05000416/LER-2008-005
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. N/A N/A | |
| Event date: | 10-26-2008 |
|---|---|
| Report date: | 12-15-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 4162008005R00 - NRC Website | |
On October 26, 2008 at 1125, Grand Gulf Nuclear Station was in Mode 1 operating at approximately 50 percent power when an automatic reactor scram was initiated by the Reactor Protection System (RPS) [JC] on detection of fast closure of the Main Turbine Control Valves (TCVs). After the scram reactor water level decreased to - 2.67 inches and was maintained with the reactor feedwater system [SJ]. The following occurrences were considered reportable:
Automatic actuation of Reactor Protection System (RPS) [JC]Automatic Scram (Reference:
10CFR50.73(a)(2)(iv) (A) & (B)(1)) Notification was made to the NRC's Emergency Notification System (ENS) reporting this condition pursuant to 10CFR50.72(b)(2)(iv)(B) and 10CFR50.72(b)(3)(iv)(A) and this event is reported under 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 50 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 October 26, 2008 at 1125 hours0.013 days <br />0.313 hours <br />0.00186 weeks <br />4.280625e-4 months <br />, Grand Gulf Nuclear Station was in Mode 1 operating at approximately 50 percent power when a reactor scram was initiated by the Reactor Protection System (RPS) on detection of fast closure of the Main Turbine Control Valves (TCVs). The fast closure of the TCVs was the result of a Main Generator [TB] trip caused by a generator loss of field condition. The loss of field occurred when the Thyristor Voltage Regulator (TVR) transferred from automatic to a failed manual controller.
After the scram reactor level decreased due to shrinkage to a level of - 2.67 inches. Water level in the Reactor Pressure Vessel (RPV) was maintained by Reactor Feed Pump Turbine (RFPT) "A" using startup level control. Since reactor feedwater was available, this was not a SCRAM with complications.
The normal heat sink (main condenser) remained available after the scram.
All control rods fully inserted to position 00. All safety systems performed as designed, there were no Emergency Core Cooling System (ECCS) [BJ, BO] actuations, no vessel isolation (Main Steam Isolation Valves remained open), and no Safety/Relief Valves (SRVs) lifted. There were no radiological or industrial safety impacts as a result of this scram.
NRC FORM 366A� (9-2007) U.S. NUCLEAR REGULATORY COMMISSION I� SEQUENTIAL Grand Gulf Nuclear Station, Unit 1 05000416 2008 --�� 005 --�� 00 __� D. CAUSE of OCCURRENCE The cause of this event was due to the Main Generator Thyristor Voltage Regulator ( TVR ) transferring from automatic control to manual control resulting in an unexpected under excited condition which tripped the main generator. It was determined that the TVR card manual reference setter was not tracking the automatic reference setter due to a defective manual reference setter motor operated potentiometer (MOP).
The root cause of this event was determined to be lack of a preventive maintenance (PM) strategy for the TVR control system to ensure reliability of the voltage control circuits.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions — The manual reference setter motor operated potentiometer for the TVR was replaced and verified to operate properly.
Long Term Corrective Actions - Condition Report GGN-2008-6241 was written and will address any additional actions.
F. SAFETY ASSESSMENT
Immediate actions performed by the Operations staff were adequate and appropriate in placing and maintaining the reactor in safe shutdown condition. No margin of safety was affected or encroached. All control rods fully inserted to position 00. All safety systems performed as designed, there were no Emergency Core Cooling System (ECCS) actuations, no vessel isolation (MSIV's remained open), and no Safety/Relief Valves (SRVs) lifted. There were no radiological or industrial safety impacts as a result of this scram.
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 isolated position. The Group 2 and 3 isolations were for valves in the same system (Residual Heat Removal).
No damage occurred as a result of the turbine trip and all equipment responses were as designed and expected.
This event did not prevent the fulfillment of a safety function therefore there were no safety system functional failures. Based on the discussion provided, the health and safety of the public was not compromised by this event.SQDF
G. ADDITIONAL INFORMATION
Previous Similar Events - Pursuant to 10CFR50.73(b)(5) this issue is considered an infrequent event.
There has not been any occurrence of the same underlying concern in the past two years at Grand Gulf Nuclear Station.