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ACCESSION NBR:9707280243 DOC.DATE: 97/07/21 NOTARIZED: NO DOCKET # FACIL 50-269 Oconee Nuclear Station, Unit 1, Duke Power Co. 05000269 AUTH.NAME AUTHOR AFFILIATION

BOND,R.T. Duke Power Co. MCCOLLUM,W.R. Duke Power Co. RECIP.NAME RECIPIENT AFFILIATION

SUBJECT: LER 97-006-00:on 970620,loss of dedicated line from Lee resulted in loss of power to Unit 1 was noted.Caused by C inappropriate personnel action.Oconee Unit 1 essential loads were restored following loss of Lee.W/970721 ltr. A

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Duke Power Company k A Duke Energy Company PrOWef Oconee Nuclear Site

A DkeE-V -pa"P.O. Box 1439 Seneca, SC 29679

W R. McCollum, Jr. (864) 885-3107 OFFICE Vce President (864) 885-3564 FAx

July 21, 1997

U.S. Nuclear Regulatory Commission Document Control Desk

Washington, D.C. 20555

Subject: Oconee Nuclear Station Docket Nos. 50-269, -270, -287

Voluntary Licensee Event Report 269/97-06 Problem Investigation Process No.: 6-097-1906

Gentlemen:

This Voluntary Licensee Event Report (LER) 269/97-06, concerning the loss of Lee, Failure of Keowee 1, and loss of Keowee 1 auxiliaries, is provided for your information.

This condition is considered to be of no significance with respect to the health and safety of the public.

Very truly yours,

R. McCollum, J

Attachment

9707280243 970721 PDR ADOCK 05000269

a 4 A 1 A *

Document Control Desk July 21, 1997 Page 2

cc: Mr. Luis A. Reyes Administrator, Region II U.S. Nuclear Regulatory Commission 61 Forsyth Street, S. W., Suite 23T85 Atlanta, GA 30303

Mr. D. E. LaBarge U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Washington, D.C. 20555

INPO Records Center 700 Galleria Parkway, NW Atlanta, GA 30339-5957

Mr. M. A. Scott NRC Resident Inspector Oconee Nuclear Station

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FACILITY NAME (1) DOCKET NUMBER (2) PAGE (3) Oconee Nuclear Station, Unit One 05000 269 1 of 18

TITLE(4) Voluntary LER on Problems During Electrical Tests: Loss of LEE, Failure of Keowee 1, and Loss of Keowee 1 Auxiliaries

EVENT DATE (5) LER NUMBER (6) REPORT DATE (7) OTHER FACILITIES INVOLVED (8) MONTH DAY YEAR YEAR SEQUENTIAL REVISION MONTH DAY YEAR FACILITY NAME DOCKET NUMBER(S) NUMBER NUMBER

Oconee, Unit Two 05000 270 06 20 97 97 - 06 - 00 07 21 97 Oconee, Unit Three 1 _____ ____ ___ __ ____________ J05000 287

OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR (Check one or more of the following) (11) MODE (9) N 20.402(b) 20.405(c) 50.73(a)(2)(iv) 73.71(b)

POWER 20.405(a)(1)(i) 50.36(c)(1) 50.73(a)(2)(v) 7371(c) LEVEL (10) 00 20.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(vii) X OTHER (Specifyin

20.405(a)(1)(iii) 50.73(a)(2)(i) 50.73(a)(2)(viii)(A) Abstract below and 20.405(a)(1)(iv) 50.73(a)(2)(ii) 50.73(a)(2)(viii)(B) in Text, NRC Form 20.405(a)(1)(v) 50.73(a)(2)(iii) 50.73(a)(2)(x) 366A)

LICENSEE CONTACT FOR THIS LER (12) NAME TELEPHONE NUMBER

AREA CODE

R. T. Bond, Safety Review Manager

COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE TO NPRDS TO NPRDS

SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MONTH DAY YEAR SUBMISSION

YES (f yes, complete EXPECTED SUBMISSION DATE) X NO DATE (15) ABSTRACT (Limit to 1400 spaces, i.e. approximately fifteen single-space typewritten lines) (16)

