05000461/LER-2016-004

LER-2016-004, Trip of Emergency Reserve Auxiliary Transformer Static VAR Compensator Causes Positive Secondary Containment Pressure Following Lightning Strike on 138 kV Offsite Source

Clinton Power Station, Unit 1
Event date:	03-30-2016
Report date:	5-27-2016
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
4612016004R00 - NRC Website

LER 16-004-00 for Clinton, Unit 1, Regarding Trip of Emergency Reserve Auxiliary Transformer Static VAR Compensator Causes Positive Secondary Containment Pressure Following Lightning Strike on 138 kV Offsite Source

ML16153A249
Person / Time
Site:	Clinton
Issue date:	05/26/2016
From:	Stoner T R Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
SRRS 5A.108, U-604286 LER 16-004-00
Download: ML16153A249 (5)

Reported lessons learned are incorporated into the licensing process and fed back to industry.

Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means 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.

PLANT AND SYSTEM IDENTIFICATION

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

EVENT IDENTIFICATION

Trip of Emergency Reserve Auxiliary Transformer Static VAR Compensator Causes Positive Secondary Containment Pressure Following Lightning Strike on 138 kV Offsite Source A. Plant Operating Conditions before the Event Unit: 1 Mode: 1 Event Date: 03/30/16 Event Time: 1545 Mode Name: Power Operation Reactor Power: 99 percent

B. DESCRIPTION OF EVENT

On March 30, 2016, at 1545 CDT, the Emergency Reserve Auxiliary Transformer (ERAT) Static VAR Compensator (SVC) [COMP] tripped when the 138 kV line cycled open and closed due to a lightning strike. Operations entered CPS procedures 4002.01, Loss of AC, and 4001.02, Automatic Isolation, and Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, AC Sources — Operating, Required Actions A.1 and A.2.

During the event, Division 1 Secondary Containment (SC) isolation dampers went shut due to the momentary loss of power and SC vacuum exceed 0 inch water gauge (WG). Emergency Operating Procedure (EOP)-8, Secondary Containment Control, and TS LCO 3.6.4.1, Secondary Containment, Required Action A.1 were entered as a result of this abnormal condition.

At 1547, the Standby Gas Treatment System (VG) Train A was started per CPS procedure 3319.01, Standby Gas Treatment, Section 8.2.1, due to SC isolation dampers going shut. At 1550, SC vacuum had been restored to TS limits. Procedure EOP-8 and TS LCO 3.6.4.1 Required Action A.1 were exited. The ERAT was restored at 1926 and TS LCO 3.8.1 Required Actions A.1 and A.2 were exited.

An investigation determined the cause of the event was most likely due to a lightning strike.

During the time frame of the event, a powerful electrical (lightning) storm was moving through the CPS service area. During the event, the ERAT relay scheme worked as designed and ERAT circuit switcher B018 remained closed allowing the ERAT to re-energize when the 138 kV breakers re-closed.

The event described in this LER was documented in Event Notification No. 51836 which was submitted to the NRC on March 30, 2016 at 2045 EST.

C. CAUSE OF EVENT

The apparent cause of the event was a voltage transient (most likely a lightning strike) caused a momentary loss of 138 kV substation feed to the ERAT. The ERAT relay scheme did not trip and the relay coordination worked as designed. ERAT circuit switcher B018 remained closed as designed.

D. SAFETY ANALYSIS

This event is reportable in accordance with 10CFR50 50.73(a)(2)(v)(C) as a condition that could have prevented fulfillment of a safety function to control the release of radioactive material because secondary containment was declared inoperable.

The VG system was fully operable at the time of the event and capable of performing its required safety function. Operations entered EOP-8 for SC vacuum less than minus 0.25 inch WG and entered TS LCO 3.6.4.1 Required Action A.1. SC vacuum was restored to within TS limits within the completion time requirement. The SC vacuum is kept slightly negative relative to atmospheric pressure to prevent leakage to the atmosphere. The Fuel Building ventilation (VF) system is a non-safety ventilation system which is normally in service to maintain secondary containment vacuum. The VG system is the safety-related system which is relied upon to perform this function following an accident. During the event the VG system was placed in service and restored Secondary Condition to operability consistent with its safety function.

Therefore, the ability of the station to maintain secondary containment vacuum in the event of an accident was never jeopardized or challenged by the VF system fan trip. Engineering analysis has determined that this event is not considered a safety system functional failure.

E. CORRECTIVE ACTIONS

No failed or malfunctioning equipment occurred during this event. All equipment functioned as designed. Actions have been created to evaluate options to improve the reliability of the 138 kV source.

F. PREVIOUS SIMILAR OCCURENCES

Licensee Event Report 2015-004-00: Trip of Emergency Reserve Auxiliary Transformer Static VAR Compensator Causes Positive Secondary Containment Pressure Following Lightning Strike on 138 kV Offsite Source.

On June 25, 2015 at 0301 CDT, the Main Control Room received numerous annunciators that indicated a trip of the ERAT XFMR Static VAR Compensator (SVC)[COMP] caused by a voltage transient on the 138 kV offsite source due to thunderstorms in the area. The Division 1 Safety Bus [BU] was manually aligned from the reserve source to its normal source. As a result of the voltage transient, the Division 1 VF system isolation dampers closed causing a trip of VF supply and exhaust fans. SC vacuum increased to greater than 0 inch WG which exceeded the TS requirement. SC vacuum was restored within TS requirements by reopening the VF isolation dampers and restarting the VF supply and exhaust fans. The ERAT SVC was returned to service at 0457 CDT.

This occurrence is directly applicable to the conditions noted in this LER.

G. COMPONENT FAILURE DATA

There were no component failures associated with this event.