05000529/LER-2013-002
| Palo Verde Nuclear Generating Station (Pvngs) Unit 2 | |
| Event date: | 12-02-2013 |
|---|---|
| Report date: | 01-31-2014 |
| Initial Reporting | |
| ENS 49600 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| LER closed by | |
| IR 05000528/2000529 (13 August 2014) IR 05000528/2014003 (13 August 2014) | |
| 5292013002R00 - NRC Website | |
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This Licensee Event Report is being submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A) to reactor protection system (RPS) (EllS: JC) due to an electrical protection trip of the circuit breaker for the 1A reactor coolant pump (RCP) (EllS: AB) motor.
This event was reported to the NRC pursuant to 10 CFR 50.72 (b)(2)(iv)(B) at 2126 on December 02, 2013, via the event notification system (EN # 49600).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The reactor coolant system (RCS) (EllS: AB) is comprised of two main flow loops each of which includes two RCPs and one steam generator (EllS: AB). The primary function of the RCPs is to provide the necessary head to maintain forced circulation of reactor coolant through the RCS during normal operations. Critical operation of the reactor requires all four RCPs to be in operation to ensure adequate RCS flow.
Each RCP is driven by a 12,000 horsepower electric motor supplied from the non-class 13.8kV electric power distribution system. Each RCP motor circuit breaker is provided with electrical protection trip features which include protection relays for under voltage, overcurrent, phase current differential, and ground. The phase current differential relay is calibrated to operate when it senses an excessive current difference between any two of the three motor phases.
The RPS is an assemblage of sensors, calculators (including the core protection calculators (CPCs)), logic circuits, and supporting equipment that monitor nuclear steam supply system (EDS: AB) parameters and ensure the reactor is rapidly and reliably shut down to protect the fuel and RCS pressure boundary and assist the engineered safety features systems in accident mitigation. A pump speed signal from each RCP is provided to each of the four CPCs to support calculation of RCS flow. If any RCP slows to less than 95 percent of rated speed, the CPC software will introduce significant mathematical penalty factors into the departure from nucleate boiling ratio (DNBR) and local power density (LPD) calculations that result in DNBR and LPD trip signals on each RPS channel. The RPS actuation then causes simultaneous trips of the reactor trip switchgear breakers (El IS: AA) which de-energize the control element drive mechanisms so that all control element assemblies are released to insert and shutdown the reactor.
3. INITIAL PLANT CONDITIONS:
On December 02, 2013, PVNGS Unit 2 was operating in Mode 1 (Power Operation) at 100 percent power and normal operating temperature and pressure. There were no structures, systems, or components out of service that contributed to the event.
4. EVENT DESCRIPTION:
At approximately 1758 on December 2, 2013, the PVNGS Unit 2 control room received alarms indicating an automatic RPS actuation due to low DNBR and high LPD trip signals from each of the four core protection calculators. No other RPS actuation signals or actuations of plant engineered safety features occurred and all control element assemblies fully inserted. Control room personnel entered the emergency operating procedures and performed the standard post trip actions. The reactor trip was determined to be uncomplicated and the reactor trip procedure was implemented to stabilize the plant in Mode 3. RCS forced circulation was maintained throughout the event as RCPs 1B, 2A, and 2B remained in operation.
PVNGS Unit 1 and Unit 3 were operating at 100 percent power at the time of the event and were not impacted by the Unit 2 reactor trip.
'I Inspection of the 1 A RCP motor circuit breaker found the related electrical protection differential relay and "86 lockout" relay both tripped. Troubleshooting of the 1 A RCP circuit breaker, cabling, and motor determined an electrical fault had occurred within the motor windings. A plant cooldown was initiated and Unit 2 was placed in Mode 5 to allow for replacement of the faulted RCP motor.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
This event did not result in a challenge to the fission product barriers or result in the release of radioactive materials to the environment. There were no actual safety consequences as a result of this event and the event did not adversely affect the health and safety of the public.
The reactor protection system functioned as designed and initiated an automatic reactor trip that expeditiously placed the plant in a safe condition. Throughout the event all vital and non-vital electrical power sources remained energized and RCS forced circulation was maintained with RCPs 1B, 2A and 2B remaining in operation.
This event did not result in conditions more severe than the limiting loss of flow event contained in Chapter 15 of the PVNGS Updated Safety Analysis Report which is based on a sheared RCP shaft accompanied by a loss of off-site power that results in the loss of all RCS forced circulation. This was determined to be an uncomplicated reactor trip which results in a very small increase in calculated core damage frequency according to the guidance in Regulatory Guide 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis.
The condition did not result in a safety system functional failure as described in 10 CFR 50.73 (a)(2)(v).
6. CAUSE OF THE EVENT:
The direct cause of the event was an electrical protection trip of the RCP 1A motor circuit breaker that initiated CPC trip signals for low DNBR and high LPD when the RCP speed dropped below 95 percent of rated speed. The RCP 1 A motor circuit breaker tripped as a result of excessive differential current between electrical phases of the motor.
The investigation determined the probable cause of the circuit breaker trip to be a high impedance fault within the C phase of the motor stator coil.
The faulted motor has been shipped to an offsite facility for dissassembly and further root cause analysis. If information is subsequently developed that significantly affects a reader's understanding or perception of this event, a supplement to this LER will be submitted.
7. CORRECTIVE ACTIONS:
To repair the 1A RCP, Unit 2 was cooled down to Mode 5 and the faulted 1A RCP motor was replaced with an on-site spare RCP motor. Following completion of appropriate retests Unit 2 was returned to full power operations.
8. PREVIOUS SIMILAR EVENTS:
No previous similar events involving an automatic RPS actuation due to a RCP motor failure have occurred at PVNGS.