On June 20, 1997, Oconee Unit 1 was at cold shutdown, Unit 2 was at 100% Full Power, and Unit 3 was at 13% Full Power. As part of a required test., the Lee Steam Station Switchyard was supplying power to Oconee Unit 1. At 1244 hours, a loss of the dedicated line from Lee resulted in a loss of power to Unit 1. Unit 1 automatically swapped to the Oconee switchyard and power was restored. Both Keowee Hydro (KH) Units (emergency generators) received a start signal, but KH-1 failed to energize the underground path. On June 23, 1997, the Lee test was repeated successfully. However, KH-1 lost auxiliary power during realignment. The loss of power from Lee resulted from inappropriate action. KH-1 failed to energize the underground path apparently due to failure of the field flash breaker to latch. The loss of auxiliary power was due to equipment failure and a deficient procedure. Corrective actions have addressed all these causes. This event did nob impact the health and safety of the public. This is a voluntary report.

NRC FORM 366

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FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) YEAR SEQUENTIAL REVISION

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Oconee Nuclear Station, Unit One 269 97 .06 00 2 OF 18

Background

At Oconee Nuclear Station, Keowee Hydro (KH) Station [EIIS:EK] serves the emergency power function typically performed by diesel generators at other nuclear stations. KH consists of two hydroelectric turbine/generator units and associated support equipment and auxiliaries. Each KH unit is provided with its own automatic start equipment. Both units undergo a simultaneous automatic emergency start and run in standby on a loss of the grid, an Engineered Safeguards (ES) actuation on any of the three Oconee Units, or an extended loss of voltage on any Oconee unit's Main Feeder Buses. On an emergency automatic start, one unit connected to the underground path supplies the Stand-by Bus while the other unit, remaining in standby, is available to supply the overhead path.

When Keowee is unavailable, a 100 KV transmission line can be dedicated to provide power from one of the Lee Steam Station combustion turbines (CT). (See Attachment 1.)

Each Oconee unit is provided with several sources of normal and backup electrical power as described below.

The Normal Source is powered from the Oconee unit's generator via an Auxiliary Transformer through the Normal ("N") Breakers.

The Start-Up Source is powered via a Start-up Transformer from the 230 KV Switchyard (SWYD) or from the "overhead" emergency power path from one KH unit, using the SWYD Yellow bus. The Start-up ("E") Breakers connect it to the unit's two Main Feeder Buses.

The Stand-By Source (Stand-by Bus) is powered from KH via an underground feeder and transformer CT4 through the "SK" Breakers or from Lee Steam Station or the Central Switchyard via transformer CT5 through the "SL" Breakers. The Stand-by ("S") Breakers connect the Stand-by Bus to the unit's two Main Feeder Buses.

Each Oconee unit's power sources are monitored by the Emergency Power Switching Logic (EPSL) and the Main Feeder Bus Monitor Panels (MFBMP). These systems will automatically initiate transfer between the Start-up and Stand-by sources as necessary to supply power. The selection logic and timing vary depending on the incident scenario.

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Oconee Nuclear Station, Unit One 269 97 06 00 3 OF 18

Technical Specification (TS) 3.7 requires both KH units and both power paths from KH to be operable. One path may be removed from service for 72 hours if the other path is verified to be operable within one hour and every eight hours thereafter. Both KH units may be inoperable for up to 72 hours for planned reasons if the standby buses are first energized from CT-5 transformer using the dedicated line from the Lee CT's. This last limiting condition for operation is reduced to 24 hours if both KH units are inoperable for unplanned reasons and the Standby Bus is energized from a dedicated Lee CT within 1 hour.

Lee Steam Station operations group is staffed with 5 operators and 1 Production Team Leader per shift. There is one control room for operation of three coal fired units. Lee also has three combustion turbines which are operated from local control panels.

Event Description

On June 20, 1997, Unit 1 was at cold shutdown, Unit 2 was at 100% Full Power, and Unit 3 was at 13% Full Power, with its turbine/generator off line.

Oconee Nuclear Station (ONS) personnel were performing PT/l/A/0610/06 "100KV Power Supply from Lee Steam Station". Because it has typically been performed during refueling conditions, this performance of the test was classified as an "infrequently performed test or evolution" which required additional procedural provisions -such as enhanced management oversight, contingency plans, etc. Lee Steam Station personnel were supporting this test by performing a Lee operating procedure titled "Emergency Power or Back-up Power to Oconee.".

