05000339/LER-2013-001
| North Anna Power Station Unit 2 | |
| Event date: | 05-10-2013 |
|---|---|
| Report date: | 07-02-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(C), 50.54(x) TS Deviation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3392013001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On May 10, 2013, at 0517 hours0.00598 days <br />0.144 hours <br />8.54828e-4 weeks <br />1.967185e-4 months <br /> with Unit 2 in Mode 1, 60 percent power, number 9 main turbine/generator bearing vibrations (EIIS SYS-TL, Component-38) began increasing from approximately 5 mils. Unit 2 was in the process of increasing power following a refueling outage when this occurred. At 0545 hours0.00631 days <br />0.151 hours <br />9.011243e-4 weeks <br />2.073725e-4 months <br /> an adjustment was performed to lower main generator reactive power. Following the adjustment, number 9 main turbine/generator bearing vibrations increased to approximately 9 mils. The main generator reactive power was immediately returned to the previous value with no corresponding improvement in vibration levels.
At 0602 hours0.00697 days <br />0.167 hours <br />9.953704e-4 weeks <br />2.29061e-4 months <br /> a slight ramp down in turbine load was commenced. The number 9 main turbine/generator bearing vibrations continued to slowly increase. At 0610 hours0.00706 days <br />0.169 hours <br />0.00101 weeks <br />2.32105e-4 months <br /> a noticeable decrease to 7 mils occurred. Concurrently, a report from the field notified the control room crew that a luminous discharge was observed inside the exciter enclosure.
At 0612 hours0.00708 days <br />0.17 hours <br />0.00101 weeks <br />2.32866e-4 months <br /> a manual reactor trip was initiated as a result of increased vibrations on the number 9 main turbine/generator bearing to 9.5 mils and a report of the luminous discharge in the Main Generator Exciter enclosure. All primary system parameters were normal for power operation at 60 percent power. All systems responded as expected following the manual trip. All control rods (EIIS SYS-AA, Component-ROD) inserted into the core at the time of the trip and decay heat was removed via the main condenser steam dumps. The auxiliary feedwater (AFW) pumps (EIIS SYS-BA, Component-P) received an automatic start signal due to the resulting low-low level in "C" steam generator (EIIS-SYS-AB, Component-SG). The AFW System operated as designed with no abnormalities noted.
The SG levels were restored to normal operating level. At 0745 hours0.00862 days <br />0.207 hours <br />0.00123 weeks <br />2.834725e-4 months <br /> the AFW pumps were secured and returned to automatic.
At 0859 hours0.00994 days <br />0.239 hours <br />0.00142 weeks <br />3.268495e-4 months <br />, when the switch for the Anticipated Transient Without Scram Accident Mitigation System Actuation Circuit (AMSAC) was taken to reset to clear "AMSAC ARMED" annunciator, the AFW system automatically actuated during simultaneous performance of 2-ES 0.1, Reactor Trip Response and 2-0P-7.11, Shutdown of Unnecessary Plant Equipment Following Entry Into EOPS. Due to the plant conditions at the time, there was low decay heat on Unit 2, which required minimizing AFW flow to the steam generators to limit cool down. Steam generator levels had not yet been restored to within the normal level band. AMSAC was placed in "BYPASSED" without first being "RESET". The AFW system was secured per steps in 2-ES-0.1 and returned to auto. This was an invalid actuation since actual plant conditions did not require the automatic start of the AFW System.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS No significant safety consequences resulted from this event since the Reactor Protection System (RPS) and the Engineered Safety Feature (ESF) System equipment responded as designed. As such, the event posed no significant safety implications and the health and safety of the public were not affected by the event.
On May 10, 2013, at 0820 hours0.00949 days <br />0.228 hours <br />0.00136 weeks <br />3.1201e-4 months <br />, a 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> report was made in accordance with 10CFR50.72(b)(2)(iv)(B) for RPS actuation and 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> report in accordance with 10CFR50.72(b)(3)(iv)(A) for the first AFW pump automatic start. This event is reportable per 10 CFR 50.73(a)(2)(iv)(A) for a condition that resulted in automatic actuation of the RPS and AFW System. The second AFW system actuation was invalid and is also being reported per 10 CFR 50.73(a)(2)(iv)(A).
3.0 CAUSE The direct cause of the event was a combination of the alignment dowel causing a ground on the number 9 bearing pedestal which allowed electrical erosion to remove material from the bearing surface causing a hydraulic rub on the number 9 bearing and improper bearing loading caused by misalignment of the number 9 bearing pedestal.
The root cause was determined to be the Fleet Turbine Group's (FTG) lack of technical expertise coupled with a less than optimal procedure for turbine activities led to an over reliance and inadequate challenging of the turbine vendor.
The cause of the second AFW system actuation when resetting AMSAC was determined to be a procedure weakness. Procedure 2-ES-0.1 did not direct the crew to observe the proper status lights that should be expected when AMSAC has been properly reset.
4.0 IMMEDIATE CORRECTIVE ACTION(S) The Control Room crew responded to the reactor trip in accordance with emergency procedure 2-E-0, Reactor Trip or Safety Injection. The post trip response progressed as expected and the Control Room crew transitioned to 2-ES-0.1, Reactor Trip Response. All equipment responded as designed.
5.0 ADDITIONAL CORRECTIVE ACTIONS Maintenance was performed to restore the equipment to proper functionality including replacement of the number 9 main turbine/generator bearing and the permanent magnet generator, fan and housing repairs, realignment of the exciter and number 9 pedestal, and proper installation of the number 9 pedestal alignment dowel. The hydrogen and air side seal oil temperature control was modified and the hydrogen and air side seal oil temperature indicators were calibrated.
6.0 ACTIONS TO PREVENT RECURRENCE The controlling procedure for reassembly of the turbine generator is being revised to ensure critical steps are noted and have the appropriate verifications to ensure that there is the proper level of vendor oversight, and ensure that any as left measurements that are unsatisfactory require a Condition Report to ensure there is the proper level of documentation and evaluation. Benchmarking is being performed to determine qualification/training requirements for turbine group personnel.
With regard to the second AFW pump start, procedure changes have been initiated to improve the wording contained in both 2-ES-0.1 and 2-0P-7.11. Lessons learned from the event will be discussed during upcoming sessions of the Licensed Operator Requalification Program (LORP).
7.0 SIMILAR EVENTS None.
8.0 ADDITIONAL INFORMATION Unit 1 was operating in Mode 1, 100 percent power on May 10, 2013 and was not affected by this event.
Description: Shoe type bearing Manufacturer: Siemens Part No.: 613F432G01