ML062840370
| ML062840370 | |
| Person / Time | |
|---|---|
| Issue date: | 10/25/2006 |
| From: | Aggarwal S NRC/RES/DFERR/DDERA/IEEB |
| To: | |
| Aggarwal S (301)415-6005 | |
| References | |
| DG-1153 RG-1.093, Rev 1 | |
| Download: ML062840370 (14) | |
Text
This regulatory guide is being issued in draft form to involve the public in the early stages of the development of a regulatory positionin 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 regulatoryanalysis 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 NRC's interactive rulemaking Web page at http://www.nrc.gov/what-we-do/regulatory/rulemaking.html. Copies of comments received may be examined at the NRC'sPublic 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 listfor 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 NRC's interactive rulemakingWeb page (see above); the NRC's public Web site under Draft Regulatory Guides in the Regulatory Guides document collection of the NRC's Electronic Reading Room at http://www.nrc.gov/reading-rm/doc-collections/; and the NRC's Agencywide DocumentsAccess and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html, under Accession No. ML062840370. U.S. NUCLEAR REGULATORY COMMISSION October 2006OFFICE OF NUCLEAR REGULATORY RESEARCH Division 1 DRAFT REGULATORY GUIDEContact: S.K. Aggarwal (301) 415-6005DRAFT REGULATORY GUIDE DG-1153(Proposed Revision 1 of Regulatory Guide 1.93, dated December 1974)AVAILABILITY OF ELECTRIC POWER SOURCESA. INTRODUCTIONIn Title 10, Part 50, of the Code of Federal Regulations (10 CFR Part 50), "Domestic Licensingof Production and Utilization Facilities," Section 50.36(c)(2) [10 CFR 50.36(c)(2)] requires thatthe technical specifications (TSs) include the limiting conditions for operation (LCO), which are defined asthe 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 reactoror follow any remedial action the TSs permit until the condition can be met.This regulatory guide describes the operating proce dures and restrictions that the staff of the U.S.Nuclear Regulatory Commission (NRC) considers acceptable for implementation if the available electricpower sources are less than the LCO. This regulatory guide is applicable to single- and multi-unit plants.With respect to available electric power sources, the LCO establish the required capabilityof the electric power system that satisfies General Design Criterion (GDC) 17, "Electric Power Systems,"
as set forth in Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50,by including the following electric power sources:(1)two physically independent circuits from the offsite transmission network, each of whichis either continuously available or can be made available within a few seconds followinga loss-of-coolant accident (LOCA)(2)redundant onsite alternating current (ac) power sources (3)redundant onsite direct current (dc) power sources to maintain core cooling,containment integrity, and other vital safety functions DG-1153, Page 2For nuclear power plants that are not licensed in accordance with GDC 17, the updated finalsafety analysis report provides the applicable design criteria. This report sets forth criteria similar toGDC 17, which requires that an offsite power system be provided to ensure the continued functioningof certain structures, systems, and components in the event of anticipated operational occurrencesand postulated events.Operating nuclear power plants for which only one of the two required offsite circuits can bemade available within a few seconds following a LOCA must justify the delayed offsite circuit availabilityto support the core cooling systems. However, the restrictions imposed on such plants regarding the lossof required sources would generally be more stringent than those recommended in this regulatory guide.The NRC issues regulatory guides to describe to the public methods that the staff considersacceptable for use in implementing specific parts of the agency's regulations, to explain techniquesthat the staff uses in evaluating specific problems or postulated accidents, and to provide guidanceto applicants. Regulatory guides are not substitutes for regulations, and compliance with regulatory guidesis not required. The NRC issues regulatory guides in draft form to solicit public comment and involvethe public in developing the agency's 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 requirementsof 10 CFR Part 50 which the Office of Management and Budget (OMB) approved under OMB controlnumber 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 currentlyvalid OMB control number.
B. DISCUSSIONElectric loads important to safety in a nuclear power plant are served by an electric power system pursuant to GDC 17. Nuclear power plants with more power sources than GDC 17 requires may be ableto withstand the loss of one or more sources and still satisfy the LCO. However, during the normal course of operation, any nuclear power plant may lose power sources to the extent that the LCO are not met. This regulatory guide addresses such cases.The TSs of operating nuclear power plants include the operational restrictions resulting fromthe loss of power sources. 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 followinga trip of the plant or other transmission contingencies identified by the grid operator. If the offsite powersystem is not capable of providing the requisite power in either situation, the system should be declaredinoperable and pertinent plant TS provisions should be followed.The operational restrictions in the TSs are based on the following three assumptions:(1)The LCO of nuclear power plants are met when all electric power sources required by GDC 17are available at the required voltage and capacity for the nuclear station and capable of withstandinga 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.