ONS had been aligned with the Lee Switchyard supplying power to the Unit 1 Main Feeder Busses via the SL and S breakers. Per step 12.8.2, substep 4, ONS had notified Lee that Central Switchyard was aligned for Oconee to receive power from the operating combustion turbine (CT). This isolated the 100 KV Lee line from the Central Switchyard.

At Lee, all three coal fired units were in operation during thiss test. Of the three combustion turbines, only 6C, the CT involved in this test, was in operation.


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The Lee Process Team Leader (PTL) (equivalent to Operations Shift Manager) and a Technical Specialist (from the station support staff) were performing steps at the local control panel for the CT. Lee Operator A (LOA)(a Control Operator) and Lee Operator B (LOB) (an Assistant Control Operator) were performing steps in the Lee control room. LOB had little experience with the Lee procedure to support Oconee, and was participating under LOA's direction as part of an informal training activity.

The other operators on this shift were performing duties outside the common control room during the test. Therefore, LOA and LOB were responsible for responding to any alarms received on any of the coal fired units, performing any required control room actions to support the test, and coordinating communications between Oconee, the personnel at the CT local control panel, and the other three operators for their tasks. Interviews subsequent to the event revealed that, while participating in this test, LOA and LOB had responded to 56 alarms on the coal units (mostly nuisance alarms on Unit 3) and 8 to 10 phone calls (several of which were related to the test).

The Lee operators had started the 6C CT and had paralleled to the grid per Enclosure 6.1 of the Lee Operating Procedure. LOA and LOB were intending to perform steps 6.1.5 and 6.1.6, which close the 89-3 switcher (connecting the CT to the line to Oconee) then open the 89-2 switcher (disconnecting the CT from the rest of the Lee switchyard).

LOA stated that he received the phone call from Oconee stating that the lineup at Central was complete. He silently read the procedure steps, extracted the actual actions to be performed, then verbally directed LOB to perform the necessary actions. At approximately 1244 hours, LOB opened 89-2 then closed 89-3 within approximately 3 seconds (opposite from the sequence required by the procedure).

This caused the CT to be separated from the grid, then reconnected. This resulted in a 50G instantaneous ground protective relay tripping open the OCB-13 Central White breaker, disconnecting the Oconee-Lee dedicated line from the rest of the grid. The breaker operations isolated the CT, and it continued to run with no load.




Y NUMBER NUMBER Oconee Nuclear Station, Unit One 269 97 06 00 5 OF 18

At Oconee, the resulting undervoltage tripped the SL breakers isolating the Unit 1 Main Feeder Busses (MFB), and initiated timing for the MFB Monitor Panel channels. After approximately 20 seconds, the Monitor Panels actuated a Keowee Emergency Start signal and, following an additional one second time delay, a Load Shed.

Upon receipt of the Load Shed signal, Emergency Power Switching Logic (EPSL) initiated the Retransfer to Start-up timers since the S breakers were closed with the Standby Bus deenergized. After a five second time delay, the S breakers opened and the E breakers closed, powering the MFBs from the Oconee switchyard.

Keowee Unit 2 (KH-2) was aligned to the overhead path. It successfully started, but did not need to close into the switchyard Yellow Bus since there was no associated Loss of Switchyard event.

Keowee Unit 1 (KH-1) was aligned to the underground path, but failed to energize CT-4. A Failure Investigation Process (FIP) team was initiated to investigate this failure.

In accordance with the contingency plan included in PT/1/A/0610/06, the Oconee operators verified that power was restored and began realigning lost loads as necessary, using the Loss Of Power Abnormal Procedure.

The Operators recognized that the failure of KH-1 to energize the underground power path made that path inoperable. This placed Oconee Units 2 and 3 into a LCO per TS 3.7.2(a)l which required that the available Keowee power path be tested within one hour. However, the test procedure was not performed within one hour because KH-2 was currently operating in stand-by with no load. Rather than shut it down to perform a test, it was considered more conservative to keep KH-2 operating until more was known about the loss of power and the failure mode of KH-1. The Operators entered a more restrictive LCO per TS 3.7.3 which required that Units 2 and 3 be at hot shutdown within 12 hours. The overhead path was tested at 1847 hours, which allowed the Oconee units to exit the 12 hour LCO.

Due to the importance of Keowee as Oconee's emergency power source, the Emergency Notification System (ENS) was used at 1605 hours to notify the NRC of the failure of KH-1 to emergency start.

