05000461/LER-2012-001
| Docket Number Sequential Revmonth Day Year Year Month Day Year 05000Number No. | |
| Event date: | 09-02-2012 |
|---|---|
| Report date: | 10-26-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 4612012001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric -- Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in test as [XX].
EVENT IDENTIFICATION
Emergency Reserve Auxiliary Transformer Trip and Subsequent Loss of Secondary Containment Differential Pressure
A. Plant Operating Conditions Before the Event
� Unit: 1 Event Date: 9/2/2012� Event Time: 2204 hours0.0255 days <br />0.612 hours <br />0.00364 weeks <br />8.38622e-4 months <br /> CDT Mode: 1 Mode Name: Power Operation �Reactor Power: 97 percent B.DESCRIPTION OF EVENT On September 1 and 2, 2012, Clinton Power Station (CPS) experienced significant precipitation (rain).
Partly due to the rain, a hard ground alarm [ALM] was received on Direct Current (DC) Motor Control Center [MCC] 1 F. To determine the location of the ground, station Electrical Maintenance technicians conducted ground fault tracing using a DC Scout ground test device.
At 2204 on September 2, 2012, when the DC Scout device was connected between the Balance of Plant (BOP) 125 Volts DC (VDC) battery [BTRY] 1F and station ground, the Emergency Reserve Auxiliary Transformer (ERAT) [XFMR] sudden pressure seal-in relay [RLY] (63SPX) sealed in causing both the ERAT and ERAT Static Var Compensator (SVC) to trip.
Immediately following the trip, the ERAT deluge system actuated and fire pumps [P] 'A' and 'B' started.
Safety related 4160 kV Bus [BU] 1A1 , which had been powered from the ERAT, momentarily lost power and transferred to the Reserve Auxiliary Transformer (RAT). Due to the momentary loss of power to Bus 1A1, Fuel Building Ventilation [VG] lost power and its dampers [DMP] closed causing a loss of Secondary Containment differential pressure. Secondary Containment differential pressure increased above the 0.25 inches vacuum required by Technical Specification (TS) 3.6.4.1, Secondary Containment. Due to high secondary containment differential pressure, operators entered Emergency Operating Procedure (EOP) - 8, Secondary Containment Control. Fuel Pool Cooling and Cleanup [DA] system pump 'A' tripped, causing the upper containment pool level to drop. However, upper containment pool level did not drop below the minimum level required by plant TS 3.6.2.4, Suppression Pool Makeup System. The Drywell fission product monitor [MON] required by TS 3.4.7, Reactor Coolant System Leakage Detection Instrumentation, isolated. Main Control Room Ventilation [VI] 'A' chiller [CHU] shut down. Diesel Generator [DG] Vent Oil Room 1A Exhaust Fan [FAN] lost power. Several radiation monitors momentarily lost power and were declared inoperable.
Secondary Containment differential pressure was restored at 2219 on September 2, 2012, when the Standby Gas Treatment System SGTS [BH] was manually started and operators exited EOP - 8 at 2238 on September 2, 2012. The fission product monitor was restored by 2309 on September 2, 2012, within the 30 day TS completion time. The Main Control Room Ventilation 'A' Chiller was restarted at 2237. The DG Vent Oil Room 1A Fan was restarted at 0104 on September 3, 2012.
Since the secondary containment differential pressure was greater than the TS required pressure, this event was reported (Event Number: 48269) on September 3, 2012, at 0417 as a condition that could have prevented the fulfillment of a safety function per 10 CFR 50.72(b)(3)(v)(C).
Corrective action program Issue Report 1408282 was initiated to evaluate this event.
A review of the ERAT trip and of the 1A1 Bus fed breaker logic indicates that the DC Scout signal used during ground fault tracing actuated the sudden pressure relay logic which then actuated the lockout relays. One lockout relay specifically blocks the synchro-verifier relay logic, thus blocking a fast transfer. This resulted in an automatic slow 1A1 bus transfer from the ERAT to the RAT. The slow transfer resulted in the momentary loss of power on the 1A1 bus and all downstream loads.
During troubleshooting activities for this event, a latent design error was identified on seal-in relays 63SPX and 63FPX. Both relays are Qualitrol Model 909-200-01 AC/DC Seal-In Relays with 125 VDC supply power. The latent design error is a ground wire installed on terminal 13 of the relay. The current vendor manual information states that terminal 13 should not be connected to earth ground when a DC power supply is used. The vendor has acknowledged that spurious operation can occur when this device is used with DC supply power and a ground on terminal 13. All indications show that the latent error has existed since original transformer construction at the factory in 1998. The current ERAT was received and installed in 1998.
C. CAUSE OF EVENT
The cause of this event was due to a latent design error that involved the wiring of terminal 13 to ground for the sudden pressure seal-in relay cards (63SPX). Updated vendor manual information states that terminal 13 should not be connected to earth ground when a DC power supply is used.
The relay manufacturer has acknowledged that spurious operation can occur when this device is used with DC supply power and a ground on terminal 13. The latent error has existed since original transformer construction at the factory.
D. SAFETY CONSEQUENCES
There were no actual nuclear safety consequences related to this event. This event resulted in the loss of secondary containment for approximately 15 minutes, from 2204 to 2219 on September 2, 2012 due to loss of power to the Fuel Building ventilation system and dampers closing. Secondary Containment differential pressure was greater than the 0.25 inches vacuum required by TS 3.6.4.1. The SGTS was manually started during this period and differential pressure was restored to within limits. The SGTS initiates automatically when conditions indicate a release of radioactive material or a loss of coolant accident to ensure that any radioactive materials that leak from the Primary Containment into the Secondary Containment following an accident are filtered by SGTS prior to release to the environment.
This event is considered to be reportable as a loss of safety function under 50.72(b)(3)(v)(C) and 50.73(a)(2)(v)(C) as an event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material.
E. CORRECTIVE ACTIONS
A Temporary Modification under Engineering Change (EC) 390386 was implemented to disable the 63SPX (Qualitrol Model No. 909-200-01) trip function and remove the ground connection from terminal 13 on the seal-in relays (63SPX and 63FPX). The trip function of the 63SPX relay can be restored during the next ERAT maintenance outage. Based on these actions the risk of an invalid ERAT trip has been mitigated.
F. PREVIOUS OCCURRENCES
12/18/2001 - ERAT TRIP - The ERAT and ERAT SVC tripped (with deluge). A Root Cause investigation was performed. The cause of the trip was an internal fault. The cause for the internal fault on the ERAT was a gradual localized breakdown of the insulating oil due to corona discharge from the spliced HV lead. The controller cards, seal-in relays, etc., were not mentioned as contributing factors and no specific issues with the seal-in relays were documented.
3/02/2002 - ERAT TRIP - The ERAT and ERAT SVC tripped (with deluge). The cause of the trip was determined to be a wet seal-in relay card. The control cabinet contained gaps that allowed rain and subsequent deluge water to enter the cabinet. A Work Request was issued to seal the ERAT control cabinet.
G. COMPONENT FAILURE DATA
Manufacturer� Nomenclature� Manufacturer Model Number Qualitrol Corporation�Sudden pressure�909-200-01 Seal-in relay (63SPX)