05000313/LER-1923-001, Automatic Reactor Trip Due to Reactor Protection System Actuation

Automatic Reactor Trip Due to Reactor Protection System Actuation

ML23104A162
Person / Time
Site:	Arkansas Nuclear
Issue date:	04/14/2023
From:	Keele R Entergy Operations
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
1CAN042301 LER 23-001-00
Download: ML23104A162 (1)

LER-1923-001, Automatic Reactor Trip Due to Reactor Protection System Actuation

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3131923001R00 - NRC Website

text

S) entergy Riley D. Keele, Jr.

Manager, Regulato ry A ssuran c e Ark a nsas Nu c lea r One Tel 4 79-85 8-7826

1CAN042301 10 CFR 50.73

April 14, 2023

ATTN: Document Control Desk U. S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject: Licensee Event Report 50-313/2023-001-00, Automatic Reactor Trip due to Reactor Protection System Actuation

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 Automatic Reactor Trip due to Reactor Protection System Actuation for Arkansas Nuclear One, Unit 1, which occurred on February 14, 2023.

There are no new commitments contained in this submittal. Should you have any questions concerning this issue, please contact me at 479-858-7826.

Sincerely,

Dig itally signed by Ri ley Keele Riley ON: cn=Riley Kee le, c=US,

ema il=rkeele@entergy.com Reason : I am approv ing this Keele documen t Date : 2023.04.14 07:35:06 -05'00'

RDK/bka

Attachment: Licensee Event Report 50-313/2023-001-00

Entergy Operations, Inc. 1448 SR 333, Russellville, AR 72802 1CAN042301 Page 2 of 2

cc: NRC Region IV Regional Administrator NRC Senior Resident Inspector - Arkansas Nuclear One NRC Project Manager - Arkansas Nuclear One Designated Arkansas State Official Attachment to 1CAN042301

Licensee Event Report 50-313/2023-001-00

Abstract

On February 14, 2023 at 1103 Central Standard Time (CST), Arkansas Nuclear One, Unit 1 (ANO-1) automatically tripped on Reactor Protection System (RPS) actuation due to a sensed loss of the 'B' and 'D' Reactor Coolant Pumps (RCPs).

The cause of this event was a blown secondary 'C' phase Potential Transformer (PT) fuse on a 6900 volt AC bus which caused the underpower relays for RCPs 'B' and 'D' to be de-energized and RPS tripped the reactor.

Prior to the plant trip, the RCP 'D' underpower relay had been replaced. Following the reactor trip, the RCP 'D' underpower relay was replaced again with a known good relay and the blown secondary PT fuse was replaced. Both were tested satisfactorily prior to plant startup on February 16, 2023.

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 85.8 percent rated thermal power (RTP) due to maintenance being performed to replace a Reactor Coolant Pump (RCP) underpower relay that had failed testing.

EVENT DESCRIPTION

On February 14, 2023 at 1103 Central Standard Time (CST), ANO-1 automatically tripped on Reactor Protection System (RPS) (JC) actuation due to a sensed loss of the 'B' and 'D' RCPs. During maintenance activities associated with the 'D' RCP underpower relay replacement, a "power to pumps" trip signal was generated by RPS due to an indication of a 6900 volt AC bus undervoltage condition associated with the bus supplying the 'B' and 'D' RCPs. The indication of a 6900 volt AC bus undervoltage was due to a blown secondary 'C' phase Potential Transformer (PT) fuse. The associated Reactor Trip Emergency Operating Procedure (EOP) was entered, all control rods fully inserted and the plant was stabilized in MODE 3 (Hot Standby). No radiological releases occurred due to this event.

This event was reported under 10 CFR 50. 72(b )(2)(iv)(B) for the RPS actuation and 10 CFR 50. 72(b )(3)(iv)(A) for the Specified System Actuation.

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 RPS.

SAFETY ASSESSMENT

RPS tripped the reactor as designed on a "power to pumps" trip signal when the secondary 'C' phase PT fuse was blown.

Following the reactor trip, the Main Feedwater system (MFW) (SJ) responded properly and supplied feedwater to both Steam Generators (SG) (AB). This allowed for the MFW system to provide normal post trip heat removal via the SGs and 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(S)

On February 8, 2023 at 0846 CST, the ANO-1 RCP 'D' underpower relay failed testing. A temporary modification was implemented on February 9, 2023 to remove the 125 volt DC source from the time delay relays feeding all RPS channels for the 'D' RCP. This temporary modification generated a signal to all four channels of RPS showing the 'D' RCP as being tripped which was a conservative configuration, but it also put AN0-1 in a conditional single point vulnerability until the failed underpower relay could be replaced and the temporary modification removed.

On February 14, 2023 at approximately 1006 CST, ANO-1 commenced a plant downpower to approximately 85 percent RTP to replace the 'D' RCP underpower relay. At 1103 CST, after the failed 'D' RCP underpower relay had been replaced, ANO-1 automatically tripped on RPS actuation due to a "power to pumps" trip signal. Post event investigation found the direct cause of the automatic plant trip to be a blown secondary 'C' phase PT fuse on the 6900 volt AC bus. When the PT fuse blew two things occurred; first, the underpower relays for the 'B' and 'D' RCPs were de-energized, causing RPS to sense only 2 RCPs running ('A' and 'C') and with reactor power greater than 55 percent, RPS initiated a reactor trip; second, the PT fuse blowing caused the 6900 volt AC bus undervoltage protection to sense an undervoltage condition that caused loads to be shed from the bus and the 'B' and 'D' RCPs were actually tripped.

The cause of the blown secondary 'C' phase PT fuse is still under investigation.

A failure to formally communicate all known risks associated with relay replacement at the appropriate level within the organization was identified as the causal factor of this reactor trip event.

CORRECTIVE ACTIONS

Following the automatic reactor trip, ANO-1 remained in MODE 3 until the replacement 'D' RCP underpower relay and secondary PT fuse were tested satisfactorily.

This event was entered into the station's corrective action program.

Corrective actions initiated to date include destructive analysis of the blown fuse and non-destructive analysis of the undervoltage relay. Further investigation of the 'D' RCP underpower relay circuit is planned for completion during the next refueling outage.

Corrective actions to address the causal factor of this event involve improvement in the station recognition and communication of risk associated with maintenance activities that could lead to a reactor trip.

PREVIOUS SIMILAR EVENTS

ANO-1 LER 2018-004-00, "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. A causal factor of this event involved the failure to identify and mitigate bus trip risk during performance of electrical testing.