05000313/LER-2021-001, Reactor Manual Trip Due to Temporary Loss of Non-Vital 4160 Volt Bus
| ML21133A020 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 05/13/2021 |
| From: | Keele R Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 1CAN052101 LER 2021-001-00 | |
| Download: ML21133A020 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3132021001R00 - NRC Website | |
text
1CAN052101 May 13, 2021 ATTN: Document Control Desk U. S. Nuclear Regulatory Commission Washington, DC 20555-0001
Subject:
Licensee Event Report 50-313/2021-001-00, Reactor Manual Trip due to Temporary Loss of Non-Vital 4160 Volt Bus Arkansas Nuclear One - Unit 1 NRC Docket No. 50-313 Renewed Facility Operating License No. DPR-51 Pursuant to the reporting requirements of 10 CFR 50.73, attached is the subject Licensee Event Report concerning the Reactor Manual Trip due to Temporary Loss of Non-Vital 4160 Volt Bus for Arkansas Nuclear One, Unit 1, which occurred on March 14, 2021.
There are no new commitments contained in this submittal. Should you have any questions concerning this issue, please contact me at 479-858-7826.
Respectfully, ORIGINAL SIGNED BY RILEY D. KEELE, JR.
Riley D. Keele, Jr.
RDK/dkb Attachment: Licensee Event Report 50-313/2021-001-00 Entergy Operations, Inc.
1448 S.R. 333 Russellville, AR 72802 Tel 479-858-7826 Riley D. Keele, Jr.
Manager - Regulatory Assurance Arkansas Nuclear One
-~ Entergy
1CAN052101 Page 2 of 2 cc:
NRC Region IV Regional Administrator NRC Senior Resident Inspector - Arkansas Nuclear One NRC Project Manager - Arkansas Nuclear One Designated State Official Arkansas
to 1CAN052101 Licensee Event Report 50-313/2021-001-00
Abstract
On March 14, 2021 at 1315 Central Daylight Time (CDT), Arkansas Nuclear One, Unit 1 (ANO-1) was manually tripped due to an indicated degraded voltage and momentary loss of the non-vital 4160VAC bus A-2. Degraded voltage of the non-vital 4160VAC bus A-2 resulted in de-energizing the vital 4160VAC bus A-4. All systems responded as designed and the plant was stabilized in MODE 3 (Hot Standby).
The cause of this event was a bent primary moveable contact in the 4160VAC Non-Vital bus A-2 Potential Transformer (PT) drawer. The damage to the primary moveable contact was corrected and repaired.
The most likely cause was the ANO-1 Stator Drop accident in 2013 and subsequent repair of the 4160VAC Non-Vital bus A-2.
There were no consequences to the general safety of the public, nuclear safety, industrial safety or radiological safety. No radiological releases occurred due to this event.
PLANT STATUS Arkansas Nuclear One, Unit 1 (ANO-1) was operating at 100 percent rated thermal power (RTP). There were no other structures, systems, or components (SSCs) that were inoperable at the time that contributed to the event.
EVENT DESCRIPTION
On March 14, 2021 at 1315 Central Daylight Time (CDT), ANO-1 was manually tripped due to an indicated degraded voltage and momentary loss of the non-vital 4160VAC bus A-2. Degraded voltage on the non-vital 4160 VAC bus A-2 resulted in de-energizing the vital 4160VAC bus A-4 [EA]. The Emergency Diesel Generator Number 2 (EDG2) [EK]
started automatically and provided power to the vital 4160 VAC A-4. All other vital and non-vital buses transferred power automatically to the Startup Transformer Number 1 (SU-1). All control rods fully inserted. The plant was stabilized in MODE 3 (Hot Standby). No radiological releases occurred due to this event.
This report was reported under 10 CFR 50.72(b)(2)(iv)(B) for the Reactor Protection System actuation and 10 CFR 50.72 (b )(3)(iv)(A) for the actuation of the Emergency Diesel Generator (EN 55138).
This report is made pursuant to 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of the Reactor Protection System and for the actuation of the Emergency Diesel Generator.
SAFETY ASSESSMENT
The manual reactor trip due to an indicated degraded voltage and momentary loss of the non-vital 4160VAC bus A-2 did not result in actual consequences to safety of the general public, nuclear safety, industrial safety or radiological safety.
The safety significance of this event is determined to be low.
EVENT CAUSE($)
Troubleshooting identified high resistance on the primary disconnect contact for "A" phase on the A-2 bus Potential Transformer (PT) drawer.
Investigation into the cause of the high contact resistance revealed a bent primary moveable contact. The "A" phase primary moveable disconnect tab was found to be bent (approximately 1/4 inch) which prevented proper contact engagement from the moveable-to-stationary surfaces after fully inserting and locking the drawer into the cabinet.
The cause of this event determined that this issue was the result of the ANO-1 Stator Drop accident and subsequent repair of the 4160VAC Non Vital bus A-2 in 2013. At that time the culture at ANO lacked proper vendor oversight to ensure overall satisfactory quality of work performed by contract personnel. This is not indicative of current practice at ANO, as this condition was identified and corrected during NRC Column IV recovery.
CORRECTIVE ACTIONS
Following the manual trip, ANO-1 remained in MODE 3 until the damage to the primary moveable contact was corrected and repaired.
05000- I 0313 I
YEAR 8-1 SEQUENTIAL NUMBER 001 REV NO.
1-0 This event was entered into the station's corrective action program. Additional corrective actions included industry benchmarking to determine the best practice for post maintenance testing (PMT) of the PT drawer. In addition, a review of the technicians required knowledge to conduct PMT to achieve consistent results was conducted. Extent of condition (EOC) inspections were performed on Unit 1 during the spring 2021 refueling outage. EOC inspections will be conducted on Unit 2 during the fall 2021 refueling outage.
PREVIOUS SIMILAR EVENTS
ANO-1 LER 2018-004, "Reactor Trip Due to the Loss of Non-Vital 4160 Volt Bus" was identified. This reactor trip was due to performing online Baker Testing on the ANO-1 Main Chiller motor, not due to equipment failure. Due to the lessons learned from this event, ANO-1 adopted the operating philosophy of inserting a manual reactor trip for the loss of A-1 or A-2 4160VAC Non-vital bus. Page 3
of 3