isare met when all the electric power sources required by GDC-17 are available.

GDC 17 are available at the required voltage and capacity for the nuclear station and capable of withstanding a system contingency such as (a) a single failure involving loss of generation by the nuclear unit, any other critical generation source, or loss of power from a transmission system element, or (b) a double failure involving a loss of power from the transmission network and the loss of one train of onsite ac power.

(2) Under certain conditions, it may be safer to continue operation at full or reduced power for a limited time than, rather than to effect an immediate shutdown on theupon loss of some of the required electric power sources. Such decisions should be based on an evaluation that balances the risks associated with immediate shutdown against those associated with continued operation. If, on balance, immediate shutdown is the safer course, the unitplant should promptly be brought promptly to an orderly shutdown, and to a cold-shutdown state as soon as possible. For example, the risks associated with an immediate shutdown on theupon loss of the onsite a.c.ac power supplysource during a period of light transmission system load with high operating reservewould tend to be less than those resulting from shutdown during a peak load period because the stability of the offsite power system would be relatively higherwith less operating reserve because the electrical grid may be able to accommodate a loss of power generation. If, 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 orderly,power system elements (generation, transmission, and capacitor banks) and prepare for orderly shutdown, provided, of course, that these activities do not risk further degradation of the electric power system or in any way jeopardize the plants safety.

(3) If the LCO has notcannot been achieved, the unitplant should be promptly brought to an orderly shutdown after thetime allowed time for continued power operation has elapsed and to a cold shutdown state as soon as possible thereafter. The premise hereof this assumption is that the time allowed for continued operation could have been used to enhance the safety of the imminent shutdown. For example, the dispatcher could take such system-wide actions as increasing generation at other plants or dropping selected loads to ensure that the shutdown does not cause power grid instability or inadequate offsite power voltage. In addition, if the lossofof power sources beyond the LCO were to DG-1153, Page 4

occur during a peak load period, the time allowed operating timefor continued operation could be used to defer the shutdown to an off-peak period when the stability of the grid should be higher.

To develop when the electric grid has sufficient operating reserve to accommodate a loss of generation.

To develop the bases for specific regulatory guidance, fivesix levels of power system degradation of the power systems are described below, in order of increasing severity:.

1. The Available A.C.Offsite AC Power Sources Are One Less Than the LCO.

This degradation level means that one of the required offsiteor onsite a.c.ac sources is not available. T and, thus, either the offsite or the onsite a.c.the offsite ac power system has no redundancy; h.

However, eachthe offsite power system retains full capability(one system with redundancy) to effect a safe shutdown and to mitigate the effects of a design -basis accidentevent. Operation could, therefore, safely continue if the availability of the remaining power sources is verified; however, since the system is degraded below the LCO, a time limit on continued operation is warranted. Operating experience indicates that the availability and reliability of a typical offsite power source isare higher than that of'a typicalthose of a typical onsite a.c. supplyac source. Thus, if risk is evaluated in terms of availability and capability, the risk associated with the loss of an offsite power source (the source with the higher availability) would appear to be more severe than the riskthat associated with the loss of an onsite a.c.ac supply (the source with the lower availability).

However, this apparent difference in severity is usually offset by maintainability considerations; that is, the the time required to detect and restore an unavailable offsite source is generally much less than that required to detect and restore an unavailable onsitea.c. supply.

ac source, especially when the grid operator utilizes real-time contingency analysis.

Based on these considerations, a general distinction does not appear to be warranted for operating restrictions associated with the loss of an offsite source and those restrictions associated withversus the loss of an onsite a.c. supplyac source. However, the loss of an offsite source due toresulting from a causen event associated with extensive consequences, such as a severe ice storm or a forest fire, would have more severe implications more severe than the loss of an onsitea.c. supplyac source. The risks associated with such an offsite loss would be compounded by three effects: because (a1) the maintainability advantage of the offsite sources would be lost, (b2) the remaining offsite circuit could be susceptible to the same cause, and (c3) the stabilityconsequential trip probability of a number of units would be higher because of the potential loss of loads, and (4) the availability and capability of the offsite power system might be affected. Thus, the loss of an offsite sourcebyresulting from such a cause should be treated as equivalent to the loss of both required offsite sources.