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Oconee Nuclear Station, Unit One 269 97 E0 00 6 OF 18

In the event of a LOCA, the Engineered Safeguards system will initiate emergency start signals to both Keowee Units and enable immediate load shed in the event of a concurrent Loss of Offsite Power. However, the Main Feeder Bus Monitor Panel provides an alternate, non-Engineered Safeguards, system to provide initiation signals for emergency start and load shed after an appropriate time delay. Therefore, in accordance with guidance given in Nureg 1022, this event was not considered reportable as an Engineered Safety Feature (ESF) actuation because it was not initiated by an ESF signal.

The loss of power to the Unit 1 MFBs existed for approximately 26 seconds. The Oconee Emergency Plan specifies that power must be lost for more that 15 minutes before an Unusual Event is declared. Therefore, the event is not reportable as an Unusual Event.

The Keowee failure was not considered reportable because it was a single component and affected only one power path.

Therefore, the ENS report was considered a voluntary report.

The FIP investigation into the failure of KH-1 to successfully start began with troubleshooting of the field flash breaker and its control circuit. The negative leg control power fuse for the KH-1 field flash breaker control circuit was found blown. The FIP team investigated further to determine the cause of the blown fuse.

The FIP team initially suspected fuse degradation as the cause due to the age and class of fuse. However, subsequent evaluation indicated that the fuse had blown.on overload. Fuse Time versus Current curve corrections for a higher ambient temperature and allowance for curve tolerances indicate that the fuse might blow in as little as 5 seconds. However, evaluation of the breaker closing times indicate the total time spent at peak current was less than five seconds unless the controls failed to interrupt current to the close coil. Therefore, the FIP concluded that it was improbable that the fuse blew due to degradation alone.

In parallel with the fuse evaluation, the field flash breaker was tested .electrically and mechanically, including Keowee manual and automatic starts and one emergency start.




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During the emergency start test it was observed that the field flash breaker cycled during the emergency start actuation. Although not an optimal design, the cycling of the field flash breaker during emergency starts did not specifically prevent the Keowee units from performing successful emergency starts as evident by tests prior to and after the event.

The FIP concluded that the breaker close coil could remain energized if the breaker failed to latch due to reaching a "trip free" condition. "Trip free" refers to a condition of the breaker brought about by attempting to close the breaker with a trip signal present. Trip latch bounce can also cause the breaker to fail in the trip free condition. Trip latch bounce is usually random.

The FIP team concluded that a trip free condition probably occurred when the breaker opened the last time and.caused the breaker close coil to remain energized until the fuse blew. The FIP team final report is still under review. The final report will contain additional technical detail to support the FIP conclusions.

On June 23, 1997, PT/l/A/610/06 was performed again since it had not been successfully completed on June 20. During this test, Lee successfully supplied power and met the procedure acceptance criteria.

However, a problem occurred during the post-test electrical system restoration. Oconee Unit 1 switchgear 1TC was deenergized during a planned dead bus transfer as part of the electrical system realignment. One of the loads fed by ITC is an underground feeder which was supplying KH-1 600 V auxiliary power loadcenter 1X via transformer CX and ACB-7. Following the dead bus transfer, ACB-7 was expected to close automatically to restore power to lX, but it did not and a lockout of 1X loadcenter occurred. The Keowee operator noted that no protective relay targets had been received. This indicated that no faults existed on lX. In accordance with the -applicable alarm response guide, the operator reset the lockout relay. Once the lockout relay was reset, ACB-5 and ACB-7 both attempted to close simultaneously. As they attempted to close, they were tripped by interlocks. Subsequently, control fuses for both breakers were found blown. KH-1 was declared inoperable at 1750 hours.

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Another FIP was initiated to investigate this failure. This FIP found that, when power was interrupted to 1TC, ACB-7 tripped open, as expected. When power was restored to 1TC and CX, ACB-7 attempted to reclose. One feature of the ACB control circuits is a "Y" relay. This is an anti-pump relay which is supposed to allow sufficient time for the breaker to fully close and latch before interrupting the circuit. In this event, the "Y" relay operated too soon because its time delay timer failed. This interrupted control power to ACB-7's close coil, causing the breaker to stop prior to full closure, and to trip open. Under the existing conditions, a mechanical linkage initiated a lockout, which tripped ACB-7 and prohibited ACB-5 and/or ACB-7 from closing.

