05000286/LER-2008-006
| Docket Number | |
| Event date: | 10-09-2008 |
|---|---|
| Report date: | 10-29-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2862008006R01 - NRC Website | |
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DESCRIPTION OF EVENT
On October 9, 2008, while at 100%-steady-State reactor poWer',CIristrumentation and Control (I&C) Technicians were performing monthly surveillancetest 3-PT-M62C, _ "480 Volt AC Degraded Grid/Undervoltage Functional Testing Bus 6A," when at approximately 12:54 hours, the normal power supply breaker {BRK} for 480 Volt AC safeguards bus 6A {ED) inadvertently opened CaUsing the-bus to de-energize The bus undervoltage control circuitry {JE} caused an automatic actuation of emergency diesel-generatorAEDG)_, 32-{EKJ-which started and re:energized the--bus" , {BU), actuation of motor driven auxiliary feedWater'(AW). pump (AFWP) 33 - and steam driven AFWP 32 {BA). In accordance with design the loads on bus 6A were stripped and then the assigned loads were sequentially re-loaded back onto the_ bus. All systems performed as designed. AFW was injected into the steam generators (SG) {AB) that resulted-in.a1-2% change in SG level, and an approximate.0.1 increase in reactor power. There was no .control rod movement-.
The event was recorded in the Indian Point Corrective. Action Program (CAP) as CR IP3-2008-02519.
The onsite AC power distribution system is composed of 480 Volt AC buses 5A, 6A, 2A and 3A which is divided into three safeguards power trains. The three safeguards power trains are train 5A (Bus 5A and EDG-33), Train 6A (Bus 6A and EDG-32), and Train 2A/3A (Bus 2A and 3A and EDG-31). The480Volt AC safeguards buses receive power from 6.9 kV bus sections through theirrespective Station Service Transformer {FK) or from three onsite EDGs. The 480 Volt safeguard buses are designed with protection against undervoltage (UV) and degraded grid voltage (DGV) using relays-that- sense_ UV or DGVconditions- Non-Safety ..Injection (SI) Blackout relays will send start signals to -bus sequencing timers for essential loads that include the motor driven AFWPs and the turbine driven AFWP. The bus undervoltage relays will initiate the opening of the power feeds from the SST and 480 Volt AC tie breaker for a DGV condition after DGV relays are timed out on a particular bus. Each of two voltage sensing relays has its own associated-timing- relayto.provide -a time-delay-tb-insure proper coordination with plant electrical transients. Actuation of the DGV relays will trip the bus supply breaker removing power to the buses which will actuate the UV relays. -When-the feeder breaker-tripsthe-bus UV-relays will initiate bus stripping, actuate EDG start:,- and provide - Signals that will begin load sequencing to reload the bus:
On October 9, 2008, at approximately 9:10 hours, three qualified I&C Technicians initiated TS required monthly functional testing of the three 480 Volt safeguards buses in accordance with surveillance test procedure 3-PT-M62A,B,C using a Fluke 189 digital volt meter. The technicians started with bus 2A/3A which was satisfactorily completed at 10:00 hours, then started testing bus 5A at 1012 hours which was satisfactorily completed at 10:38 hours. After a lunch break, at approximately 12:35 hours, using the same Fluke digital volt meter used in the previous tests, the three I&C technicians started testing bus 6A in accordance with surveillance procedure 3-PT-M62C to test the 480 Volt AC undervoltage/degraded grid protection system for Bus 6A. The test is performed to demonstrate that the 480 Volt AC undervoltage/degraded grid protection system functions properly. The 480 Volt AC bus 6A undervoltage/degraded grid protection functions are tested locally at switchgear {SWGR} in the 480 Volt Switchgear Room on the 15 foot elevation of the Control Building {NA).
Cause of Event
The initial root cause analysis (RCA) did not identify a cause but determined the breaker trip was caused by energizing its shut trip coil of the normal supply breaker for 480 volt safeguards bus 6A. The most probable cause was concluded to be an intermittent fault in the digital volt meter (DVM) (Fluke 189) used to verify that the contacts in the DGV Agastat relay (62-1/6A) were energized and ready for actuation. Subsequent analysis by an independent vendor determined the DVM operated properly in all functions and ranges and calibration results were within specifications and no anormalies were noted. The root cause analysis considered human error, plant equipment malfunction or designed actuation, and test equipment fault. Human error and malfunction of installed equipment or designed actuation was evaluated and determined not to be the cause. A subsequent re-analysis for a similar event on January 9, 2009, that was performing the same test could not identify a cause. A monitoring plan was developed to collect circuit parameters in the 52/6A trip circuitry during testing. Monitoring was performed in regularly scheduled tests with additional monitoring and testing performed in the spring 2009 refueling outage. Despite rigorous investigation, the root cause is indeterminate. The probable cause was determined to be a meter lead short from terminal 4 to terminal 2 of DGV time delay relay 62-1/6A. A meter lead short from relay 62-1/6A terminal 4 to terminal 2 would not have resulted in an arc as determined in a failure analysis of the relay and leads. An evaluation of organizational and programmatic weaknesses identified inadequate work practices during performance of test 3-PT- M62C where the test leads used had the potential for shorting the terminals of Agastat time delay relays. The potential for causing a short with a meter lead exists anytime an exposed meter lead is long enough to short between terminals.