DG-1153, Page 5

2. The Available Offnsite A.C.AC Power Sources Are TwoOne Less Than the LCO.

This degradation level means that one train of the onsite ac power system is not available for safe shutdown or to mitigate the effects of an event. In a single-unit plant, this typically means that one diesel generator is inoperable. In certain multi-unit plants that share onsite ac supplies, this means that the available onsite supply does not have the capacity to mitigate the effects of events in both units.

Since any inadvertent generator trip could potentially result in a total loss of ac power, the time allowed for continued operation should be severely restricted. In the absence of one onsite power source, the intent is twofold:

(1) Avoid the risk associated with immediate shutdown.

(2) Minimize the risk associated with this level of degradation by severely limiting its exposure time.

Licensees should make a concerted effort to restore the onsite ac power source during this restricted time period, and should inform the grid operator to ensure that the offsite power system can accommodate the imminent shutdown.

The Available Offsite AC Power Sources Are Two Less Than the LCO This degradation level means that the offsite power system does not have the capability tocannot effect a safe shutdown and to mitigateand mitigate the effects of an accidentevent; however, the onsite a.c.ac system hasis not been degraded. This degradationThus, this level generally corresponds to total loss of-theof the immediately accessible offsite power sources.

Because of the normally high availability ofthe offsite sources, this level of degradation may appear to be more severe than the next two degradation levelslisteddiscussed. However, two factors tend to decrease the severity of this degradation levelas, compared to the next twodegradation levels: (a). First, the configuration of the redundanta.c.onsite ac power system that remains available for this degradation level is not susceptible to a single bus or switching failure, whereas the next degradation level listed (No.

3) may be so susceptible, and (b). Second, the time required to detect and restore -anan unavailable offsite power source is generally much less than that required to detect and restore an unavailable onsite a.c.ac power source.

This level of degradation can be caused by anya variety of several events, including the loss of two offsite circuits, an unstable offsite power systemunavailable electric grid, or any other condition that renders offsite power unavailable for safe shutdown and emergency purposes. Since the onsite power system hasis not been degraded and sincea loss of offsite power simultaneous loss of offsite power andwith a LOCA werewas postulated as a design basis, a brief interval of continued operation is justified if an alternative source of ac power, independent of grid condition, is readily available and can act as a substitute train of ac power. (Note: S that some nuclear power plants are designed to cause anautomatic shutdown or to initiate load rejection at this level of degradation. Plants designed to cause an automatic shutdown at this level of degradation need no further discussion; however, those that initiate load reject. loadion are considered to remain operating within the context of this regulatory guide.)

DG-1153, Page 6

3. The Available Offsite and Onsite A.C.AC Power Sources Are Each One Less Than the LCO.

This level of power system degradation level results infrom the loss of individual redundancy in both the offsite and onsite ac power system and the onsite a.c. power systemsystems. However, since power system redundancy is provided by two diversedifferent sources of power, the reliability, and, hence, the safety, of this degradation level appears to be slightly higher than that of the previous degradation level. This apparent improvement could, However, h be offset bythe susceptibility of this power system configuration to a single bus or switching failure could offset this apparent improvement. For example, the failure of an emergency power distribution bus that is energized by either the single available offsite circuit or the single available onsitea.c.ac supply could render all emergencya.c. power ineffective.

ac power from that source ineffective. Moreover, if the offsite and onsite power were available to only one train, a bus fault could render all emergency power unavailable.

Based on these considerations, the operating restrictions imposed onat this level of degradation should be similar to those of the previous degradation level.

However, the allowed operating time should be shortened because the onsite a.c.ac power system has been degraded, and the simultaneousa loss of the offsite power andsimultaneous with a LOCA (or any event that causes a generator trip) is a design -basis event.

4. The Available Onsite A.C. ElectricAC Power SuppliesSources Are Two Less Than the LCO.

This degradation level means that the onsite a.c.ac power system does not have the capability tocannot effect a safe shutdown and to mitigateand mitigatethe effects of an accident. In a single -unit plant, this condition usually meansthe unavailability ofthat two diesel generatorsare inoperable. In multi--unit plants that share onsite a.c.ac supplies, this degradation level means that the available onsite supplies, if any, do not have the capacity to mitigate the effects of anaccidentevent in one unit and to safely shutdownand safely shut down the other unit(s).

