05000461/LER-2013-008
| Clinton Power Station, Unit 1 | |
| Event date: | 12-08-2013 |
|---|---|
| Report date: | 06-28-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4612013008R01 - NRC Website | |
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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 Division 1 Transformer Leads to Isolation of Instrument Air Supply to Containment, Lowering Scram Pilot Air Header Pressure, and Manual Reactor Scram A. Plant Operating Conditions before the Event Unit: 1 Event Date: 12/08/13 Mode: 1 Mode Name: Power Operation
B. DESCRIPTION OF EVENT
Event Time: 2036 Reactor Power: 97.3 percent On 12/8/2013 at 2026 hours0.0234 days <br />0.563 hours <br />0.00335 weeks <br />7.70893e-4 months <br />, with the plant in Mode 1 (Power Operation) at 97.3 percent reactor power, multiple alarms [ALM] were received in the Main Control Room (MCR) due to the trip of 4160 volt [EB] 1A1 breaker [BKR] 1 APO7EJ which resulted in a loss of power to Division 1 480 volt unit substations 1A [ED] and Al . Area operators were immediately dispatched to investigate the trip of the breaker. Many Division 1 components lost power. Operators entered the action requirements for numerous Technical Specification Limiting Conditions for Operation. Major impacts to the station as discussed in this report include: loss of Instrument Air (IA) [LE] supply to Containment loads, affecting the Control Rod Drive system (CRD) [AA], main steam isolation valves (MSIVs) [ISV] [SB], and the Reactor Water Cleanup system (RWCU) [CE]; loss of Secondary Containment [VG] differential pressure; loss of Low Pressure Core Spray system (LPCS) [BM] capability; and loss of Residual Heat Removal (RHR) [BN] Train A capability.
Operators in the MCR noted the loss of power caused IA containment isolation valves to close so operators began monitoring the control rod drive (CRD) scram pilot air header pressure for potential of control rods [ROD] to drift due to the loss of air. At 2035 hours0.0236 days <br />0.565 hours <br />0.00336 weeks <br />7.743175e-4 months <br />, the MCR received an alarm for Rod Control and Information System inoperable due to the loss of power to the Division 1 Rod Action Control System (RACS). The loss of instrument air affected various other containment loads, including the MSIVs and the RWCU system.
At 2036 hours0.0236 days <br />0.566 hours <br />0.00337 weeks <br />7.74698e-4 months <br />, when the pre-established scram pilot air header low pressure alarm limit was reached, operators immediately placed the reactor mode switch [HS] into the shutdown position, initiating a manual reactor scram. At 2037 hours0.0236 days <br />0.566 hours <br />0.00337 weeks <br />7.750785e-4 months <br />, reactor pressure vessel water level dropped to the low reactor water Level 3 setpoint (normal result of a scram from high power) and operators entered Emergency Operating Procedure (EOP) -1, Reactor Pressure Vessel (RPV) Control. At 2042 hours0.0236 days <br />0.567 hours <br />0.00338 weeks <br />7.76981e-4 months <br />, operators verified all control rods fully inserted into the core using the Division 2 RACS.
Clinton Power Station, Unit 1 05000461 At 2037 hours0.0236 days <br />0.566 hours <br />0.00337 weeks <br />7.750785e-4 months <br />, normally closed Group 2 (Residual Heat Removal (RHR) [BO]), Group 3 (RHR), and Group 20 (miscellaneous systems) containment isolation valves received signals to close as expected and operators subsequently verified the valves were closed.
At 2043 hours0.0236 days <br />0.568 hours <br />0.00338 weeks <br />7.773615e-4 months <br />, operators manually initiated the standby gas treatment system (SGTS) [BH] to restore Secondary Containment differential pressure that became positive when the Fuel Building ventilation system isolated due to the loss of 480 volt power.
At 2052 hours0.0238 days <br />0.57 hours <br />0.00339 weeks <br />7.80786e-4 months <br />, a report from the field indicated that the 480 volt Unit Sub A 4160 / 480 volt stepdown transformer [XFMR] OAPO5E2 was the source of the fault that caused the trip of the 4160 volt breaker 1 APO7EJ. A subsequent visual inspection of the transformer identified damage to the A and B phase windings.
