05000416/LER-2008-002
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/Anumber No. N/A | |
| Event date: | 03-21-2008 |
|---|---|
| Report date: | 05-15-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| LER closed by | |
| IR 05000416/2008004 (17 October 2008) | |
| 4162008002R00 - NRC Website | |
On March 21, 2008 at 1525 hours0.0177 days <br />0.424 hours <br />0.00252 weeks <br />5.802625e-4 months <br /> Grand Gulf Nuclear Station experienced an automatic reactor shutdown (scram) from 100 percent power due to a main generator trip caused by a "C" phase unit differential lockout. The main generator trip resulted in a main turbine control valve fast closure which initiated the reactor protection system (RPS) and resulted in the automatic reactor shutdown. The initiation of RPS resulting in an automatic reactor scram is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A).
B. INITIAL CONDITIONS
At the time of the event, the reactor was in OPERATIONAL MODE 1 with reactor power at approximately 100 percent. There were no additional inoperable structures, systems, or components at the start of the event that contributed to the event.
C. DESCRIPTION OF OCCURRENCE
On March 21, 2008 at 1525 hours0.0177 days <br />0.424 hours <br />0.00252 weeks <br />5.802625e-4 months <br /> Grand Gulf Nuclear Station was in Mode 1 operating at 100 percent power when the main generator [TB] tripped. The main generator trip initiates a main turbine control valve (TCV) fast closure which in turn resulted in a RPS full automatic reactor scram. All control rods fully inserted into the core and all other systems performed as designed.
The minimum reactor water level recorded during this event was -8.2 inches wide range therefore no Emergency Core Cooling System (ECCS) initiation occurred (ECCS initiation is at -41.6 inches). Six safety relief valves (SRV) lifted momentarily and then closed at the onset of the event. After SRV closure, reactor pressure was controlled with the main turbine bypass valves and reactor level was maintained the entire time using normal condensate and feedwater. This was not a scram with complications.
D. CAUSE of OCCURRENCE Indications are that the main generator trip was the result of a "C" phase unit differential lockout. No definitive cause for the "C" phase unit differential lockout was identified. Three potential causes were identified. The first is possible failure of the current transformers (CTs) associated with the "C" phase unit differential scheme. The second is a possible internal problem with the "C" phase unit differential relay. And the third is intermittent CT secondary circuit problem (wiring and terminations).
Immediate Corrective Actions —. As a result of trouble shooting the following immediate corrective actions were taken. The Low voltage CT circuit wiring and terminations were inspected to the extent possible. The Main Generator Neutral bushing "C" phase CT in the circuit had been replaced in the previous refuel outage, however, as a precaution, another CT was wired into the circuit. As a precaution, the suspect "C" phase unit differential relay was replaced. An inspection of the high voltage circuit was also conducted as part of troubleshooting. No evidence of an actual fault condition was found. No evidence of animal intrusion in the high voltage bus work was found. Additionally, the calibration of the "A" and "B" relays was checked and found to be acceptable.
Long Term Corrective Actions - Condition Report GGN-2008-01476 was written and will address any additional actions.
F. SAFETY ASSESSMENT
Immediate actions performed by operators were adequate and appropriate in placing and maintaining the reactor in a safe shutdown condition. No margin of safety was affected or encroached. All safety systems responded as designed in this event.
The reactor water level did not reach Level 2 (-41.6 inches), therefore no Emergency Core Cooling System (ECCS) initiations occurred. The Group 2 and 3 Primary Containment Isolation initiated at Level 3 (+11.4 inches) however no valves changed position because these valves were already in the normal closed position. The Group 2 and 3 isolations were for valves in the same system (Residual Heat Removal system).
Six safety relief valves opened momentarily then reclosed at the onset of this event to control reactor pressure. Reactor pressure was controlled with the main turbine bypass valves thereafter and reactor level was controlled with normal condensate and feedwater. The normal heat sink (main condenser) remained available during this event, therefore this was not a scram with loss of heat sink.
The event did not prevent the fulfillment of a safety function therefore it was not a safety system functional failure. Based on the discussion provided, the health and safety of the public was not compromised by this event.
G. ADDITIONAL INFORMATION
Previous Similar Events - Pursuant to 10CFR50.73(b)(5) a review was conducted to determine whether this was an infrequent event. There has not been any occurrence of the same underlying concern in the past two years at Grand Gulf Nuclear Station.