Since the offsite power system is the only source of a.c.ac power forat this level of degradation, the risk associated with continued operation for a very short time could be less than that associated with immediate shutdown. (the iImmediate shutdown could cause grid instability which could resultresulting in a total loss ofa.c.ac power.). However, since any inadvertent generator trip could alsopotentially result in a total loss of a.c.ac power, the time allowed for continued operation should be severely restricted. In addition, all work that could potentially trip the unit should be suspended. The intent here is twofold: (a) to avoid (1) Avoid the risk associated with immediate shutdown and (b) to subsequently minimize.

(2) Minimize the risk associated with this level of degradation by severely limiting its exposure time.

DG-1153, Page 7

ALicensees should make aconcerted effort should be made to restore at least one onsitea.c.

supplyac power source during this restricted time period, andtake system-wide actionsshould be initiatedto ensure that the offsite power system can accommodate the imminent shutdown.

5. The Available Onsite D.C. SuppliesDC Power Sources Are One Less Than the LCO.

This degradation level means that the available d.c.dc power suppliessources do not have the required redundancy; however, the d.c.remaining train(s) of the dc power system has (have) full functional capability to effect a safe shutdown andto mitigate the effects of anaccidentevent. Since a subsequent degradation in an onsite ac or dc system could jeopardize plant safety (e.g., a subsequent single failure could render the entire power system ineffective on a generator trip), the time allowed for continued operation should be severely restricted.If the affected d.c. supplydc source is restored within this time period, unrestricted operation maybe resumed. If not, the unit shouldpromptly be brought promptly to an orderly shutdownand to a cold shutdown state as soon as possible . The required functions of thed.c.dc system should be critically monitored during the shutdown period and corrective actions taken, if required, to ensure safety.

The regulatory positions in Section C of this regulatory guide are based on the grid operators capabilities to ensure the safety of the shutdown.

adequacy of the offsite power system through contingency evaluations, as well as the plants capabilities to manage risk-significant maintenance and outages.

C. REGULATORY POSITION The intent of each of the following regulatory positions1 is to implement theensure that a nuclear power plant is in an acceptablysafest operating mode whenever the available electric power sources are less than the LCO. Accordingly,this section discusses thevarious levels of degradation of the electric power system are listed below, in order of increasing degradation; the TSs should incorporate the regulatory position given for each degraded level should be incorporated in the Technical Specifications. Whenever the Technical SpecificationsTSs allow unrestricted operation to be resumed, such resumption should be contingent on the verification of the integrity and capability of the restored sources. WSimilarly, whenever the Technical SpecificationsTSs allow power operation to continue during a specific degradation level, such continued power operation should be contingent on (a1) an immediate verification of the availability, capability, and integrity of the remaining sources,; (b2) reevaluation of the availability of the remaining diesel-generatordiesel generator(s) at time intervals not to exceed eight24 hours,; (c3) verification that the required maintenance activities do not further degrade the power system or in any way jeopardize plant safety,; and (d4) compliance with the additional conditions stipulated for each specific degradation level.

1. The operating time limits delineated below are primarily for corrective maintenance activities only.

The conduct of other maintenance or surveillance activities should be evaluated to determine their compliance with 10 CFR 50.65, Requirements for Monitoring the Effectiveness of Maintenance at Nuclear Power Plants. Many plants are now attempting online testing.

1. The Available Offsite AC Power Sources Are One Less Than the LCO 1

See Decision Flow Diagram for Availability of Electric Power Sources (6 sheets) appended to this regulatory guide.

These sheets graphically depict the regulatory position for each of the six levels of degradation covered by this guide.

DG-1153, Page 8

If the available a.c.offsite ac power sources are one less than the LCO, power operation may continue for a period that should not exceed 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months if the electric gridsystem stabilitycapability and reserves are such that a subsequent single failure (including a trip of the unit's generator, but excluding an unrelated failure of the remaining offsite circuit if this degraded state was caused by the loss of an offsite source) would not cause a total loss of offsite power.

