05000461/LER-2017-010

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LER-2017-000, Division 1 Transformer Failure Leads to Instrument Air Isolation to Containment Requiring a Manual Reactor Scram
Clinton Power Station, Unit 1
Event date: 12-09-2017
Report date: 02-05-2018
Reporting criterion: 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material

10 CFR 50.73(a)(2)(iv)(A), System Actuation
4612017000R00 - NRC Website
LER 17-010-00 for Clinton Power Station, Unit 1 Regarding Division 1 Transformer Failure Leads to Instrument Air Isolation to Containment Requiring a Manual Reactor Scram
ML18040A463
Person / Time
Site: Clinton Exelon icon.png
Issue date: 02/05/2018
From: Stoner T R
Exelon Generation Co
To:
Document Control Desk, Office of Nuclear Reactor Regulation
References
U-604402
Download: ML18040A463 (5)


comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB contrcl number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. LER NUMBER

2017 - 00 010

PLANT AND SYSTEM IDENTIFICATION

General Electric -- Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in text as [XX].

EVENT IDENTIFICATION

Division 1 Transformer Failure Leads to Instrument Air Isolation to Containment Requiring a Manual Reactor Scram

A. Plant Operating Conditions Before the Event

Unit: 1 Event Date: 12/09/17 Mode: 1 Mode Name: Power Operation Event Time: 1347 Reactor Power: 97 percent

B. Description of Event

At 1347 CDT on December 9, 2017, the Main Control Room received annunciators that indicated a trip of the 4160 V [EB] 1A1 breaker [BKR]1 APO7EJ and the loss of the 480V transformer 1A [ED] and Al.

Numerous Division 1 components lost 480V power (powered from unit substations 1A and Al). The Division 1 containment Instrument Air isolation valves had failed closed by design due to the loss of power. Due to the loss of containment instrument air, several control rods began to drift into the core as expected and, by procedure, the reactor mode switch was placed in the shutdown position at 1353 and a manual reactor scram was performed.

Due to the loss of power, the Fuel Building ventilation dampers failed closed by design. With the normal ventilation system secured, secondary containment differential pressure rose to slightly greater than 0 inches water gauge which exceeded the Technical Specification Surveillance Requirement 3.6.4.1.1 limit of greater than or equal to 0.25 inches vacuum water gauge at 1348. The Control Room entered Emergency Operating Procedure-8, Secondary Containment Control. Secondary Containment differential pressure was restored within Technical Specification requirements at 1351 by starting the Division 2 Standby Gas Treatment System (SGTS).

Inspection of 480V transformer 1A (1AP11E) found an area on the upper end of the B phase coil that was consistent with a phase to ground fault.

On December 14, the transformer was replaced and the plant returned to Mode 1 operations on December 15.

This event is reportable under 10 CFR 50.73(a)(2)(iv)(A) as a manual actuation of the Reactor Protection System (RPS), 10 CFR 50.73(a)(2)(v)(C) as a Condition that Could Have Prevented Fulfillment of a Safety Function and 10 CFR 50.73 (a)(2)(ii)(B) as the plant being in an unanalyzed condition. The event described in this report is considered an unplanned scram with complications due to the loss of the Division 1 480V power.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. LER NUMBER

2017 - 00 010

C. Cause of the Event

A phase to ground fault was identified on the B Phase of the 480V transformer 1A (1AP11E). The cause of the transformer failure is currently under investigation and will be provided in a supplemental Licensee Event Report.

D. Safety Consequences The trip of 4160V circuit breaker and the failure of the 480V transformer placed the station in a potential scram condition due to loss of instrument air to the containment and scram pilot air header. Manual operator actions were taken to shut down the reactor prior to an automatic scram and place the plant in a safe and stable condition. The loss of 480 volt power caused the Fuel Building Ventilation System to isolate resulting in positive secondary containment pressure. Operators placed the Division 2 SGTS in service to restore secondary containment negative pressure. All Division 2 and Division 3 Emergency Core Cooling Systems remained operable and available throughout this event for accident mitigation if required. No plant safety limits were exceeded and no Emergency Core Cooling System actuations occurred.

E. Corrective Actions

On December 14, the faulted transformer was replaced and the plant returned to Mode 1 operations on December 15. Additional corrective actions will be determined following completion of the causal evaluation.

F. Previous Similar Occurrences Containment, Lowering Scram Pilot Air Header Pressure, and Manual Reactor Scram On December 8, 2013 at

2026 hours
0.0234 days
0.563 hours
0.00335 weeks
7.70893e-4 months

with the plant in Mode 1 at 97.3 percent reactor power, operators received multiple alarms due to the trip of 4160 volt 1A1 breaker which resulted in a loss of power to two Division 1 480 volt unit substations. Operators were immediately dispatched and found a 4160/480 volt stepdown transformer Al (0APO5E) failed. Many Division I components lost power. The loss of power caused an instrument air (IA) containment isolation. The loss of IA affected various containment loads, including the scram pilot air header, the main steam isolation valves and the reactor water cleanup system. At

2036 hours
0.0236 days
0.566 hours
0.00337 weeks
7.74698e-4 months

, the scram pilot air header low pressure alarm was received and in response to an anticipated automatic reactor scram, operators immediately initiated a manual reactor scram. All control rods fully inserted into the core.

comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. LER NUMBER

2017 - 00 010 The cause of the transformer failure was a turn to turn failure of the high side windings due to insulation breakdown over time, prior to its expected end of life. An installed spare was connected to replace the failed Division 1 transformer.

G. Component Failure Data

Component Description: I-T-E Dry Type Transformer; 4160V/480V; 750KVA Manufacturer: GOULD-BROWN-BOVERI