05000461/LER-2016-005

LER-2016-005, Insulator Failure On the Reserve Auxiliary Transformer Results In A Loss of Secondary Containment Vacuum

Docket-Number
Event date:	04-02-2016
Report date:	05-31-2016
Reporting criterion:	10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material
4612016005R00 - NRC Website

LER 16-005-00 for Clinton Power Station, Unit 1, Regarding Insulator Failure on the Reserve Auxiliary Transformer Results in a Loss of Secondary Containment Vacuum

ML16155A080
Person / Time
Site:	Clinton, Docket-Number
Issue date:	05/31/2016
From:	Stoner T R Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
U-604287 LER 16-005-00
Download: ML16155A080 (5)

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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

Insulator Failure on the Reserve Auxiliary Transformer Results in a Loss of Secondary Containment Vacuum A. Plant Operating Conditions before the Event Unit: 1 Mode: 1 Event Date: 04/02/16 Event Time: 1257 Mode Name: Power Operation Reactor Power: 99 percent

B. DESCRIPTION OF EVENT

On April 2, 2016, at approximately 1257 CDT the Reserve Auxiliary Transformer (RAT) and Static VAR Compensator (SVC) tripped due a failure of the 'A' phase insulator 345 kV Circuit Switcher 4538 in the switchyard. The Main Control Room (MCR) entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.1, AC Sources-Operating, Required Action A.1 and A.2. The CPS area was experiencing very strong winds at the time of the failure. The bus bar connected to the failed insulator has a configuration that is susceptible to wind induced loads. The bus bar is supported at the plant north end by the insulator stack and on the south end by a flexible link attached to the 345 kV Circuit Switcher 4538. In the middle of the bus bar is a wire connection to the overhead 345 kV lines that supply power from the North Bus. This configuration allows the wind to induce loads on the overhead 345 kV lines, the electrical drop connection (wire attached to the bus bar), and on the physical bus bar itself.

As a result of the voltage transient, caused by automatic transfer to the Emergency Reserve Auxiliary Transformer (ERAT), the Division 1 Fuel Building Ventilation (VF) system isolation dampers closed causing a trip of VF supply and exhaust fans. The effect of this condition was that Secondary Containment (SC) rose to slightly greater than 0 inches water gauge (WG) and exceeded the TS limit of 0.25 inch water gauge. Operations personnel entered Emergency Operating Procedure (EOP) -8, Secondary Containment Control, and TS LCO 3.6.4.1, Secondary Containment, Required Action A.1. The Standby Gas Treatment System was started and SC vacuum was restored within TS limits at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />.

Clinton Power Station, Unit 1 05000461 Additional actuations occurred resulting from the RAT trip. MCR intake radiation monitors 1RIX- PROO1A/C and 8A/C had momentary power interruptions and were temporarily inoperable. The MCR ventilation (VC) Division 2 auto transferred to high radiation mode. Division 1 diesel generator (DG) Vent Oil Room exhaust fan tripped and was restarted. Subsequent review concluded that these systems responded as expected due to the momentary loss of power and did not adversely affect the safe operation of the plant or the restoration of SC.

The RAT was successfully returned to service following replacement of the broken 345 kV Circuit Switcher 4538 insulator stack (`A' phase). The 'B' phase was also replaced (the 'C' phase had been previously replaced due to an identified crack). Alignment testing was performed on the 4538 Circuit Switcher as well as thermography and a visual inspection of the high voltage cable connections.

C. CAUSE OF EVENT

The cause of the event was identified as the failure of the 'A' phase 345 kV 4538 Circuit Switcher insulator due to a manufacturing defect. The CPS area was experiencing very strong winds at the time of the insulator 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.

The defective insulator caused a trip of the RAT which 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 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 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 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.

E. CORRECTIVE ACTIONS

CPS performed an analysis of remaining installed Ohio Brass insulators. A total of 196 Ohio Brass insulator stacks were identified as installed on site in the switchyard. An Operational Decision Making (ODM) process was perfcirmed and developed a replacement strategy. All Ohio Brass insulators were risk reviewed for the potential failure impact and a replacement strategy was created to replace the high risk and critical insulators. Replacement of all remaining Ohio Brass insulators in the CPS switchyard will be completed by the end of C1R17.

F. PREVIOUS SIMILAR OCCURENCES

No previous events were identified associated with an insulator failure.

G. COMPONENT FAILURE DATA

The component failure associated with this event was the defective Ohio Brass insulator.

Component Failure Data:

Part: Insulator Manufacturer: Ohio Brass