If these conditions for continued power operation are met and the affected source is restored within 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months , unrestricted operation may resume. IConversely, if the conditions for continued power operation are met, but the source is not restored within 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months , the unit should be brought toto hot shutdown (Mode 3) for boiling-water reactors (BWRs) in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months , hot standby (Mode 3) for pressurized-water reactors (PWRs) in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months ,and cold shutdown statefor either within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

If the conditions for continued 2. The Available Onsite AC Power Sources Are One Less Than the LCO If the available onsite ac power sources are one less than the LCO,power operation cannot be met, the unit should be ramped down immediately to the minimum power level required for stable operation (preferably that required to accommodate the unit's auxiliary loads only). The ramping rate should be at the maximum permitted by the Technical Specifications without resorting to blowdown. While the Unit is operating at this reduced power level, it should provide minimum real power to the grid (preferably zero);

however, the generator may supply reactive power to the grid within its rating, as required to enhance grid stability. The unit may operate at this reduced power level for a period may continue for a period that should not exceed 4872 hours0.0564 days 1.353 hours 0.00806 weeks 0.00185 months . If the affected source is restored within 48 hours5.555556e-4 days 0.0133 hours 7.936508e-5 weeks 1.8264e-5 months , unrestricted operation could be resumed. If not, the unit should be brought to a cold shutdown state within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

2. , provided the redundant diesel generator is assessed within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to be free from common-cause failure.

3. The Available Offsite AC Power Sources Are Two Less Than the LCO If the available offsitea.c.ac power sources are two less than the LCO, power operation may continue for 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months if it appears likely that at least one of the offsite sources can be restored within 24 hour2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months sthat time. If these conditions for continued power operation are met and both offsite sources are restored within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months , unrestricted operation may be resumed. If only one offsite source is restored within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months , power operation may continue for a total time that should not exceed 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months in accordance with the conditions described in Regulatory Position I (loss of one a.c. source)1.

IConversely, if no offsite source is restored within the first 24-hour period of continued power operation, the unitplant should be promptly be shut down and brought to a cold shutdown state, or the lowest attainable pressure-temperature state, within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

If the conditions for continued power operation cannot be met, the unit should be ramped down to the minimum power level as described in Regulatory Position 1. The unit may operate at this reduced power level for a period that should not exceed 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months . If both offsite sources are restored within the 24-hour period, unrestricted operation could be resumed. If only one offsite source is restored within this 24-hour period, reduced power operation may continue for an additional 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months subsequent to the restoration of one source. If the other offsite source is restored within this additional time, unrestricted operation could be resumed; if not, the unit should be promptly shut down and brought to a cold shutdown state, or to the lowest attainable pressure-temperature state, within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months . If no offsite source is DG-1153, Page 9

restored within the first 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months of reduced power operation, the unit should be shut down as previously described.

3.in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months to hot shutdown for BWRs (Mode 3) and hot standby for PWRs (Mode 3).

4. The Available Offsite and Onsite AC Power Sources Are Each One Less Than the LCO If the available offsite and onsitea.c.ac power sources are each one less than the LCO, power operation may continue for 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months if (a1) the reserves and system stability are such that a subsequent single failure (including a trip of the unit's generator, but excluding an unrelated failure of the remaining offsite circuit) would not causea total loss of offsite power,and (b2) it appears likely that at least one of the affected sources can be restored within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months .

If these conditions for continued power operation are met and both sources are restored within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months , unrestricted operation maybe resumed. If either an offsite or an onsitea.c.ac source is restored within 12 hour1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months s12 hours, power operation may continue for a total time that should not exceed 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months in accordance with the condition described in Regulatory Position 1 for the loss of one ac source.

Conversely, if neither an offsite source nor an onsite source is restored within the first 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months of continued power operation, the plant should be brought to hot shutdown (Mode 3) for BWRs in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months , hot standby (Mode 3) for PWRs in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months , and cold shutdown for either within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

5. The Available Onsite AC Power Sources Are Two Less Than the LCO If the available onsite ac electric power sources are two less than the LCO, power operation may continue for a period that should not exceed 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months . If both onsite ac electric power sources are restored within these 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months , unrestricted operation may resume. If only one onsite ac power source is restored within these 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months , power operation may continue for a total time that should not exceed 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months , in accordance with the conditions described in Regulatory PositionI1 for the loss of onea.c.ac source.

