05000413/LER-2021-005, Automatic Actuation of the 1B Emergency Diesel Generator Load Sequencer on Bus Under-voltage Condition

Automatic Actuation of the 1B Emergency Diesel Generator Load Sequencer on Bus Under-voltage Condition

ML21355A165
Person / Time
Site:	Catawba
Issue date:	12/21/2021
From:	Simril T Duke Energy Carolinas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RA-21-0322 LER 2021-005-00
Download: ML21355A165 (6)

LER-2021-005, Automatic Actuation of the 1B Emergency Diesel Generator Load Sequencer on Bus Under-voltage Condition

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(1), Submit an LER, Invalid Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
4132021005R00 - NRC Website

text

el_~ DUKE

~ ENERGY RA-21-0322 December 21, 2021 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555

Subject:

Duke Energy Carolinas, LLC Catawba Nuclear Station, Unit 1 Docket No. 50-413 Licensee Event Report (LER) 413/2021-005-00 Tom Simril Vice President Catawba Nuclear Station Duke Energy CN01VP I 4800 Concord Road York, SC 29745 o: 803.701.3340 f: 803.701.3221 10 CFR50.73 Pursuant to 10 CFR 50.73(a)(1) and (d), attached is LER 413/2021-005-00, entitled" Automatic Actuation of the 1 B Emergency Diesel Generator Load Sequencer on Bus Undervoltage Condition". This report is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A).

There are no regulatory commitments contained in this letter or its attachment.

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

If questions arise regarding this LER, please contact Ari D. Tuckman of Regulatory Affairs at (803) 701 -3771.

Sincerely, Tom Simril Vice President, Catawba Nuclear Station Attachment

United States Nuclear Regulatory Commission Page 2 December 21, 2021 xc (with attachment):

L. Dudes Regional Administrator U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, GA 30303 Z. Stone NRC Project Manager (CNS)

U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mailstop O-8G9A Rockville, MD 20852 J. Austin (without enclosure)

NRC Senior Resident Inspector

Abstract

On October 27, 2021, at approximately 1430, with Unit 1 in Mode 6 at 0% power, the 1 B Emergency Diesel Generator (EOG) Load Sequencer was actuated by a valid undervoltage condition on the 1 B 4160v Essential Bus that occurred during 1 B sequencer timer calibration activities. Valid actuation signals were sent to both the 1 B EOG and the Unit 1 Auxiliary Feedwater (CA) System. Neither system automatically started as they were both removed from service for maintenance activities at the time. Decay heat removal was maintained via 1 A train systems.

During performance of the 1 B EOG sequencer timer calibration, jumpers equipped with mini grabbers are used to place the sequencer into a test configuration. The cause of the event is the mini grabber placed during the testing activity lost adequate connection which allowed the load sequencer to realign from the test condition.

This event was reported to the NRG as an eight-hour, non-emergency Event Notification 55549 on October 27, 2021, per 1 0CFR 50.72(b)(3)(iv)(A), Specified safety system actuation.

BACKGROUND YEAR 2021 SEQUENTIAL NUMBER

- 005 REV NO.

- 00 rT°he following systems and component information is provided to assist readers in understanding the event described in this LER. Applicable Energy Industry Identification [EIIS] system and component codes are enclosed with brackets. Catawba Nuclear Station unique system and component identifiers are contained within parentheses.

rT"his event is being reported under the following criterion:

1 OCFR 50. 73(a)(2)(iv)(A), for any event or condition that resulted in manual or automatic actuation of the PWR auxiliary feedwater system and the emergency AC electrical power system.

Engineered Safety Features Actuation System [JE](ISE), Diesel Generator Load Sequencer [EK]

(EQB)(EQC)

The onsite standby power source for each 4160 volt Engineered Safety Features (ESF) bus at Catawba is a dedicated Diesel Generator [DG] (DG). For each unit, DGs A and Bare dedicated to ESF buses ETA and ETB, respectively. Each DG starts automatically on a Safety Injection (SI) signal (i.e., low pressurizer pressure or high containment pressure) or on an ESF bus degraded voltage or undervoltage signal. After the DG has started, it will automatically tie to its respective bus after offsite power is tripped as a consequence of ESF bus undervoltage or degraded voltage, independent of or coincident with an SI signal. With no SI signal, there is a ten-minute delay between the degraded voltage signal and the DG start signal. The DGs will also start and operate in the standby mode without tying to the ESF bus on an SI signal alone. Following the trip of offsite power, a sequencer strips loads from the ESF bus. When the DG is tied to the ESF bus, loads are then sequentially connected to its respective ESF bus by the automatic load sequencer. The sequencing logic controls the permissive and starting signals to motor breakers to prevent overloading the DG by automatic load application.

In the event of a loss of preferred power, the ESF electrical loads are automatically connected to the DGs in sufficient time to provide for safe reactor shutdown and to mitigate the consequences of a Design Basis

~ccident (DBA) such as a Loss of Coolant Accident (LOCA).

Certain required unit loads are returned to service in a predetermined sequence in order to prevent overloading

~he DG in the process. Approximately one minute after the initiating signal is received, all loads needed to recover the unit or to maintain it in a safe condition are returned to service.

