05000280/LER-2008-001
| Surry Power Station | |
| Event date: | 04-20-2008 |
|---|---|
| Report date: | 06-16-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2802008001R00 - NRC Website | |
1.0 DESCRIPTION OF THE EVENT On April 18, 2008, prior to startup from a 4-day outage for repairs of a Feedwater heater [EllS-SJ, HX] tube leakage, a turbine balance move was completed in accordance with recommendations provided by the turbine vendor. While the previous operating vibrations were acceptable, the balance move was recommended to eliminate vibrations during turbine rollup through critical speed.
At 2143 hours0.0248 days <br />0.595 hours <br />0.00354 weeks <br />8.154115e-4 months <br /> on April 20, 2008, with reactor power at 54%, ramping of the unit was secured due to high vibrations on the #4 turbine bearing [El IS-TA, TRB]. The #4 turbine bearing vibrations indicated 13.76 mils and was increasing. The operating team initiated 0 AP-23, "Rapid Load Reduction", to perform a rapid load reduction in response to the high turbine vibrations. As the ramping rate decreased, turbine vibrations continued to fluctuate between 14 and 13.8 mils. At 2216 hours0.0256 days <br />0.616 hours <br />0.00366 weeks <br />8.43188e-4 months <br />, with reactor power at 37% and sustained turbine vibrations greater than 14 mils, the reactor was manually tripped as directed by the annunciator response procedure. The Main Turbine tripped as a result of the reactor trip.
The operating staff stabilized the unit at Hot Shutdown and verified the proper response of the automatic protection systems following manual actuation of the reactor trip. Following the trip, all three auxiliary feedwater (AFW) pumps [EIIS-BA, P] automatically initiated, as designed, on low-low steam generator level. The Reactor Coolant System (RCS) cooled down to a minimum temperature of 538 DEGF due to the combination of low decay heat and full auxiliary feedwater flow. At 2223 hours0.0257 days <br />0.618 hours <br />0.00368 weeks <br />8.458515e-4 months <br />, operators manually secured AFW flow to all 3 steam generators. RCS temperature began to increase following reduction of AFW flow and eventually recovered to 547 DEGF at 2311 hours0.0267 days <br />0.642 hours <br />0.00382 weeks <br />8.793355e-4 months <br />. The steam generators were restored to normal operating levels and the AFW pumps were secured. Main Feedwater flow was established on Feedwater Bypass Valves at 2257 hours0.0261 days <br />0.627 hours <br />0.00373 weeks <br />8.587885e-4 months <br />.
At 0037 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br /> on April. 21, 2008, non-emergency four-hour and eight-hour notifications were made to the NRC pursuant to 10 CFR 50.72(b)(2)(iv), any event that results in activation of the reactor protection system when the reactor is critical, and 10 CFR 50.72(b)(3)(iv), any event that results in the activation of the auxiliary feedwater system.
Updated vendor turbine balance information was received and independently verified by Dominion personnel. The turbine was rebalanced and the unit placed on line April 21, 2008 at 2348 hours0.0272 days <br />0.652 hours <br />0.00388 weeks <br />8.93414e-4 months <br />. The unit was successfully ramped to 100% reactor power with acceptable turbine vibrations.
This report is being submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A) for a manual actuation of the reactor protection system and automatic actuation of the auxiliary feedwater system.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS When the vibrations experienced during the unit restart were observed, the ramp was stopped with reactor power at 54% and a rapid ramp to reduce reactor power was initiated.
Turbine load was reduced until the unit reached 37% reactor power. Sustained vibrations were above 14 mils and, as required by the annunciator response procedure, the reactor was manually tripped. The reactor trip and associated safety system actuations occurred as designed, resulting in no significant safety consequences or implications. There were no radiation releases and no personnel injuries or contamination events. Therefore, the health and safety of the public were not affected.
3.0 CAUSE During turbine roll up following the 2007 Unit 1 refueling outage (RFO), high vibrations were observed on the #4 turbine bearing at 945 rpm (critical speed). A balance shot was performed on the #5 turbine bearing, as directed by the turbine vendor, and turbine was taken to 1800 rpm with acceptable vibration results. At 30% reactor power, vibrations were observed on the #5 and #6 bearing, however, as the unit ramp increased, the vibrations slowly trended down. The unit reached 100% reactor power on 12/3/07 at 1721 hours0.0199 days <br />0.478 hours <br />0.00285 weeks <br />6.548405e-4 months <br /> with acceptable turbine vibrations.
The turbine vendor recommended that, while the vibrations were currently acceptable, additional balance moves should be made at the next opportunity to eliminate vibrations during turbine rollup through critical speed. The vendor balance information was published in the recommendations section of the Surry 2007 Unit 1 Refueling Outage (RFO) Report.
A root cause evaluation (RCE) determined that the vendor recommendations to perform a balance move on bearings #4 and #5 on Low Pressure turbine #1 (LP1) and on bearing #6 on LP2 were incorrect. The report should have recommended a balance move on bearings #3 and #4 on LP1 and on bearing #6 on LP2. The vendor attributed the report error as being a typographical error in their startup report. Also, a contributing cause for this event was the lack of a systematic process for conducting independent reviews of vendor provided balance move recommendations.
4.0 IMMEDIATE CORRECTIVE ACTION(S) A Post Trip Review was conducted and the report reviewed and approved.
The balance weights were independently verified by station personnel to be installed in the correct positions as recommended by the vendor report. The vendor was contacted and the error in the balance report was identified. A new balance plan correcting the error was provided by the vendor and the turbine was rebalanced by repositioning the balance weights. All balance weights were independently verified in their correct as left position.
On 4/21/08, the turbine was rolled up to 1800 rpm and vibrations were found to be � acceptable. The unit was placed on line at 2348 hours0.0272 days <br />0.652 hours <br />0.00388 weeks <br />8.93414e-4 months <br /> and ramped to 100% reactor power on 4/22/08 with vibrations remaining acceptable.
5.0 ADDITIONAL CORRECTIVE ACTIONS A root cause evaluation (RCE) was initiated.
6.0 ACTIONS TO PREVENT RECURRENCE turbine balance moves made after the 2007 Unit 1 refueling outage. To prevent recurrence, the vendor, with participation by Dominion personnel, will complete an evaluation of the event and implement internal corrective actions to prevent future occurrences.
7.0 SIMILAR EVENTS There were no similar events where the vendor supplied information on turbine balance moves that was implemented and later found to be incorrect.
8.0 MANUFACTURER/MODEL NUMBER Westinghouse Electric Corporation/BB-81 9.0 ADDITIONAL INFORMATION Unit 2 was at 96% power in an end of core life coast down and remained unaffected by the Unit 1 event.