05000341/LER-2010-001
| Docket Number03 25 2010 2010 - 001 -000 05 19 2010 05000 | |
| Event date: | 03-25-2010 |
|---|---|
| Report date: | 05-19-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 45789 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3412010001R00 - NRC Website | |
Initial Plant Conditions:
Mode 1 Reactor Power 63 percent
Description of the Event
At 16:27 EDT on March 25, 2010, the reactor mode switch [HS] was taken to shutdown following an automatic scram due to a main turbine [TA] trip. The Operations Department was in the process of raising power prior to the event. The Main Turbine Trip relays actuated causing a turbine control valve fast closure which initiated a scram signal. Investigation determined that the Main Turbine Trip relays actuated due to a generator differential current relay trip in the Main Generator Z phase protection circuit. The Z phase relay actuation was caused by a shorted current transformer wire in the Main Generator Z phase line terminal bushing enclosure.
The scram was uncomplicated. The reactor protection system (RPS) [JD] performed as expected, and all control rods were fully inserted into the core. Reactor water level reached a low of approximately 135 inches above top of active fuel and recovered to normal level of 173 to 214 inches automatically with the reactor feedwater system [SJ] following the scram without operator intervention. Reactor water Level 3 isolations occurred and Reactor water level was maintained above Level 2 as expected. No primary containment isolations or safety injection system initiations associated with Level 2 occurred.
Reactor pressure control was maintained by the turbine bypass valves. The peak Reactor Pressure was 1000 prig, within normal pressure control limits and no safety relief valves (SRVs) actuated.
There was no maintenance or testing in progress that would affect generator differential relaying. Safety related plant equipment responded as expected to the reactor scram. At the time of the scram, all Emergency Core Cooling Systems (ECCS) and Emergency Diesel Generators (EDGs) [DG] were operable.
This report is being made in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in an unplanned manual or automatic actuation of the reactor protection system (RPS) when the reactor was critical.
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 automatic reactor trip. Important safety-related and non-safety related equipment performed as discussed in the description of the event, and plant response was as expected. There was no significant increase in reactor pressure, and the reactor core was adequately covered and cooled throughout the event.
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 automatic actuation of the reactor protection system and a reactor scram. A 4-hour non-emergency notification was made to the NRC Operations Center at 18:18 EDT on March 25, 2010 (EN 45789) in accordance with 10 CFR 50.72(b)(2)(iv)(B) for an actuation of the reactor protection system.
Cause of the Event
The automatic reactor scram was due to Main Turbine Trip relays actuating causing a turbine control valve fast closure. Investigation determined that the Main Turbine Trip relays actuated due to a generator differential current relay trip in the Main Generator Z phase protection circuit. The cause of the Z phase relay actuation was the result of a shorted current transformer (CT) wire in the Main Generator Z phase line terminal bushing enclosure.
One of the wires connected to the generator differential protection current transformer, installed on the Main Generator Z phase line terminal bushing, shorted to ground inside the CT/bushing enclosure. The impedance/current imbalance in the Z phase differential protection circuit, caused by the ground short condition, resulted in actuation of the Z-87G protection relay, energizing the Main Turbine Trip relays and causing the turbine control valve fast closure logic to initiate an automatic reactor scram.
The CT wire failure was due to abrasion at the conduit entrance to the CT/bushing enclosure. The conduit bulkhead connecter utilizes a nylon insulated throat that was found degraded at the location where the wire shorted to ground.
Corrective Actions
The shorted CT wire was located and repaired in accordance with plant design specifications. Protective layers of tape were added to this and other similar generator CT wiring in locations where the wiring interfaces with the conduit entries as a preventive measure for all output and neutral phases. This event is documented and evaluated in the Fermi 2 corrective action program. Other actions are being considered to address this event. These actions will be tracked and implemented by the corrective action program.
Additional Information
A. Failed Components: The conduit that failed was a Thomas & Betts (T&B) insulated throat fitting used for jacketed flexible liquid tight conduit entry.
B. Previous LERs on Similar Problems: None