At 2114 hours0.0245 days <br />0.587 hours <br />0.0035 weeks <br />8.04377e-4 months <br />, operators manually opened the outboard instrument air containment isolation valve 1IA012A in accordance with the loss of power off-normal procedure to restore the instrument air supply to the containment.
At 0121 hours0.0014 days <br />0.0336 hours <br />2.000661e-4 weeks <br />4.60405e-5 months <br /> on 12/9/13, the plant was in a stable condition and operators exited EOP-1.
RPV pressure control was maintained using main steam line drains to the main condenser [COND] and RPV water level control was maintained using condensate / condensate booster systems [SD] and the CRD system.
This event is reportable under the provisions of: 10 CFR 50.73(a)(2)(iv)(A) due to the unplanned actuations of the Reactor Protection System [JC] (RPS) and containment isolation valves; 10 CFR 50.73(a)(2)(v)(C) due a loss of normal ventilation and differential pressure to Secondary Containment; and 10 CFR 50.73(a)(2)(v)(D) as a result of the loss of Division 1 480 volt power causing the loss of accident mitigation abilities of the LPCS system. Event Notification Number 49617 was made to the NRC on 12/9/13 at 0015 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> Central Standard Time.
This event was entered into the Clinton Power Station corrective action program under Issue Report 1594407.
C. CAUSE OF EVENT
A definitive root cause cannot be determined. The transformer vendor was contacted to assist in determining the most likely cause for the failure. Photographs of the failed transformer were provided to the vendor. The vendor compared these photographs to photographs of other failed transformers and indicated that based on their visual assessment, the most probable cause of the OAPO5E2 transformer failure is a turn to turn failure of the high side windings due to insulation breakdown over time.
The OAPO5E2 transformer was installed in 1980 with a 40 year life expectancy (2020). The transformer failed seven years prior to its expected end of life. Based on the vendor's dry transformer performance history, the failure of OAPO5E2 transformer is considered a low probability event within the expected 40 year life cycle.
Following the submittal of LER 2013-008, CPS requested the dry transformer vendor to examine the unit to establish a clear cause of failure. The transformer was disassembled and three coils of the unit were removed from the core and shipped to the transformer vendor for failure analysis. The vendor's inspection identified two failures apparent in the windings, a Phase B arc and a Phase A turn to turn short circuit. The Phase B arc caused insulation damage and some erosion of the conductor in one turn of the High Voltage winding. The vendor was unable to determine a definitive root cause of the unit failure.
Based on the transformer vendor's failure analysis, the cause of failure of OAPO5E2 described in high side windings due to insulation breakdown over time.
D. SAFETY ANALYSIS
The failure of OAPO5E2 transformer and subsequent trip of 4160 volt circuit breaker 1APO7EJ 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 needed. No plant safety limits were exceeded and no Emergency Core Cooling System actuations occurred.
E. CORRECTIVE ACTIONS
An Engineering Change document was developed to utilize the spare OAPO5E7 transformer.
Maintenance personnel performed necessary work to transfer leads, cables, conduit, etc. from the OAPO5E2 transformer to the OAPO5E7 transformer. The OAPO5E7 transformer was energized and the station restored Division 1 to service and restarted the unit.
F. PREVIOUS SIMILAR OCCURENCES
CPS experienced one other dry type transformer failure on 2/3/1996. This transformer was a non- safety radiological waste building transformer and its fault was non-consequential. Conditions (transformer load and cycling due to cold outside temperatures) were the most likely cause of the 2/3/1996 transformer failure. A transformer autopsy was determined to be cost prohibitive and a definitive cause of the 2/3/1996 transformer failure was not identified. The 12/8/2013 failure was reviewed and determined to be unlike the 2/3/1996 failure.
G. COMPONENT FAILURE DATA
Component Description: I-T-E Dry Type Transformer; 4160V/480V; 750KVA Manufacturer: GOULD-BROWN-BOVERI Model:VU-9 Year Built: 1980