IConversely, if neither an offsite source nor anno onsite source isac source can be restored within the first 122 hours0.00141 days 0.0339 hours 2.017196e-4 weeks 4.6421e-5 months of continued power operation, the unit should be brought to a cold shutdown state within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

If the conditions for continued power operation cannot be met, the unit should be ramped down to the minimum power level as described in Regulatory Position 1. The unit may operate at this reduced power level for a period that should not exceed 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months . If both sources are restored within this 12-hour period, unrestricted operation may be resumed. If either an offsite or an onsite a.c. source is restored within this 12-hour period, reduced power operation may continue for an additional 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months subsequent to the restoration of one source. If the other source is restored within these additional 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months , unrestricted operation may resume; if not, the unit should be brought to a cold shutdown state within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months . If neither an offsite nor an onsite a.c. source is restored within the first 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months of reduced power operation, the unit should be brought to a cold shutdown state within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months . 4.

If the available onsite a.c. electric supplies are two less than the LCO, power operation may continue for a period that should not exceed two hours. If both onsite a.c. electric power supplies are restored within these two hours, unrestricted operation may be resumed. If only one onsite a.c. supply is restored within these two hours, power operation may continue for a total time that should not exceed 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months in accordance with the conditions described in Regulatory Position 1 for the loss of one a.c. source. If no onsite a.c. supply is restored within the first two hours of continued power operation, the unit should be brought to a cold shutdown state within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

DG-1153, Page 10

5.6. The Available Onsite DC Power Sources Are One Less Than the LCO If the available onsited.c. suppliesdc power sources are one less than the LCO, power operation may continue for a period that should not exceed two2 hours. If the affected d.c. supplydc source is restored within these two2 hours, unrestricted operation may be resumed. If not, the unitplant should be brought promptly to a controlled shutdown and to a cold shutdown state within the next 36 hour4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months shot shutdown (Mode 3) for BWRs in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months , hot standby (Mode 3) for PWRs in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months , and cold shutdown within the next 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months . The required functions of the d.c.dc system should be critically monitored during the shutdown process, and necessary actions taken, (such as cross-connecting a supply to a load, if required,, or shedding optional loads) to ensure a safe shutdown.

The operating time limits delineated above are explicitly for corrective maintenance activities only. The operating time limits should not be construed to include preventive maintenance activities which require the incapacitation of any required electric power source. Such activities should be scheduled for performance during cold shutdown and/or refueling periods.

D. IMPLEMENTATION The purpose of this section is to provide information to applicants and licensees regarding the Regulatory NRC staff's plans for utilizingusing this draft regulatory guide.

This regulatory guide reflects current regulatory practice. Therefore, except in those; therefore, no backfitting is intended or approved in connection with its issuance.

The NRC has issued this draft guide to encourage public participation in its development.

Except in those cases in which thean applicant or licensee proposes or has previously established an acceptable alternative method for complying with specified portions of the Commission's the NRCs regulations, this guide will be used by the Regulatory staff in evaluatingthe all methods to be described in the active guide will reflect public comments and will be used in evaluating (1) submittals in connection with applications forconstructionpermit applications for which the issue date of the Safety Evaluation Report (SER) is July 1, 1974, or after.

DECISION FLOW DIAGRAMS DID NOT CONVERT permits, standard plant design certifications, operating licenses, early site permits, and combined licenses; and (2) submittals from operating reactor licensees who voluntarily propose to initiate system modifications following the issuance of this guide if there is a clear nexus between the proposed modifications and the subject for which guidance is provided herein.

REGULATORY ANALYSIS

1. Statement of the Problem The U.S. Nuclear Regulatory Commission (NRC) issued Regulatory Guide 1.93 in December 1974 to ensure that nuclear power plants are in an acceptably safe operating mode whenever the available DG-1153, Page 11

electric power sources are less than the limiting conditions for operation (LCO). This regulatory guide needs a general update to reflect the operating experience gained over the past 30 years, 10 CFR 50.63 (the Station Blackout Rule), 10 CFR 50.65 (the Maintenance Rule), issues related to grid reliability, and deregulation of the electrical industry.

2. Objective The objective of this action is to provide current guidance for an acceptably safe operating mode whenever the available electric power sources are less than the LCO.

