05000315/LER-2011-001
| Donald C. Cook Nuclear Plant Unit 1 | |
| Event date: | 09-07-2011 |
|---|---|
| Report date: | 11-07-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3152011001R00 - NRC Website | |
Conditions Prior to Event 100 percent reactor power.
Description of Event
On September 07, 2011, at 0854 hours0.00988 days <br />0.237 hours <br />0.00141 weeks <br />3.24947e-4 months <br />, Donald C. Cook Nuclear Plant (CNP) Unit 1 Reactor [AC] tripped automatically due to a trip of the main turbine [TRB]. All control rods [AA] fully inserted and the auxiliary feedwater system (AFW) [BA] started and performed as designed.
The reactor trip was uncomplicated and all major plant components functioned as designed; as such, there were no safety system functional failures. The reactor trip was reported in accordance with 10 CFR 50.72(b)(2)(iv)(B) and the AFW actuation was reported in accordance with 10 CFR 50.72(b)(3)(iv)(A). The reactor trip and AFW actuation pre reportable as a Licensee Event Report (LER) in accordance with 10 CFR 50.73(a)(2)(iv)(A).
Cause of Event
The main turbine tripped due to an automatic turbine trip signal generated by the main turbine thrust bearing wear detection system [JJ]. The initial investigation has concluded that there was no actual thrust bearing wear condition. The cause of the inadvertent trip signal has been determined to be a result of inadequate installation of sensing equipment. This inadequate installation resulted in a spurious trip signal common to both channels. Corrective actions have been taken to correct the installation. A Root Cause Evaluation (RCE) is in progress, and a supplement to this LER will be submitted following the evaluation if results are substantially different than what is being reported here.
Analysis of Event
The event is not considered to be risk significant as there were no risk significant equipment failures to pose elevated risk. It is recognized that there was an actuation of a main turbine protective circuit; subsequently, all systems responded as designed.
Based on review of the control room log and Plant Process Computer [CPU] information, along with the post-trip review from which the information above was obtained, all plant systems performed as designed to shut down the unit and remove decay heat. No risk-significant equipment functions were affected or failed and no significant operator actions outside those required for normal trips were required.
Corrective Actions
Completed Corrective Actions
The following corrective actions were taken to correct the installation inadequacy:
The thrust probes have been routed through separate conduits to provide circuit separation.
The thrust probe connectors have been sealed to keep lubricating oil out.
The thrust probe cabling in the junction boxes has been wrapped with an EMI (Electro Magnetic Interference) mesh tape to protect circuits from cross-communication interference.
Planned Corrective Actions
None.
Previous Similar Events
LERs for CNP Unit 1 and Unit 2 for the past three years were reviewed for similar events. While there have been manual reactor trips, there have been no automatic reactor trips due to actuation of main turbine trip circuitry.