05000287/LER-2018-002, Actuation of the Keowee Hydroelectric Station Due to Loss of AC Power to the Unit 3 Main Feeder Buses
| ML18199A105 | |
| Person / Time | |
|---|---|
| Site: | Oconee  (DPR-055) |
| Issue date: | 07/05/2018 |
| From: | Dunton C Duke Energy Carolinas |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RA-18-0076 LER 2018-002-00 | |
| Download: ML18199A105 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(8) |
| 2872018002R00 - NRC Website | |
text
RA-18-0076 July 5, 2018 Attn: Document Control Desk U.S. Nuclear Regulatory Commission.
11555 Rockville Pike Rockville, MD 20852-27 46 Duke Energy Carolinas, LLC Oconee Nuclear Station Unit 3 Docket Numbers: 50-287 Renewed Operating Licenses: DPR-55 10 CFR 50.73 Carrie T. Dunton Director, Nuclear Site Support Oconee Nuclear Station Duke Energy ON01VP 17800 Rochester Hwy Seneca, SC 29672 o. 864.873.3477 f: 864.873.4208 carrie.dunton@duke-energy.com
Subject:
Licensee Event Report 287 /2018-002, Revision 00 - Actuation of the Keowee Hydroelectric Station Due to Loss of AC Power to the Unit 3 Main Feeder Buses Licensee Event Report 287/2018-002, Revision 00, is being submitted pursuant to the requirements of 10 CFR 50. 73 to provide notification of the subject event.
There are no regulatory commitments associated with this LER.
If there are questions, or further information is needed, contact Stephen C. Newman, Lead Nuclear Engineer, Regulatory Affairs, at (864) 873-4388.
Sincerely, Carrie T. Dunton Director, Nuclear Site Support Oconee Nuclear Station Enclosure NRG FORM 366 (11-2015)
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I -
cc (w/Enclosure ):
Ms. Catherine Haney, Administrator, Region II U.S. Nuclear Regulatory Commission Marquis One Tower 245 Peachtree Center Ave., NE, Suite 1200 Atlanta, GA 30303-1257 Ms. Audrey L. Klett, Project Manager (by electronic mail only)
U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 0-08B1A Rockville, MD 20852-2738 Mr. Eddy Crowe NRC Senior Resident Inspector Oconee Nuclear Station
NRC FORM366 (04-2018)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03/31/2020 LICENSEE EVENT REPORT (LER)
(See Page 2 for required number of digits/characters for each block)
(See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/readinq-rm/doc-collectionslnuregslstaff/sr1022ir3D
- 1. Facility Name Oconee Nuclear Station Unit 3
- 4. Title
, the NRG may not conduct or sponsor, and a erson is not reauired to resoond to the information collection.
~- Docket Number
. Page 05000287 1 OF4 Actuation of the Keowee Hvdroelectric Station Due to Loss of AC Power to the Unit 3 Main Feeder Buses
- 5. Event Date
- 6. LER Number
- 7. Report Date
- 8. Other Facilities Involved Sequential Rev Facility Name Docket Number Month Day Year Year Month Day Year N/A Number No.
05000 05 10 2018 2018 002 00 07 5
2018 Facility Name Docket Number N/A 05000
- 9. Operating Mode Abstract (Limit to 1400 spaces, i.e., approximately 14 single-spaced typewrttten linies)
On May 10, 2018, at 16:03, with Unit 3 in MODE 6 (refueling) with the unit's startup transformer (CT-3) carrying the unit's power loads, a CT-3 lockout occurred. With the transformer locked out, Unit 3 experienced a loss of all alternating current (AC) power to the unit's main feeder buses (MFBs) which subsequently resulted in an autostart of both Keowee Hydroelectric Station (KHS) Units. Approximately 18 seconds later, emergency AC power was restored to the MFBs via the KHS Unit 2 underground powerpath and CT-4 transformer. During the 38-second power outage, the Unit 3 decay heat removal and spent fuel cooling systems were unavailable; however, the MFB Monitoring Panel responded as designed to restore AC power.
This event was reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as a valid actuation of one of the systems listed in 10 CFR 50.73(a)(2)(iv)(B)(8), i.e., the Keowee Hydroelectric Station.
.The cause of the CT-3 lockout was due to transient disturbances from external voltages/currents being impressed upon the direct current (DC) system due to an internal failure of the 62GZ relay concurrent with multiple DC grounds. A Cause Evaluation is ongoing to determine if planned corrective actions are warranted. The overall plant risk due to the event was insignificant and there was no impact on public health and safety.
NRG FORM 366 (02-2018)
EVALUATION:
Background
YEAR 2018 SEQUENTIAL NUMBER 002 REV NO.
00 During start-ups, shutdowns, and outage periods where the Auxiliary Transformer (3T on Unit 3) [EIIS: XFMR] is not available, power is supplied from the Switchyard through the Start-up Transformer (CT-3 on Unit 3).
Emergency power can be provided to any or all three Oconee Nuclear Station (ONS) Units from the Keowee Hydroelectric Station (KHS)KHS [EIIS: EK]. There are two emergency power paths:
- 1. the overhead path, that includes the Start-up Transformer (CT-1, CT-2, or CT-3) on each ONS unit; and
- 2. the underground path, through CT -4.
In addition, there is an offsite powerpath from either the Central Switchyard or Lee Steam Station via transformer CT-5. Transformers 3T or CT-3 supply power at 6900 Volts M.
