05000281/LER-2015-001
| Surry Power Station, Unit 2 | |
| Event date: | 07-21-2015 |
|---|---|
| Report date: | 09-18-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2812015001R00 - NRC Website | |
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1.0 DESCRIPTION OF THE EVENT On July 21, 2015 with Unit 1 at Hot Shutdown, Unit 2 was starting up following a shutdown to repair leakage to a pressurizer spray valve. With the reactor critical and stable at approximately 6% power, the turbine [EIIS-TA] was latched at 04:50 to perform the Unit 2 Turbine Overspeed Protection Control (OPC) system circuitry test. Operators were using operating procedure 2-OP-TM-001, "Turbine-Generator Startup to 20%-25% Turbine Power" when, by procedure, they entered a terminal speed of 550 rpm into the turbine control system [EIIS-IT] and initiated turbine rollup.
This started the reference counter [EIIS-IT-CTR] pulsing up to achieve the desired turbine endpoint speed. The reference counter receives input signals and converts them into an analog voltage signal which inputs the desired end-point speed of the turbine to the circuit, and controls the final position of the governor valves [EllS-SB- PCV].
Immediately after turbine rollup was initiated, the overspeed switch [EIIS-IT-SIS] was taken to the overspeed simulation position to test the circuit and the governor valves closed as expected. With the governor valves closed for the test, an error signal between turbine speed and reference signal was generated. When the overspeed switch was returned to the normal position, the actual turbine speed was 140 rpm with a reference signal of 230 rpm. The governor valves rapidly opened due to the speed error and caused a rapid pressurization of the turbine impulse chamber. A turbine trip signal was generated due to exceeding the 15% impulse pressure with the generator offline. Since indicated impulse pressure was greater than 10%, a reactor trip was generated by the turbine trip.
All systems functioned as required. Initiation of auxiliary or emergency systems was not required. Decay heat removal was maintained by dumping steam to the Main Condenser [EIIS-WI], and steam generator water level was maintained by the Main Feedwater system [EIIS-SJ]. Unit 2 was stabilized at hot shutdown.
At 0740 hours0.00856 days <br />0.206 hours <br />0.00122 weeks <br />2.8157e-4 months <br /> on July 21, 2015, a non-emergency, four-hour notification was made to the NRC pursuant to 10CFR50.72(b)(2)(iv)(B), any event or condition that results in actuation of the reactor protection system (RPS) when the reactor is critical.
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.
Surry Power Station 2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event resulted in no safety consequences or implications. Appropriate operator actions were taken in accordance with emergency operating procedures and the unit remained in a stable condition. Station equipment relied upon to mitigate the event was available and responded as designed. Therefore, the health and safety of the public were not affected.
3.0 CAUSE The direct cause of the trip was the governor valves opening due to a speed error between the turbine speed and reference speed. The root cause of this event is inadequate instructions. The initial plant and test conditions established by 2-OP- TM-001 were modified in 2003 and again in 2010 which introduced the potential to generate a speed error.
4.0 IMMEDIATE CORRECTIVE ACTION(S) A Unit 2 Event Review Team was established to perform an investigation of the event.
5.0 ADDITIONAL CORRECTIVE ACTIONS A root cause evaluation was performed to identify the cause(s) and recommend corrective actions.
6.0 ACTIONS TO PREVENT RECURRENCE Performance of OPC testing will be removed from operating procedures 1/2-OP-TM- 001 and placed in test procedures that are not performed as part of routine turbine startup.
7.0 SIMILAR EVENTS None 8.0 MANUFACTURER/MODEL NUMBER Not applicable 9.0 ADDITIONAL INFORMATION Unit 1 was at Hot Shutdown and remained unaffected by the Unit 2 reactor trip.