3. Technical Approach This proposed Revision 1 of Regulatory Guide 1.93 was developed to achieve the objective outlined above. As such, this guide is consistent with current regulatory practice and the standardized technical specifications.

4. Conclusion The NRC should issue this regulatory guide to enhance the licensing process. The staff has concluded that the proposed action will reduce unnecessary burden on both the NRC and its licensees, and will result in an improved and more uniform process for implementing the safest operating mode when the available power sources are not available. Moreover, the staff sees no adverse effects associated with issuing this regulatory guide.

BACKFIT ANALYSIS Because this proposed Revision 1 of Regulatory Guide 1.93 reflects current regulatory practice, it does not require a backfit analysis as described in 10 CFR 50.109(c).

DG-1153, Page 12

Decision Flow Diagram For Availability Of Electric Power Sources Start (Regulatory Position 1 - The Available Offsite AC Power Sources Are One Less Than the LCO)

All required (NO) offsite AC sources available?

(NO)

Only one offsite 1 (To Sheet 3)

AC source not available?

(YES)

(To Sheet 6)

(NO)

All required DC Conditions (NO) sources 4 satisfied for available? power operation to continue?

(YES) (From Other Sheets) C (YES)

Power operation may continue for 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months (Max. Total Time)

(NO)

(From Other Sheets) Source restored (From Other within 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months ?

Sheets)

A B

(YES)

LCO satisfied - Bring to hot shutdown for BWRs Unrestricted operation (Mode 3), hot standby for PWRs may continue or be (Mode 3) in 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months . Bring to resumed. cold shutdown within 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months .

(END) (END) 9 Sheet 1 of 6

Decision Flow Diagram For Availability Of Electric Power Sources Start (Regulatory Position 2 - The Available Onsite AC Power Sources Are One Less Than the LCO)

All required (NO) onsite AC sources available?

(NO)

Only one onsite 1 (To Sheet 3)

AC source not available?

(YES)

(NO)

Redundant Diesel Generator is available?

(To Sheet 6)

(NO)

All required DC sources 4 available? (YES)

(YES) Conditions (NO) satisfied for power operation to continue?

(YES)

A C B (To Sheet 1) 10 Sheet 2 of 6

Decision Flow Diagram For Availability Of Electric Power Sources (From Sheet 1) 1 (Regulatory Position 3 - The Available Offsite AC Power Sources Are Two Less Than the LCO)

Two offsite (NO) sources not 2 (To Sheet 4) available?

(YES)

Conditions (NO) satisfied for power operation to continue?

(YES)

Power operation may continue for 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months (Max. Total Time)

(NO)

Both sources restored within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months ?

(NO)

One source (YES) restored within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months ?

(YES)

A C B (To Sheet 1) 11 Sheet 3 of 6

Decision Flow Diagram For Availability Of Electric Power Sources (From Sheet 3) 2 (Regulatory Position 4 - The Available Offsite and Onsite AC Power Sources Are Each One Less Than the LCO)

(NO)

One offsite & one 3 (To Sheet 5) onsite source not available?

(YES)

Conditions (NO) satisfied for power operation to continue?

(YES)

Power operation may continue for 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months (Max. Total Time)

(NO)

Both sources restored within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months ?

(NO)

One source (YES) restored within 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months ?

(YES)

A C B (To Sheet 1) 12 Sheet 4 of 6

Decision Flow Diagram For Availability Of Electric Power Sources (From Sheet 4) 3 (Regulatory Position 5 - The Available Onsite AC Electric Supplies Are Two Less Than the LCO)

Two onsite AC sources not available?

(YES)

Power operation may continue for 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months (Max. Total Time)

(NO)

Both sources restored within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months ?

(YES) (NO)

One source restored within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months ?

(YES)

A C B (To Sheet 1) 13 Sheet 5 of 6

Decision Flow Diagram For Availability Of Electric Power Sources (From Sheet 1) 4 (Regulatory Position 6 - The Available Onsite DC Supplies Are One Less Than the LCO)

Only one DC source not available?

(YES)

Power operation may continue for 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months (Max. Total Time)

(NO)

DC source restored within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months ?

(YES)

A B (To Sheet 1) 14 Sheet 6 of 6

ML063250360

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