DESCRIPTION OF EVENT
On May 10, 2018, at 16:03, with Unit 3 in MODE 6 (refueling) with the unit's startup transformer (CT-3) carrying the unit's power loads, a CT-3 lockout occurred. With the transformer locked out, Unit 3 experienced a loss of all alternating current (AC) power to the unit's main feeder buses (MFBs) [EIIS: BU]. Although not an Engineered Safeguards (ES) [EIIS: JE] generated signal, approximately 20 seconds after the MFB Monitoring Panel sensed a loss of power, it sent a signal to start the Keowee Units. Approximately 18 seconds later, emergency AC pow~r was restored to the MFBs via the KHS [EIIS: EK] Unit 2 underground powerpath and CT -4 transformer. During the 38-second power outage, the Unit 3 decay heat removal [EIIS: BP] and spent fuel cooling [EIIS: DA] systems were unavailable.
Prior to and at the time of the lockout, Oconee personnel were conducting electrical testing following the implementation of a protective relaying upgrade project. This testing was determined not to be the direct cause of the lockout; however, 'the testing did energize the 62GZ relay [EIIS: RL Y]. Evidence indicates that the 62GZ relay had existing internal degradation that occurred at some time prior to the outage. The cause of this prior degradation is unknown.
Reportability
This event was reportable pursuant to 1 O CFR 50. 73( a)(2)(iv)(A) as a valid actuation of one of the systems listed in 10 CFR 50.73(a)(2)(iv)(B)(8), i.e., the Keowee Hydroelectric Station (KHS).
Pursuant to 1 O CFR 50. 72 requirements and the guidance provided in NUREG-1022 (R3), the KHS start signal was a valid actuation.
Additionally, the loss of power to the MFBs resulted in an Emergency Plan declaration of an Unusual Event at 16:25 per Emergency Action Limit-CU2.1, "AC power capability, Table C-3, to essential 4160 V buses MFB-1 and MFB-2 reduced to a sinQle power source for Qreater than or equal to 15 minutes AND Anv additional sinQle power Page 2 of 4 (04-2017)
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- 1. FACILITYNAME
- 2. DOCKETNUMBER YEAR Ocqnee Nuclear Station Unit 3 05000-287 2018
- 3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.
00 source failure will result in loss of all AC power to SAFETY SYSTEMS." Oconee Units 1 and 2 were unaffected by this event.
CAUSAL FACTORS The cause of the CT-3 lockout was due to transient disturbances from external voltages/currents being impressed upon the DC system due to an internal failure of the 62GZ relay concurrent with multiple DC grounds.
CORRECTIVE ACTIONS
Immediate:
- 1. Stopped the relay functional testing and entered the appropriate abnormal operating procedures.
- 2. Restored decay heat removal and spent fuel cooling capability.
- 3. Closed the unit 3 equipment hatch.
Subsequent:
- 1. The damaged 62GZ control relay was replaced.
- 2. DC grounds were isolated and repaired.
Planned:
- 1. A Cause Evaluation is ongoing to determine if planned corrective actions are warranted.
SAFETY ANALYSIS
A qualitative risk evaluation was performed to consider the potential impacts of this event on plant safety.
ONS-3 was in Mode 6 when a CT3 lockout actuation occurred on May 10, 2018, resulting in a loss of AC power to the Unit 3 Main Feeder Susses (MFBs). The MFB Monitor Panel initiated a Keowee start signal.
All required KHS equipment responded as designed and power was restored in approximately 38 seconds.
The temporary loss of AC power resulted in a loss of decay heat removal that was automatically restored from KHS Unit 2. The Oconee Unit 3 Spent Fuel Pool (SFP) cooling flow was also lost but was manually restored by procedure in approximately 6 minutes.
As documented on the May 10, 2018, Defense-in-Depth Status Sheet, the*"time to core boiling" was approximately 66 minutes based on the expected heat load for the number of days after shutdown. This time available supports that operators could have reliably performed necessary recovery actions if automatic restoration of power or LPI flow had failed. Similarly, the "time to boil" for the SFP was approximately 28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br />. Page 3 of 4 (04-2017) l/,S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-\\)104 EXPIRES: 3/31/2020
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YEAR Oconee Nucl,ear Station Unit 3 05000-287 2018
- 3. LER NUMBER SEQUENTIAL NUMBER 002 REV NO.
00 Procedural actions available to recover from a potential failure of KHS Unit 2 include alignment of backup power from Transformer CT-5, swapping the KHS Unit 1 alignment from overhead path to underground path, alignment of 4160V cross-tie from Unit 2 Startup Transformer CT-2, and alignment of gravity flow from the Borated Water Storage Tank to the core. Although the containment equipment hatch was open at the time of the lockout, containment closure was reestablished in approximately 22 minutes providing significant margin prior to core boiling.
Based the reliability of the Keowee power system, diverse power sources, and redundancy of cooling systems, the impact of this event on plant risk was very low when available recovery actions are considered. Thus, it is concluded that the impact of this event on overall plant risk is insignificant and had no impact on public health and safety.
ADDITIONAL INFORMATION
A review of Duke Energy's Corrective Action Program and industry Operating Experience (OE) databases was conducted using applicable keyword searches, i.e., "CT-3, KHS, KHU, lockout, actuation," identified one (1) similar Oconee LER that occurred in 2006. LER 287/2006-01, Rev. 0, reported an event involving a KHS actuation because of a CT-3 lockout on Unit 3. In that instance, it was concluded that the differential relays which precipitated the lockout were very sensitive to mechanical impact/jarring. The corrective actions associated with that LER would not have prevented the current lockout event.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX]. This event is considered INPO Consolidated Events System (ICES) Reportable. There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event. Page 4 of 4