05000416/LER-2013-001
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 01-04-2013 |
|---|---|
| Report date: | 03-05-2013 |
| 4162013001R00 - NRC Website | |
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 (EllS:JC) (RPS). Telephonic notification was made to the U.S. Nuclear Regulatory Commission (NRC) Emergency Notification System on January 5, 2013, 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 94 percent. There were no additional inoperable structures, systems, or components at the start of the event that contributed to this event.
C. DESCRIPTION OF OCCURRENCE
At 23:37 Central Standard Time on January 4, 2013, Grand Gulf Nuclear Station experienced an unexpected Reactor SCRAM caused by a Main Generator (EIIS:TB) trip.
The plant was operating in Mode 1 at 94 percent thermal power. All safety systems responded per design. Safety Relief Valves (EIIS:RV) (SRVs) opened at the onset of the event to control reactor pressure and reseated properly. All control rods (EIIS:ROD) inserted when the signals generated by the RPS were received. There were no Emergency Core Cooling System actuations. The shift immediately entered the appropriate Off Normal Event Procedures. The plant was stabilized with pressure control on the main turbine bypass valves (El IS:PCV) and level control on the start-up level control valve (El ISICV). High pressure feedwater heater start-up outlet valve (start-up outlet valve) 1N21F010B did not open when the start-up level control valve was placed in service but did not prevent Operations from controlling the reactor water level.
The plant responded to the trip as designed with the exception of the one start-up outlet valve noted above.
D. CAUSE
The cause of the SCRAM was the Main Generator 'A' Phase Neutral Current Transformer (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 inadequate work instructions not specifying the clearance during installation.
E. CORRECTIVE ACTIONS
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 Entergy 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. All safety systems responded as designed. The breaker thermal for the 1N21F010B start-up outlet valve was reset and returned to service and did not prevent Operations from controlling the reactor water level.
Immediate actions performed by the Operations staff were adequate and appropriate in placing and maintaining the reactor in a safe shutdown condition.
Nuclear safety was not significantly compromised because safety related equipment necessary to safely shutdown the unit performed its safety function.
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.
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
The cause was the same as the December 29, 2012, SCRAM, which is addressed in LER-2012-008-00 and CR-GGN-2012-13290. The instruments installed following the December 29, 2012, SCRAM enabled the determination of the cause of both SCRAMS.
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 start-up outlet valve 1N21F010B not opening was determined to be a thermal overload breaker trip. The breaker thermal was reset and the valve was returned to service. CR-GGN-2012-13297 documents this issue and the corrective actions that will be taken as part of a work order during the next refueling outage.