05000416/LER-2012-008
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 12-29-2012 |
|---|---|
| Report date: | 02-27-2013 |
| 4162012008R00 - NRC Website | |
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A. REPORTABLE OCCURRENCE
This Licensee Event Report (LER) is being submitted pursuant to Title 10 Code of Federal Regulations (10 CFR) 50.73(a)(2)(iv)(A) for an automatic actuation of the Reactor Protection System (EIIS:JC) (RPS). Telephonic notification was made to the U.S. Nuclear Regulatory Commission (NRC) Emergency Notification System on December 29, 2012, within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> of the event pursuant to 10 CFR 50.72(b)(3)(iv)(A).
B. INITIAL CONDITIONS
At the time of the event the reactor was in operational mode 1 with reactor power at 100 percent. There were no additional inoperable structures, systems, or components at the start of the event that contributed to this event. Post event it was discovered that Safety Relief Valve (EIIS:RV) (SRV) 1B21F047A failed to reclose automatically as anticipated when the reseat reactor pressure decayed below the setpoint. The SRV closed automatically within approximately ten minutes.
C. DESCRIPTION OF OCCURRENCE
On December 29, 2012, at 00:18 Central Standard Time (CST), Grand Gulf Nuclear Station was operating in Mode 1 at 100 percent thermal power when an unexpected Reactor SCRAM occurred due to a Main Generator (EIIS:TB) trip. All systems responded as expected with the exception of Safety Relief Valve (SRV) 1B21F047A, which was slow to close automatically, and high pressure feedwater heater start-up outlet valve (start-up outlet valve) 1N21F010B, which did not open automatically when the start-up level control valve (EIIS:LCV) was placed in service. SRVs opened at the onset of the event to control reactor pressure, as designed. Operations immediately entered the appropriate Off Normal Event Procedures (ONEPs). Reactor water level was controlled using the normal condensate/feedwater system (EIIS:SD/SJ) throughout the event. All control rods (EIIS:ROD) inserted after the appropriate channels of the main turbine control fast closure signals generated by the RPS were received. There were no Emergency Core Cooling System (ECCS) actuations. The plant was stabilized with pressure control on the main turbine bypass valves (EIIS:PCV), and level controlled on the start-up level control valve. The plant responded to the trip as designed with the exception of the one SRV and the one start-up outlet valve noted above.
D. APPARENT CAUSE
The cause of the event was not determined initially since the relay issue or fault was evaluated to be intermittent in nature. Following a second SCRAM on January 4, 2013, it was determined that the Main Generator 'X Phase Neutral Current Transformer D. APPARENT CAUSE (continued) (EIIS:XCT) (CT) experienced partial grounding due to inadequate clearance between the micarta plate bolts and bottom of the CT allowing the conductors to come in contact with a bolt providing a shunt path to ground. This was caused by inadequate workmanship and work instructions not specifying the clearance during installation.
E. CORRECTIVE ACTIONS
Operating Experience (OE) searches, Entergy Transmission, Power Control System (PCS)/General Electric (GE) vendor support, modification history, and Entergy Fleet challenges were used to identify possible causes and/or failure modes that could be analyzed without destructive testing or destructive modification. Initial, testing, troubleshooting, and inspection did not confirm a specific failure mode, so the relay issue or fault was determined to be intermittent in nature. These actions isolated the possible failure modes to an unnecessary trip of Unit Phase Differential Relay 1N41M701A (EIIS:87), an internal fault of a Current Transformer (EIIS:XCT) (CT) or a fault in the CT to relay circuitry, but did not identify a failure mode.
Active components such as the 1N41M701A, B, and C Unit Phase Differential Relays were replaced to eliminate relay intermittent operation issues. Monitoring equipment was installed in the CT Circuitry to monitor plant performance and to capture data from any future transients that might occur intermittently.
After the root cause was identified following the SCRAM on January 4, 2013, the following corrective actions were identified:
Removal and/or thread cutting of the micarta plate bolts to ensure minimum cold clearance of 0.5 inch between the CT and the micarta plate bolts was completed on January 6, 2013.
For each main generator CT, the Post Maintenance Test special requirement planning notes will be revised to ensure that 0.5 inch cold clearance is maintained between the micarta plate bolts and the main generator CTs.
Plant personnel will perform boroscopic inspection of CTs and add work instruction steps that include hold points to ensure adequate cold clearance of 0.5 inch is maintained between the micarta plate bolts and the main generator CTs.
Applicable drawings will be revised to incorporate a minimum cold clearance of 0.5 inch to be maintained between the CT and micarta plate bolts.
F. SAFETY ASSESSMENT
The event posed no threat to public health and safety as the RPS performed as designed. Although the event was a SCRAM with Complications, the SRV closed within approximately ten minutes. The 1N21F01013 start-up outlet valve was reset and returned to service and did not prevent Operations from controlling the reactor water level. All other safety systems responded as designed.
Immediate actions performed by the Operations staff were adequate and appropriate in placing and maintaining the reactor in a safe shutdown condition. Reactor pressure peaked at 1090.4 pounds per square inch gauge (psig), minimum reactor pressure was 671.7 psig, and reactor water minimum level was recorded as 7.9 inches wide range.
Thus, these two critical parameters did not challenge any design or safety limit.
Nuclear safety was not significantly compromised because safety related equipment necessary to safely shutdown the unit performed its safety function.
SRV 1B21F047A lifted as expected along with ten other SRVs on Reactor Pressure.
However, SRV 1B21F047A failed to reclose automatically as anticipated when the reseat pressure was reached. Operator actions were taken to close the valve by placing the handswitch to Close and pulling the fuse (EIIS:FU) to the division 1 solenoid (EIIS:PSV). Pressure eventually bled off the exhaust port allowing the SRV to close automatically. Had the pressure not bled off, the SRV would have remained open, thereby depressurizing the Reactor Vessel (EllS:RPV). Should this have occurred, the Operating crew would not have had control of reactor pressure. The depressurization would also have violated the cool-down limits of the vessel.
During the event, no Technical Specification defined Safety Limits were challenged.
Radiological Safety was not affected since there was no radiological release to the public during the event. Actuation of SRVs did cause the Drywell Fission Product Monitor Particulate channel to alarm, as expected. This elevated particulate condition soon returned to the pre-SCRAM level. Containment airborne radiation levels did not rise to a level which required posting the containment due to SRV actuation.
Response of the crew did not challenge established industrial safety protocol or requirements. There was no impact to the safety of the public, industrial safety or radiological safety as a result of this event.
G. ADDITIONAL INFORMATION
A second SCRAM occurred January 4, 2013. The cause was the same as the December 29, 2012, SCRAM and is addressed in LER-2013-001-00. The instruments installed during the December 29, 2012, SCRAM enabled the determination of the cause of both SCRAMs.
Prior to December 29, 2012, there had been no SCRAMs in the past 3 years due to CT grounding. CR-GGN-2008-01476 documents a CT-related SCRAM but was not caused by CT grounding. The CT that caused both the December 29, 2012, SCRAM and the January 4, 2013, SCRAM was installed between February and April 2012.
The cause of SRV 1B21F047A closing slowly was determined to be a foreign material exclusion plug that was left in the valve. CR-GGN-2013-00100 documents this issue and the corrective actions being taken.
The cause of start-up outlet valve 1N21F010B not opening was determined to be an electrical overload breaker trip. The breaker was reset and the valve was returned to service. CR-GGN-2012-13297 documents this issue and the corrective actions that were taken.