05000281/LER-2014-002
| Surry Power Station, Unit 2 | |
| Event date: | 10-13-2014 |
|---|---|
| Report date: | 1-2-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2812014002R00 - NRC Website | |
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1.0 DESCRIPTION OF THE EVENT On October 13, 2014 at 07:58, with Units 1 and 2 at 100% power, Unit 2 experienced a reactor trip due to a spurious opening of the Unit 2 "B" reactor trip breaker (RTB). A loose screw at terminal 1 on the "B" RTB reactor protection trip relay [EIIS-JC-RLY] RT2YB in the reactor protection system caused a fault in the circuit to the "B" RTB under-voltage coil, tripping the reactor without a reactor first out annunciator, and initiating a turbine trip.
The opening of "B" RTB removed power from the Control Rod Drive Mechanisms causing the control rods to drop and the delta flux to become very negative inducing a penalty on the overpower differential temperature (OPDT) and overtemperature differential temperature (OTDT) setpoints. The second "A" RTB opened due to the OPDT and OTDT set points being exceeded. As a result, Unit 2 was initially thought to have tripped due to the spurious OPDT signal as reported in the 10CFR50.72 reports.
The plant responded to the reactor/turbine trip as designed. All three auxiliary feedwater (AFW) pumps [EIIS-BA-P] automatically initiated on low-low steam generator (SG) water level providing flow to the SGs [EllS-AB-SG]. A rapid increase in SG pressure on the relief valve setpoints resulted in the lifting of all three SG power operated relief valves (PORVs) [EllS-SB-RV] as designed. The combination of the PORVs lifting, the actuation of the steam dumps and the initiation of AFW resulted in the Unit 2 reactor coolant system (RCS) cooling down below the nominal temperature of 547°F to 542°F. At 08:28, main feedwater flow was re-established to control SG level. AFW was subsequently secured by procedural guidance and Unit 2 was placed in hot shutdown using normal operating procedures.
The Source Range Nuclear Instrument (SRNI) detectors [EIIS-IL-RI] did not energize automatically because the Intermediate Range Nuclear Instruments N-35 and N-36 did not decay to were energized manually in accordance with abnormal procedures.
At 11:13, a four-hour report was made pursuant to 10CFR50.72(b)(2)(iv)(B) due to valid automatic actuation of Reactor Protection Systems and an eight-hour report was made pursuant to 10CFR50.72(b)(3)(iv)(A) due to automatic actuation of the AFW System.
This report is being submitted pursuant to 10CFR50.73(a)(2)(iv)(A) as an event that resulted in the automatic actuation of the Reactor Protection System and the AFW System.
Surry Power Station 05000 - 281 2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event resulted in no safety consequences or implications. There was no testing or surveillances in progress when the reactor trip occurred. Appropriate operator actions were taken in accordance with emergency operating procedures and the unit was quickly brought to a stable condition. Station equipment relied upon to mitigate the event responded as designed. Therefore, the health and safety of the public were not affected.
3.0 CAUSE The direct cause for the automatic reactor trip was determined to be a loose screw at terminal 1 on Unit 2 "IV RTB reactor protection trip relay RT2YB.
4.0 IMMEDIATE CORRECTIVE ACTION(S) The loose screw at the terminal on the Unit 2 "B" RTB reactor protection trip relay was tightened and the breaker was closed successfully.
5.0 ADDITIONAL CORRECTIVE ACTIONS The remaining electrical connections on the U2 reactor trip breaker "A" & "B" relay control RTB relay control circuitry will be checked for tightness at the next available opportunity.
The SRNI detectors not energizing automatically and the Unit 2 RCS cooling below the nominal temperature of 547°F to 542°F have been entered into the corrective action program.
6.0 ACTIONS TO PREVENT RECURRENCE Torque requirements for terminal screw sizes #2, #4 and #6 will be incorporated into the Surry Electrical Installation Specification, and applicable procedures will be identified and revised to include the torque requirements.
7.0 SIMILAR EVENTS None 8.0 MANUFACTURER/MODEL NUMBER Westinghouse Electric Corporation/ BFD22S 9.0 ADDITIONAL INFORMATION Unit 1 was at 100% power and remained unaffected by the Unit 2 reactor trip.