Once the lockout occurred, a 30 second timer timed out allowing automatic transfer logic to provide a close permissive to ACB-5. ACB-7 still had a close permissive due to being previously closed and in automatic. Therefore, when the lockout was reset, both ACB-5 and ACB-7 had close signals present. As the breakers began to close, the interlocks between ACB-5 and ACB-7 actuated and tripped both ACBs. However, the ACBs tripped before the "Y" relay in each ACB could energize and drop out the "X" relay and closing coil. This is a trip free condition as described above. This left the close coil circuits energized and the 10 amp close coil fuses (Gould OT10) opened as designed.

Based on these findings, the "Y" timer was replaced on ACB-7. Additionally, a spare breaker with a new "Y" timer was.installed on ACB-5. The "Y" timers on ACB-6 and ACB-8 (on KH-2) were verified operable. The close coil circuit fuses in ACB-5 and ACB-7 were replaced. The alarm response manual was revised to require placing the automatic transfer logic in manual prior to resetting the lockout relay. This will preclude both auxiliary power breakers from having simultaneous close signals as occurred in this event.

The failed "Y" timer was examined, and it was determined that a time delay adjustment potentiometer had a short, which resulted in a very low time

i delay which allowed the timer to deenergize the close coil prior to the '. 7breaker fully closing.

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FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) YEAR SEQUENTIAL ' REVISION

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Conclusions

Loss of Power from Lee:

The root cause of the loss of power from Lee was determined to be inappropriate personnel action due to failure to follow the procedure correctly.

Lee Operator A (LOA) recognized that the event had occurred and realized that his thumb (which he was using as a placekeeping aid) had moved on the procedure page, allowing him to get on the wrong step. As a result, he had misread the switcher sequence and misdirected LOB. LOB performed as directed.

There were several factors contributing to this inappropriate action.

1. It is apparent that there were a high number of job distractions during the test. LOA and LOB did not seek redirection of the available resources. For example, they could have called (or requested that supervision call) one of the other operators to take over monitoring of the coal fired units or they could have decided that it was just "too busy" to allow LOB to participate, and freed him to monitor the other units.

2. Based on interviews after the event, LOA and LOB did not appear to use human performance aids (such as STAR, touch the label, repeat back, independent verification, etc.) which have become expected in the nuclear culture.

3. Although the procedure steps provided adequate direction, the overall procedure format did not include Human Performance enhancements typically used in the nuclear culture.

Keowee Unit 1 Failure (June 20):

. The FIP team concluded the fuse probably experienced an overload condition due to the breaker being in a trip free condition with a close signal applied.




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Keowee Unit 1 Auxiliary Power Failure (June 23)

There were two root causes associated with this event.

1. Equipment failure of the "Y" relay timer adjustment potentiometer in the ACB-7 control circuit. This is considered the random failure of an electrical component. There is no history of similar failures to indicate that this is a recurring problem.

2. Deficient Procedure: The Lockout alarm response manual did not provide adequate direction for the condition where both breakers would have close signals when the lockout was reset.

Inappropriate action, specifically procedure adherence, is an identified problem at Oconee. However, prior events affecting Oconee have not involved Lee Steam Station personnel.

There have been several events involving unanticipated system interactions. However, none have involved Keowee within the last two years.

There have been no other events involving deficient alarm response manuals within the last two years.

Therefore, none of these events are considered recurring.

None of these events involved personnel injury, radioactive releases or exposures.

Corrective Actions

Immediate

1. Oconee Unit 1 essential loads were restored following the loss of Lee on June 20, 1997.'

f2. The test procedure was exited and Oconee Unit 1 was returned to its pretest alignment.




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Subsequent

1.The Keowee Unit 1 blown field flash breaker control fuse was replaced. Keowee Unit 1 was restored to operable status after appropriate troubleshooting and testing.

2.Management implemented an interim policy to replace the field flash breaker control fuses after each Keowee emergency start. Subsequently, a design modification was implemented to eliminate the unnecessary cycling of field flash breakers during Keowee emergency starts.

3. The failed "Y" timer in ACB-7 and the blown fuses in ACB-5 and ACB-7 were replaced. Also, a spare breaker with a new "Y" timer was installed in ACB5 and the "Y" timers in ACB-6 and ACB-8 were verified operable.

