05000416/LER-2007-003
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/Anumber No. N/A | |
| Event date: | 08-21-2007 |
|---|---|
| Report date: | 10-15-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4162007003R00 - NRC Website | |
On August 21, 2007 at 1415 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.384075e-4 months <br /> Grand Gulf Nuclear Station experienced an automatic reactor shutdown (SCRAM) from 100 percent power due to decreasing reactor water level caused by partial loss of feed water. Reactor water level decreased to Level 3 (+11.4 inches) which per design, initiated the Reactor Protection System (RPS) SCRAM signal. The initiation of RPS resulting in an automatic reactor SCRAM is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A) as a 60 day Licensee Event Report.
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 August 21, 2007 Instrumentation and Controls (I&C) technicians were taking power supply voltages and alternating current ripple measurements in the "A" Bailey INFI-90 digital Feed Water [SJ] control panel. The technicians had completed taking a reading and were in the process of removing their probes from the panel when they noticed an arc followed by the sound of relays changing position resulting in an unexpected Power Failure Interrupt (PFI) signal being generated. This signal drove the "A" Reactor Feed Pump governor control valve to minimum position which lead to a reactor SCRAM on Level 3 (+11.4 inches) low reactor water level. The "A" Feed Pump continued to run which prevented the logic for the Reactor Recirculation System from detecting this condition and initiating a Reactor Recirculation Flow Control Valve "run back" to minimum position to reduce reactor power within the capacity of one Reactor Feed Pump to maintain reactor vessel level. Control room operators tripped the "A" Reactor Feed Water Pump to initiate a Reactor Recirculation Flow Control Valve run back however the timing of these efforts did not prevent the reactor water level from reaching the Level 3 (+11.4 inch) Reaction Protection System SCRAM set point.
The normal heat sink (main condenser) remained available therefore, no Main Steam Safety Relief Valves (MSRV) actuations occurred during the event. All control rods fully inserted and all safety systems functioned as designed and responded properly. This was not a SCRAM with complications.
J Investigation to this point has not determined a definitive cause for this event. The panel was thoroughly inspected and no evidence of any failed or damaged components was found. The condition could not be duplicated.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions — Completed inspection of the power entry panel.
The bus monitoring module, auto transfer module, power supply, and the power entry panel were replaced.
Long Term Corrective Actions - Condition Report GGN-2007-04128 was written and will address any additional action that may be necessary as a result of future testing.
F. SAFETY ASSESSMENT
All safety systems responded as designed in this event. No Emergency Core Cooling System (ECCS) initiations were required to maintain reactor vessel inventory. While the Group 2 and 3 Primary Containment Isolation set point of +11.4 inches was reached, no valves changed position because these valves were already in the normal isolated position.
The health and safety of the public was not compromised by this event. This condition did not prevent the fulfillment of a safety function therefore it was not a safety system functional failure.
G. ADDITIONAL INFORMATION
Previous Similar Events - Pursuant to 10CFR50.73(b)(5) this issue is to be an infrequent event. There has not been any occurrence of the same underlying concern in the past two years at Grand Gulf Nuclear Station.