DG-1153, Page 3(2)Under certain conditions, it may be safer to continue operation at power for a limited time,rather than to effect an immediate shutdown upon loss of some required electric power sources. Such decisions should be based on an evaluation that balances the risks associated with immediateshutdown against those associated with continued operation. If, on balance, immediate shutdownis the safer course, the plant should promptly be brought to an orderly shutdown. For example,the risks associated with immediate shutdow n upon loss of the onsite ac power source duringa period of light transmission system load with high operating reserve would tend to be less thanthose resulting from shutdown during a peak load period with 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 usedto restore the lost power system elements (generation, transmission, and capacitor banks)and prepare for orderly shutdown, provided, of course, that these activities do not risk furtherdegradation of the electric power system or in any way jeopardize the plant's safety.(3)If the LCO cannot be achieved, the plant should be brought to an orderly shutdown after the time allowed for continued power operation has elapsed. The premise of this assumption is thatthe time allowed for continued operation could have been used to enhance the safety of the imminent shutdown. For example, the dispatcher could take such systemwide actionsas increasing generation at other plants or dropping selected loads to ensure that the shutdowndoes not cause power grid instability or inadequate offsite power voltage. In addition, if the loss of power sources beyond the LCO were to occur during a peak load period, the time allowed for continued operation could be used to defer the shutdown to an off-peak periodwhen the electric grid has sufficient operating reserve to accommodate a loss of generation.To develop the bases for specific regulatory guidance, six levels of power system degradationare described below, in order of increasing severity.The Available Offsite AC Power Sources Are One Less Than the LCOThis degradation level means that one of the required offsite ac sources is not available and, thus,the offsite ac power system has no redundancy. However, the offsite power system retains full capabilityto effect safe shutdown and mitigate the effects of a design-basis event. Operation could, therefore,safely continue if the availability of the remaining power sources is verified; however, since the systemis degraded below the LCO, a time limit on continued operation is warranted. Operating experienceindicates that the availability and reliability of a typical offsite power source are higher than thoseof a typical onsite ac source. Thus, if risk is evaluated in terms of availability and capability, the riskassociated with the loss of an offsite power source (the source with higher availability) would appear to bemore severe than that associated with the loss of an onsite ac supply (the source with lower availability). However, this apparent difference in severity is usually offset by maintainability considerations;that is, the time required to detect and restore an unavailable offsite source is generally much less than that required to detect and restore an unavailable onsite ac source, especially when the grid operatorutilizes real-time contingency analysis.Based on these considerations, a general distincti on does not appear to be warranted for operatingrestrictions associated with the loss of an offsite source versus the loss of an onsite ac source. However,the loss of an offsite source resulting from an event associated with extensive consequences, such asa severe ice storm or forest fire, would have more severe implications than the loss of an onsite ac source. The risks associated with such an offsite loss would be compounded because (1) the maintainabilityadvantage of the offsite sources would be lost, (2) the remaining offsite ci rcuit could be susceptibleto the same cause, (3) the consequential 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 beaffected. Thus, the loss of an offsite source resulting from such a cause should be treated as equivalent to the loss of both required offsite sources.