Corrective Actions
- - The following corrective actions have been performed under Entergy's Corrective Action Program to address the possible cause and prevent recurrence:
- Test procedures 3-PT-M62A,B,C were revised so that the voltage check of the degraded grid voltage Agastat are not taken across contacts.
- An independent vendor performed an analysis of. the Fluke 189 digital volt meter used during the test. The- vendor analysis determined the. DVM operated properly in all functions and ranges and calibration results were within specifications and no anormalies were noted.
- Degraded Grid time delay relay 62-1/6A was replaced and satisfactorily testing in accordance with 3-PT-M62C. The relay and the DVM leads used during the test were sent to an independent vendor for performance of a failure analysis. The results of the independent vendor analysis concluded neither the meter leads or relay 62-1/6A were faulty.
- Degraded Grid Voltage Protection Tests (3-PT-M62A, 3-PT-M62B, and 3-PT-M62C) were revised to specify the use of test leads that do not have the potential to create a short circuit between terminals of the Agastat time delay relays.
- Maintenance and Operations personnel were briefed on the possibility of meter lead probes causing a short circuit during meter reading activities and that pre-job briefs should include this precaution. The brief included the expectation that during an activity that identifies that terminals could be shorted by standard meter lead probs, different leads should be used.
- Insulated test leads by Pomona were purchased to help reduce the possibility of a short circuit.
Event Analysis
The event is reportable under 10CFR50.73(a)(2)(iv)(A). The licensee shall report any event or condition that resulted in the manual or automatic actuation -of any system listed in 10CFR50.73(a)(2)(iv)(B). The systems to which the requirements of 10CFR50.73(a)(2)(iv)(A) apply include; (#6) PWR auxiliary or emergency feedwater system, and (#8) Emergency AC electrical power systems including emergency diesel generators (EDG).
This event meets the reporting criteria because the 32 EDG actuated to start and the 32 and 33 AFWP actuated to start when the UV control circuit on 480 Volt AC Bus 6A actuated. On October. 9, 2008, the normal power supply to safeguards bus 6A was inadvertently de-energized and in accordance with design the 32 EDG, AFWP-32 and 33 automatically started at approximately 12:54 hours. At 12:58 hours, the 33 AFWP was secured and at 13:19 hours the 32 AFWP was secured. At 14:35 hours, 480. Volt AC Bus 6A was returned to its normal power supply and the condition for Technical Specification 3.8.1 was exited. At 14:41 hours, the 32 EDG was secured and returned to its normal standby condition. All required safety systems performed as designed. As a result of the event, there were no safety systems that were not capable of performing their safety function. In accordance with reporting guidance in NUREG-1022, an additional random single failure need not be assumed in that system during the condition. Therefore, there was no safety system functional failure reportable under 10 CFR 50.73(a)(2)(v).
Past Similar Events
A review was performed of Licensee Event Reports (LERs) for the past three years for any events reporting inadvertent Engineered Safety Feature actuation during testing. The review identified three LERs: LER-2005-003, LER-2008-003, and LER 2008-004. LER-2005-003 reported an inadvertent actuation of AFWPs 31 and 33 on May 16, 2005, during Reactor Protection Logic Functional testing. The cause of the event was human error due to inadequate work practices where the technician performing the test failed to adhere to the procedure. LER-2008-003 reported the actuation of an EDG due to the inadvertent action .of the. UV sensing circuit on bus 5A. The cause of the inadvertent actuation was procedure use and adherence. LER-2008-004 reported inadvertent actuation of AFWPs 31 and 33 during Reactor Protection Logic Channel Functional testing caused by incorrect jumper connection due to personnel error. The events reported in LER-2005-003, LER-2008-003, and LER-2008-004 do have a similar cause to this 'event because those events were determined to be caused by human performance errors although for this event there was insufficient objective evidence that a meter lead short did occur therefore, a Human Performance error could not be concluded as the cause of the event.
Safety Significance
This event had no effect on the health and safety of the public. There were no actual safety consequences for the event because there were no accidents or transients requiring the EDGs. Required power from both offsite sources and onsite emergency power were available and the actuation circuitry and EDG performed in accordance with design and minimum safeguards power was available to power safety loads. There was no significant core reactivity change as there was no automatic movement of the control rods, reactor power increased approximate 0.1%, and there was .a 1-2% change in SG level. The changes were well, within the actuation limits of the reactor protection system.
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