05000461/LER-2017-001

LER-2017-001, Failure of the 138 kV Offsite Power Source Results in a Loss of Secondary Containment Vacuum

Clinton Power Station, Unit 1
Event date:	02-24-2017
Report date:	04-21-2017
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
4612017001R00 - NRC Website

LER 17-001-00 for Clinton, Unit 1, Regarding Failure of the 138 kV Offsite Power Source Results in a Loss of Secondary Containment Vacuum

ML17114A470
Person / Time
Site:	Clinton
Issue date:	04/21/2017
From:	Stoner T R Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
U-604340 LER 17-001-00
Download: ML17114A470 (6)

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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

Failure of the 138 kV Offsite Power Source Results in a Loss of Secondary Containment Vacuum A. Plant Operating Conditions before the Event Unit: 1 Event Date: 02/24/17 Event Time: 2239 Mode: 1 Mode Name: Power Operation Reactor Power: 99 percent

B. DESCRIPTION OF EVENT

On February 24, 2017, at approximately 2239 hours0.0259 days <br />0.622 hours <br />0.0037 weeks <br />8.519395e-4 months <br /> CDT the Emergency Reserve Auxiliary Transformer (ERAT) and the associated Static VAR Compensator (SVC) tripped due to a loss of the 138 kV line owned and maintained by Ameren Illinois. The Main Control Room (MCR) entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, AC Sources-Operating, Required Actions A.1 and A.2. The Clinton area was experiencing very strong winds at the time of the 138 kV power loss and the ERAT trip.

As a result of the voltage transient caused by automatic transfer to the Reserve Auxiliary Transformer (RAT), the Division 1 Fuel Building Ventilation (VF) system isolation dampers closed causing a trip of the VF supply and exhaust fans. The effect of this condition was that Secondary Containment (SC) vacuum rose to slightly greater than 0 inches water gauge (WG) and exceeded the TS limit of -0.25 inch WG. Operations personnel entered Emergency Operating Procedure (EOP) -8, Secondary Containment Control, and TS LCO 3.6.4.1, Secondary Containment, Required Action Al.. As a result of the loss of power and subsequent bus transfer, the following additional system actuations occurred:

Division 1 MCR Ventilation Chiller tripped Division 1 Diesel Generator room Exhaust Fan 1VDO2CA tripped, Division 1 Shutdown Service Water Pump auto started, Fission Product Monitoring (FPM) skid isolation valves shut, Division 1 Hydrogen Monitoring system isolations valves shut, Containment Service Air system Outboard Isolation 1SA029 shut Nuclear Systems Protection System Inverter Trouble alarms annunciation (due to loss of syncronization).

The system actuations were addressed and plant equipment was restored to service in accordance with plant procedures. The Standby Gas Treatment System (VG) was manually started and SC vacuum was restored within TS limits at approximately 2242 hours0.0259 days <br />0.623 hours <br />0.00371 weeks <br />8.53081e-4 months <br />. The momentary loss of power did not adversely affect the safe operation of the plant or the restoration of SC.

During the investigation, Ameren determined that high winds caused a line fault external to the Station which resulted in the 138 kV supply line trip. At 0053 hours6.134259e-4 days <br />0.0147 hours <br />8.763227e-5 weeks <br />2.01665e-5 months <br /> on February 25, 2017 Ameren successfully re-energized the 138 kV line. The ERAT was returned to service and declared OPERABLE at 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br />.

C. CAUSE OF EVENT

The cause of the event was identified by Ameren as an air flow spoiler failure. The radial feed design of 138 kV offsite power to CPS is susceptible to single point failures. The Clinton area was experiencing very strong winds at the time of the air flow spoiler failure.

D. SAFETY ANALYSIS

This event is reportable in accordance with 10 CFR 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 SC was declared inoperable during the event.

The broken air flow spoiler which caused a trip of the 138 kV supply line and the ERAT is a condition bounded by the analysis in Updated Safety Analysis Report section 15.2.6, "Loss of AC Power." The condition described in this report is less severe and involved no safety consequences. The AC electrical power sources are designed with sufficient redundancy to ensure the availability of necessary power to plant systems, structures, and components.

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 -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 associated completion time requirement. The SC vacuum is kept slightly negative relative to atmospheric pressure to prevent leakage to the atmosphere. The VF system is a non-safety ventilation system which is normally in service to maintain SC 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 manually placed in service and restored SC to operability consistent with its safety function. Therefore, the ability of the station to maintain SC 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.

The event discussed in this report does not constitute a safety system functional failure.

E. CORRECTIVE ACTIONS

The 138 kV line was successfully re-energized on February 25, 2017 at 0053 hours6.134259e-4 days <br />0.0147 hours <br />8.763227e-5 weeks <br />2.01665e-5 months <br />. Following the restoration of the 138 kV line, the ERAT transformer and associated static VAR Compensator were returned to service. An additional corrective action includes installing a modification to preclude tripping of VF due to a momentary loss of power.

F. PREVIOUS SIMILAR OCCURENCES

There were no previous occurrences involving the loss of 138 kV line and ERAT trip due to a broken air flow spoiler. However, several similar occurrences involving to loss of the 138 kV line have occurred. These events are summarized below.

License Event Report 2016-004-00: Trip of Emergency Reserve Auxiliary Transformer Static VAR Compensator Causes Positive Secondary Containment Pressure Following Lightning Strike on 138 kV Offsite Source On 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. During the event, Division 1 Secondary Containment (SC) isolation dampers went shut due to the momentary loss of power and SC vacuum exceed 0 inches water gauge (WG). EOP-8 and TS LCO 3.6.4.1, 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 procedures. 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 hours0.0223 days <br />0.535 hours <br />0.00318 weeks <br />7.32843e-4 months <br /> and TS LCO 3.8.1 Required Actions A.1 and A.2 were exited.

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 transformer 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 inches 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.

G. COMPONENT FAILURE DATA

The component failure which caused the 138 kV supply trip was owned by Ameren. There were no CPS component failures associated with this event.