DG-1153, Page 4The Available Onsite AC Power Sources Are One Less Than the LCOThis degradation level means that one train of the onsite ac power system is not availablefor safe shutdown or to mitigate the effects of an event. In a single-unit plant, this typically meansthat one diesel generator is inoperable. In certain multi-unit plants that share onsite ac supplies, this meansthat 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 timeallowed 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 thisrestricted time period, and should inform the grid operator to ensure that the offsite power systemcan accommodate the imminent shutdown.The Available Offsite AC Power Sources Are Two Less Than the LCOThis degradation level means that the offsite power system cannot e ffect a safe shutdownand mitigate the effects of an event; however, the onsite ac system is not degraded. Thus, this levelgenerally corresponds to total loss of the immediately accessible offsite power sources.Because of the normally high availability of offsite sources, this level of degradation may appearto be more severe than the next two degradation levels discussed. However, two factors tend to decreasethe severity of this degradation level, compared to the next two. First, the configuration of the redundantonsite ac power system that remains available for this degradation level is not susceptible to a single busor switching failure, whereas the next degradation level may be susceptible. Second, the time requiredto detect and restore an unavailable offsite power source is generally much less than that requiredto detect and restore an unava ilable onsite ac power source.This level of degradation can be caused by a variety of events, including the loss of two offsitecircuits, an unavailable electric grid, or any other condition that renders offsite power unavailablefor safe shutdown and emergency purposes. Since the onsite power system is not degraded and a lossof offsite power simultaneous with a LOCA was postulated as a design basis, a brief interval of continuedoperation is justified if an alternative source of ac power, independent of grid condition, is readily availableand can act as a substitute train of ac power. (Note that some nuclear power plants are designed to causean automatic shutdown or initiate load rejection at this level of degradation. Plants designed to causean automatic shutdown at this level of degrada tion need no further discussion; however, those thatinitiate load rejection are considered to remain operating within the context of this regulatory guide.)
DG-1153, Page 5The Available Offsite and Onsite AC Power Sources Are Each One Less Than the LCOThis level of power system degradation results from the loss of individual redundancy in boththe offsite and onsite ac power systems. However, since power system redundancy is provided by twodifferent 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. However, the susceptibility of this power systemto a single bus or switching failure could offset this apparent improvement. For example, the failureof an emergency power distribution bus that is energized by either the single available offsite circuitor the single available onsite ac supply could render all emergency ac power from that source ineffective. Moreover, if the offsite and onsite power were available to only one train, a bus fault could renderall emergency power unavailable.Based on these considerations, the operating restrictions imposed at this level of degradationshould be similar to those of the previous degradation level. However, the allowed operating timeshould be shortened because the onsite ac power system has been degraded, and a loss of offsite powersimultaneous with a LOCA (or any event that causes a generator trip) is a design-basis event.The Available Onsite AC Power Sources Are Two Less Than the LCOThis degradation level means that the onsite ac power system cannot effect a safe shutdownand mitigate the effects of an accident. In a single-unit plant, this usually means that two diesel generatorsare inoperable. In multi-unit plants that share onsite ac supplies, this degradation level means thatthe available onsite supplies, if any, do not have the capacity to mitigate the effects of an event in one unitand safely shut down the other unit(s).Since the offsite power system is the only source of ac power at this level of degradation,the risk associated with continued operation for a very short time could be less than that associated withimmediate shutdown. (Immediate shutdown could cause grid instability resulting in a total loss of ac power.) However, 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 addition, all work that couldpotentially trip the unit should be suspended. 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 at least one onsite ac power source during thisrestricted time period, and take systemwide actions to ensure that the offsite power system can accommodatethe imminent shutdown.The Available Onsite DC Power Sources Are One Less Than the LCOThis degradation level means that the available dc power sources do not have the requiredredundancy; however, the remaining train(s) of the dc power system has (have) full functional capabilityto effect a safe shutdown and mitigate the effects of an event. Since a subsequent degradation in an onsiteac or dc system could jeopardize plant safety (e.g., a subsequent single failure could render the entirepower system ineffective on a generator trip), the time allowed for continued operation should be severelyrestricted. If the affected dc source is restored within this time period, unrestricted operation may resume. If not, the unit should promptly be brought to an orderly shutdown. The required functions of the dc systemshould 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 adequacy of the offsite power system through contingency evaluations,as well as the plants' capabilities to manage risk-significant maintenance and outages.