~uxiliary Feedwater System [BA](CA)

!The Auxiliary Feedwater System assures sufficient feedwater supply to the steam generators in the event of loss the condensate/feedwater system, to remove energy stored in the core and primary coolant. The CA system may also be required in some other circumstances such as evacuation of the main control room or Page 2 of 4

u_s_ NUCLEAR REGULATORY COMMISSION APPROVED BYOMB: NO. 3150-0104 EXPIRES: 08/31/2023 (08-2020)

Estimated burden per response to comply with this mandatOI)' collection request 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed bock to industry. Send comrents LICENSEE EVE NT REPORT (LER) regarding burden estimate to the FOIA, Library, and Information Collections Brfllch {T-6 A10M), U.S.

CONTINUATION SHEET Nuclear RegulatOI)'

Commission, Washington, DC 20555-0001, or by e-mail l:J lnfocollects.R9source@nrc.gov, and the 0MB reviewer at: 0MB Office of lnfoomation and Reguebly Affairs, (3150-0104), Attn: Desk ail: oira submission@omb.eop.gov. The NRC may not conduct or (See NUREG-1022, R.3 for instruction and guidance for completing this fonn sponsor, and a person~ not required to respond to, a collection of information unless the document https://www nrc qov/readinq-nn/doc-colections/nureqs/staff/sr1022/r3/)

requesting or requiring the collection displays a currently valid OMBconb"ol number.

3. LERNUMBER YEAR Catawba Nuclear Station, Unit 1 05000-413 2021 SEQUENTIAL NUMBER

- 005 REV NO.

- 00 cooldown after a loss-of-coolant accident for a small break, including maintaining a water level in the steam generators following such a break.

The CA system is designed to start automatically in the event of a loss of offsite electrical power, trip of both main feedwater pumps, safety injection signal, or low-low steam generator water level; any of which may result in, coincident with, or be caused by a reactor trip. The CA system will supply sufficient feedwater to maintain the reactor at hot standby for two hours followed by cooldown of the reactor coolant system to the temperature at which the residual heat removal system may be operated.

The motor driven auxiliary feedwater pumps will automatically start and provide the minimum required feedwater flow within one minute following any of these conditions:

Trip of both main feedwater pumps AMSAC Actuation (ATWS Mitigation System and Actuation Circuitry)

Two out of four (2/4) low-low level alarms in any one SG Initiation of a Safety Injection signal Loss of power to the 4160V essential bus

EVENT DESCRIPTION

lAt the time of the event, Unit 1 was in mode 6, 0% power for a scheduled refueling outage and Unit 2 was operating in mode 1 at 100% power.

On October 27, 2021 at approximately 1430, with Unit 1 in Mode 6 at 0% power, during planned 1 B EOG sequencer timer calibration, the 4160 Volt essential power bus 1 ETB de-energized when the feeder breaker from transformer SATB opened. This undervoltage condition on the essential power bus actuated the 1 B EOG load sequencer which sent valid start signals to the 1 B EOG and the Unit 1 CA system. Neither system automatically started as they were both removed from service for planned maintenance activities at the time.

CAUSAL FACTORS A fault tree was performed to investigate all potential equipment failures that could have resulted in this event and all were refuted. The investigation concluded that a jumper which was placed with a mini grabber lost adequate connection resulting in test relays changing state and re-establishing the normal system configuration. Additionally, the procedure directing the timer calibration evolution did not have proper verification controls during performance to manage/eliminate risk to prevent the loss of the essential bus. Page 3 of 4 (08-2020)

U.S. NUCLEAR REGULATORY COMMISSION APPROVED BYOMB: NO. 3150-0104 EXPIRES: 08/31/2023 Estimated burden per response to comply with this mandat()()' collection request 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed ba::k to industry. Send conrrents LICENSEE EVENT REPORT (LER) regarding burden estimate to the FOIA, Library, and Information Collections Braich (T-6 A10M), U.S.

CONTINUATION SHEET Nuclear Regulat()()'

Commission, Washington, DC 20555-0001, or by e-mail b

lnfocollects.~source@nrc.gov, and the 0MB reviewer at: 0MB Office of lnfonmation and Reguatxy Affairs, (3150-0104), Attn: Desk ail: oira submission@omb.eop.gov. The NRG may not conduct or (See NUREG-1022, R.3 for instruction and guidance for completing this form sponsor, and a person is not required to respond to, a collection of information unless the doourrent https://www.nrc.gov/read inq-nn/doc-colections/nuregs/staff/sr1022/r3/)

requesting or requiring the collection displays a currently valid OMBconb"ol number.

3. LERNUMBER Catawba Nuclear Station, Unit 1 05000-413

CORRECTIVE ACTIONS

1.

Power was restored to 1 ETB (complete)

YEAR 2021 SEQUENTIAL NUMBER

- 005 REV NO.

- 00

2. Revise all unit and train related procedures to ensure proper controls are in place for timer calibration to detect if the sequencer is removed from test condition and prevent opening the incoming and alternate breakers. (complete)

3. Issue standing order to restrict mini grabber jumper usage unless proper controls and oversight have been implemented. (complete)

4. Revise administrative maintenance procedure to restrict use of mini grabber leads for output blocking.

(planned)

SAFETY ANALYSIS

At the time of the event, Unit 1 was in Mode 6 at 0% power. Both the 1 B Emergency Diesel Generator and Auxiliary Feedwater systems were previously removed from service for maintenance. Unit 1 core cooling and spent fuel cooling remained in service via 1A train equipment upon the loss of power to the 1 B train essential 4160 Volt bus.

This event is considered low safety significance as decay heat removal was not challenged and all equipment operated as designed.

ADDITIONAL INFORMATION

There have been no previous Licensee Event Reports at Catawba Nuclear Station in the last 3 years with the same causal factor as this issue. Page 4 of 4