05000416/LER-2013-002
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 01-14-2013 |
|---|---|
| Report date: | 03-15-2013 |
| 4162013002R00 - 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 14, 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 100 percent. There were no inoperable structures, systems, or components at the start of the event that contributed to this event.
C. DESCRIPTION OF OCCURRENCE
At 18:05 Central Standard Time on January 14, 2013, Grand Gulf Nuclear Station experienced an automatic Reactor SCRAM caused by a Turbine Trip due to Main Generator (EDS:TB) lockout. The plant was operating in Mode 1 at 100 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 (EDS: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 and Emergency Procedures. The plant was stabilized with pressure control on the main turbine bypass valves (EIIS:PCV) and level control on the start-up level control valve (El IS:LCV). High pressure feedwater heater start-up outlet valve (start-up outlet valve) 1N21F010B did not open when placing the start-up level control valve 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 this event is a vulnerability in the design configuration of the horizontal bushing in an energized section of the isophase bus, in close proximity to a degraded top viewing port, that allowed water accumulation that created a ground condition. A contributing cause was weaknesses in the response to previous condition reports (CRs), where it was concluded that the water in the isophase bus duct system was the result of condensation. Corrective measures were focused on the water source from condensation and did not consider the possibility of rain water intrusion due to the system being pressurized when in operation. One corrective action was to inspect the viewing ports on top of the ductwork. However, visual inspections were not performed on the top due to documentation of objective evidence that no water was observed after heavy rains.
E. CORRECTIVE ACTIONS
To correct the condition that caused this event, additional portions of the isophase bus with seal off bushings were de-energized. Covers were installed over the isophase bus duct viewing ports.
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 1N21 F01 0B 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. The highest noted pressure was approximately 1110 pounds per square inch gauge (psig). The lowest noted pressure was approximately 930 psig. The lowest noted reactor water level was 0.89 inch on narrow range.
Although a reactor SCRAM is an initiator, nuclear safety was not 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 health and safety of the public, industrial safety or radiological safety as a result of this event.
G. ADDITIONAL INFORMATION
The isophase bus ducts were modified during the extended power uprate project in 2012. No previous SCRAMs at the site were caused by water leaking into an isophase bus.
The cause of start-up outlet valve 1N21 F01 0B 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.