1See "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 6C. REGULATORY POSITIONThe intent of each of the following regulatory positions 1 is to ensure that a nuclear power plantis in an acceptably safe operating mode whenever the available electric power sources are less than the LCO. Accordingly, this section discusses the various levels of degradation of the electric power system, in orderof increasing degradation; the TSs should incorporate the regulatory position for each degraded level. Whenever the TSs allow unrestricted operation to resume, such resumption should be contingent onverification of the integrity and capability of the restored sources. Similarly, whenever the TSs allowpower operation to continue during a specific degradation level, such continued power operationshould be contingent on (1) immediate verification of the availability, capability, and integrityof the remaining sources; (2) reevaluation of the availability of the remaining diesel generator(s) at timeintervals not to exceed 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />; (3) verification that the required maintenance activities do not furtherdegrade the power system or in any way jeopardize plant safety; and (4) compliance with the additionalconditions stipulated for each specific degradation level.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 Maintenanceat Nuclear Power Plants." Many plants are now attempting online testing.1.The Available Offsite AC Power Sources Are One Less Than the LCOIf the available offsite ac power sources are one less than the LCO, power operation may continuefor a period 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 electric grid system capability and reserves are such thata subsequent single failure (including a trip of the unit's generator, but excluding an unrelated failureof 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 restoredwithin 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, unrestricted operation may resume. Conversely, if the conditions for continuedpower 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 broughtto hot shutdown (Mode 3) for boiling-water reactors (BWRs) in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, hot standby (Mode 3)for pressurized-water reactors (PWRs) in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, and cold shutdown for either within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.2.The Available Onsite AC Power Sources Are One Less Than the LCOIf the available onsite ac power sources are one less than the LCO, power operation may continuefor a period that should not exceed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, provided the redundant diesel generator is assessedwithin 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to be free from common-cause failure.3.The Available Offsite AC Power Sources Are Two Less Than the LCOIf the available offsite ac power sources are two less than the LCO, power operation may continuefor 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> if it appears likely that at least one of the offsite sources can be restored within that time. If these conditions for continued power operation are met and both offsite sources are restored within24 hours, unrestricted operation may resume. If only 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 operation may continue for a total time 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 conditions described in Regulatory Position 1. Conversely, if no offsite source is restored within DG-1153, Page 7the first 24-hour period of continue d power operation, the plant should promptly be shut down in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> to hot shutdown for BWRs (Mode 3) and hot standby for PWRs (Mode 3).4.The Available Offsite and Onsite AC Power Sources Are EachOne Less Than the LCOIf the available offsite and onsite ac power sources are each one less than the LCO, power operationmay continue for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> if (1) 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 cause a total loss of offsite power, and (2) it appears likely that at least one of the affected sources can be restored within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.If these conditions for continued power operation are met and both sources are restored within12 hours, unrestricted operation may resume. If either an offsite or an onsite ac source is restored within12 hours, power operation may continue for a total time 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 withthe 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 <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of continued power operation,the plant should be brought to hot shutdown (Mode 3) for BWRs in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, hot standby (Mode 3) for PWRs in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, and cold shutdow n for either within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.5.The Available Onsite AC Power Sources Are Two Less Than the LCOIf the available onsite ac electric power sources are two less than th e LCO, power operationmay continue for a period that should not exceed 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. If both onsite ac electric power sources are restored within these 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, unrestricted operation may resume. If only one onsite ac power sourceis restored within these 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, power operation may continue for a total time that should not exceed72 hours, in accordance with the conditions described in Regulatory Position 1 for the loss of one ac source. Conversely, if no onsite ac source can be restored w ithin the first 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> of continued power operation,the unit should be brought to cold shutdown within the next 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.6.The Available Onsite DC Power Sources Are One Less Than the LCOIf the available onsite dc power sources are one less than the LCO, power operation may continuefor a period that should not exceed 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />. If the affected dc source is restored within these 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />,unrestricted operation may resume. If not, the plant should be brought to hot shutdown (Mode 3)for BWRs in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, hot standby (Mode 3) for PW Rs in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, and cold shutdown within the next36 hours. The required functions of the dc system should be critically monitored during the shutdownprocess, and necessary actions taken (such as cross-connecting a supply, or shedding optional loads) to ensure safe shutdown.