4. The alarm response guide for this lockout was revised to require placing the automatic transfer logic in manual prior to resetting the lockout relay. This will preclude both auxiliary breakers from having a close signal at the same time.

Planned

1. Oconee Site will provide assistance to Lee Steam Station as needed to: A) implement human factors improvements affecting procedures related to backup power to Oconee in the following areas: * procedure formatting * error reduction techniques (e.g., STAR) * reduction/avoidance of distractions

B) evaluate the Oconee/Lee organizational interface to maintain an appropriate level of awareness and to apply Oconee/nuclear human performance programs/enhancements at Lee to the extent appropriate to assure Lee's function as a back-up power supply to Oconee.

2. Evaluate alternative procedure sequencing which could reduce the impact of upsets at Lee upon Oconee.

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Oconee Nuclear Station, Unit One 269 . 97 06 00 12 OF 18

3. Review other Safety Related breaker lockout functions and associated alarm response guides, both at Keowee and Oconee, for the potential for similar adverse reactions when resetting lockouts. Revise the alarm response guides as necessary following this review.

Planned corrective actions 1,2, and 3 are considered NRC commitment items. No other commitments are included in this report.

Safety Evaluation

Loss Of Power at Lee:

The event occurred because power was interrupted from the Lee 100 KV line while it was supplying power to Oconee Unit 1. While this loss was not expected, it was anticipated in the test procedure contingency plan. Expected alternate power sources included Keowee Hydro Unit 1 (KH-1) via the underground path, the Start-up Source via the Oconee Switchyard, or KH2 via the overhead path. Although KH-1 failed, KH-2 could have been realigned to the underground path if necessary. Also, Lee or Central could have been restored as sources.

The event caused a momentary interruption of AC power to Oconee Unit 1. At the time of the event, the unit was at cold shutdown with the Reactor Coolant System (RCS) filled, approximately 39 psig of nitrogen in the Pressurizer, and 100F. The unit had been shutdown for 6 days and had 7 to 8 MW of decay heat.

Normal decay heat cooling was in service. By procedure, Unit 2 was supplying power to the operating Low Pressure Service Water Pump so that it would not be affected by dead bus transfers expected later in the test procedure. The Low Pressure Injection pump in service was non-load shed, but did require manual action to restart. This was accomplished with in 1 to 2 minutes. The temperature of the RCS was observed to increase from 102.5 to 103.0 F.

Under the existing conditions, the time to boil following loss of decay heat removal was approximately 1 hour, and time to core uncovery was greater than 8 hours, assuming no heat transfer from the RCS. The Steam Generators, by procedure requirement, were partially filled such that


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Oconee Nuclear Station, Unit One 269 97 .06 00 13 OF 18

inventory boil-off would provide additional margin to core uncovery. In addition, emergency feedwater was available for alignment from either of the other two Oconee units, and auxiliary service water was available from the Oconee Stand-by Shutdown Facility.

A risk calculation was performed for this event and the estimated core damage probability for this event was approximately 3E-07.

Keowee Unit 1 Inoperability:

The impact of the loss of Lee event on Oconee Units 2 and 3 was minor. There was no effect on actual operation, but both units were placed in a 12 hour Limiting Condition for Operation (LCO) until KH-2 was tested to the overhead path.

The failure to start of the Keowee unit connected to the underground during a LOCA or LOCA/LOOP is included in the Oconee licensing basis as a single failure. The Keowee Emergency Start signal will be initiated by either Engineered Safeguards or Loss of Switchyard, whichever actually occurs first. The Keowee unit on the overhead path will start and energize the overhead path. As occurred in this event, the individual Oconee Unit's Emergency Power Switching Logic (EPSL) will sense voltage on the overhead path, and will transfer to that source. In the event of a LOCA/LOOP, operating LOOP units are expected to close into the start-up source prior to Main Feeder Bus time out. The LOCA unit breakers will close approximately five seconds after the path is energized. (See Attachment 2 Keowee EPSL Sequence of Events.)

In conclusion, in the actual event, power and decay heat removal were restored promptly. Even in the postulated event that a LOCA or LOCA/LOOP had occurred on operating units in a similar scenario, sufficient redundancy existed to assure that emergency power would have been provided within UFSAR assumed time limits. Therefore, the health and safety of the public were not affected by this event.

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NUMBER NUMBER Oconee Nuclear Station, Unit One 269 97 06 00 L4 OF 18

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FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)

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