05000341/LER-2008-001
| Docket Number01 31 2008 2008 -8001 -8800 03 27 2008 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 43948 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3412008001R00 - NRC Website | |
Initial Plant Conditions:
Mode� 1 Reactor Power� 100 percent
Description of the Event
At 15:44 hours EST January 31, 2008, the reactor mode switch was taken to shutdown in response to the trip of both reactor recirculation pumps. This inserted a manual reactor scram within 9 seconds of the trip of the recirculation pumps. The reactor protection system (RPS) [JD] performed as expected, and all rods were fully inserted into the core. Post scram feedwater logic [FCO] actuated, as expected. Reactor water level reached a low of approximately 157 inches above top of active fuel and recovered to normal automatically without operator intervention. Subsequent to the event, the main steam isolation valves (MSIVs) remained open and reactor water level was maintained in the normal band of 173 to 214 inches. Reactor water was supplied by the condensate [SD] and reactor feedwater systems [SJ], and the resultant reactor steam was sent to the condenser [SG] via the main turbine bypass lines. Pressure control was maintained by the main turbine bypass valves. Reactor dome pressure peaked at about 1077 psig. With reactor pressure maintained below the Safety Relief Valve (SRV) setpoints, none of the SRVs lifted. Reactor water Level 3 isolations [JM] occurred as expected. These included isolation Group 4 (Residual Heat Removal Shutdown Cooling and Head Spray), Group 13 (Drywell Sumps), and Group 15 (Traversing In-core Probe System) isolations. There was no maintenance or testing in progress that would explain the pump trips. No operator performance issues were identified in response to this event.
At the time of the scram all emergency core cooling systems (ECCS) and Emergency Diesel Generators (EDGs) [DG] were operable with the exception of the Division 1 Residual Heat Removal (RHR) [BO] system which was in surveillance testing. It was available if needed. The operating RHR pump was stopped and the system returned to standby by approximately 15:55 hours. Reactor water level was maintained above Level 2, and as expected, none of the isolations and safety injection systems initiations associated with Level 2 occurred. Safety related plant equipment responded as expected to the manual reactor scram.
Subsequent troubleshooting determined that the recirculation pump trips were received from the 65G bus undervoltage relay logic.
Significant Safety Consequences and Implications This event posed no significant safety implications because the reactor protection and safety related systems functioned as designed following the manual reactor trip. The transient was compared to the UFSAR transient analysis for the trip of both recirculation pumps, and the actual response was enveloped by the UFSAR analysis.
Therefore the health and safety of the public were not affected by this event.
This event is being reported under 10 CFR 50.73(a)(2)(iv)(A), as an event or condition that resulted in manual actuation of the reactor protection system that resulted in a reactor scram. A 4-hour non-emergency notification was made to the NRC at 18:06 EST on January 31, 2008 (EN 43948) in accordance with 10 CFR 50.72(b)(2)(iv)(B) for an actuation of the reactor protection system.
Cause of the Event
The manual scram was inserted in response to a trip of both recirculation pumps in accordance with plant procedures. The cause of the recirculation pump trips was determined to be an intermittent failure of the 65G bus undervoltage trip logic. This could have been caused by a failure of the undervoltage relay, or by degraded connections in the relay circuit resulting in the relay sensing a low voltage condition and tripping in response to that condition. Field investigation determined that the intermittent failure was likely caused by a degraded relay cutoff switch in the undervoltage trip logic. The specific cause, however, could not be identified by failure analysis.
Corrective Actions
Since the failure was determined to be intermittent, additional corrective measures were taken to address possible faults based on a failure modes and effects analysis. The undervoltage relay and its cutoff knife switches were replaced, and the original components were sent out for analysis. The undervoltage circuit fuses were replaced, fuse clips tightened, connections cleaned, continuity checked, and the circuit was meggered to identify any potential faults prior to the restart of the unit. The failure analysis was inconclusive.
This event is documented and evaluated in the Fermi 2 corrective action program. Other actions are being planned to address this event. These actions will be tracked and implemented by the corrective action program.
Additional Information
A. Failed Components: Unknown B. Previous LERs on Similar Problems:
There have been no previous events involving a manual trip in response to an undervoltage trip of the recirculation pumps.