DG-1153, Page 8 D. IMPLEMENTATIONThe purpose of this section is to provide information to applicants and licensees regardingthe NRC staff's plans for using this draft regulatory guide. This regulatory guide reflects currentregulatory practice; 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 an applicant or licensee proposes or has previously establishedan acceptable alternative method for complying with specified portions of the NRC's regulations,the methods to be described in the active guide will reflect public comments and will be used in evaluating(1) submittals in connection with applications for construction permits, standard plant design certifications,operating licenses, early site permits, and combined licenses; and (2) submittals from operating reactorlicensees 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 guidanceis provided herein.REGULATORY ANALYSIS1.Statement of the ProblemThe U.S. Nuclear Regulatory Commission (NRC) issued Regulatory Guide 1.93 in December 1974to ensure that nuclear power plants are in an acceptably safe operating mode whenever the availableelectric power sources are less than the limiting conditions for operation (LCO). This regulatory guideneeds 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.ObjectiveThe objective of this action is to provide current guidance for an acceptably safe operating modewhenever the available electric power sources are less than the LCO.3.Technical ApproachThis proposed Revision 1 of Regulatory Guide 1.93 was developed to achieve the objectiveoutlined above. As such, this guide is consistent with current regulatory practice and the standardized technical specifications.4.ConclusionThe NRC should issue this regulatory guide to enhance the licensing process. The staff hasconcluded 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 modewhen the available power sources are not available. Moreover, the staff sees no adverse effectsassociated with issuing this regulatory guide.BACKFIT ANALYSISBecause this proposed Revision 1 of Regulatory Guide 1.93 reflects current regulatory practice,it does not require a backfit analys is as described in 10 CFR 50.109(c).
9StartAll required offsite AC sources available?(NO)(YES)Only one offsite AC source not available?(NO)(To Sheet 3)(NO)(NO)(YES)(YES)(YES)All required DC sources available?Conditions satisfied for power operation to continue?Power operation may continue for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />(Max. Total Time)(To Sheet 6)(From Other Sheets)(From Other Sheets)(YES)(NO)(From Other Sheets)(END)(END)Source restored within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />?Bring to hot shutdown for BWRs (Mode 3), hot standby for PWRs (Mode 3) in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Bring to cold shutdown within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.LCO satisfied -Unrestricted operation may continue or be resumed.Decision Flow Diagram For Availability Of Electric Power Sources(Regulatory Position 1 -The Available Offsite AC Power Sources Are One Less Than the LCO)Sheet 1 of 6 1 4 C A B 10StartAll required onsite AC sources available?(NO)(YES)Only one onsite AC source not available?(NO)(NO)(NO)(YES)(YES)All required DC sources available?Conditions satisfied for power operation to continue?(To Sheet 6)(YES)Decision Flow Diagram For Availability Of Electric Power Sources(Regulatory Position 2 -The Available Onsite AC Power Sources Are One Less Than the LCO)(To Sheet 3)
A C B(To Sheet 1)Sheet 2 of 6 1 4Redundant Diesel Generator is available?(YES)(NO) 11Two offsite sources not available?(NO)(YES)(NO)(YES)Both sources restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />?Conditions satisfied for power operation to continue?Power operation may continue for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />(Max. Total Time)(NO)One source restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />?(Regulatory Position 3 -The Available Offsite AC Power Sources Are Two Less Than the LCO) 1(From Sheet 1)(To Sheet 4)(YES)(NO)(YES)A C B(To Sheet 1)Sheet 3 of 6 2Decision Flow Diagram For Availability Of Electric Power Sources 12One offsite & one onsite source not available?(NO)(YES)(NO)(YES)Both sources restored within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />?Conditions satisfied for power operation to continue?Power operation may continue for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />(Max. Total Time)(YES)(NO)One source restored within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />?(Regulatory Position 4 -The Available Offsite and Onsite AC Power Sources Are Each One Less Than the LCO) 2(From Sheet 3)(NO)(YES)A C B(To Sheet 1)(To Sheet 5)Sheet 4 of 6 3Decision Flow Diagram For Availability Of Electric Power Sources 13Two onsite AC sources not available?(YES)(YES)Both sources restored within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />?Power operation may continue for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />(Max. Total Time)(NO)One source restored within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />?(Regulatory Position 5 -The Available Onsite AC Electric Supplies Are Two Less Than the LCO) 3(From Sheet 4)(NO)(YES)A C B(To Sheet 1)Sheet 5 of 6Decision Flow Diagram For Availability Of Electric Power Sources 14Only one DC source not available?(YES)(YES)DC source restored within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />?Power operation may continue for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />(Max. Total Time)(Regulatory Position 6 -The Available Onsite DC Supplies Are One Less Than the LCO) 4(From Sheet 1)(NO)A B(To Sheet 1)Sheet 6 of 6Decision Flow Diagram For Availability Of Electric Power Sources