05000336/LER-2009-001
| Docket Number | |
| Event date: | 07-03-2009 |
|---|---|
| Report date: | 08-19-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3362009001R00 - NRC Website | |
1. Event Description At 1304 on July 3, 2009, with the Millstone Power Station Unit 2 at 100% power in Mode 1, the reactor automatically tripped due to a high pressurizer pressure reactor trip signal. There was a grid disturbance coincident with the time of the trip. Approximately 5 seconds prior to the trip, the four main turbine stop valves and four intermediate stop valves went fully closed. With the main turbine stop valves closed, the reactor coolant system (RCS) pressure increased to the high trip setpoint. The reactor automatically tripped and both pressurizer power operated relief valves (PORVs) [PCV] lifted to relieve pressure as designed. The reactor trip generated a turbine trip signal.
This event is being reported pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B).
2. Cause The most probable cause of this event is that the 24VDC power supply to the electro-hydraulic control (EHC) of the turbine valves did not tolerate the July 3, 2009 grid disturbance. As a result of the grid disturbance, the master trip solenoid valves (MTSVs) provided fluctuating emergency trip system (ETS) hydraulic pressure to the turbine valves.
This fluctuating hydraulic pressure caused the turbine stop and intermediate stop valves to close unexpectedly.
3. Assessment of Safety Consequences This event is considered to be of low safety significance. The loss of external load was initiated by the closure of the turbine stop valves. By design, the closure of the stop valves does not result in an immediate turbine trip which would cause a reactor trip (the reactor trip on turbine trip is not credited in the Final Safety Analysis Report (FSAR) Chapter 14 Safety Analysis). With the turbine stop valve closure, the plant experienced a decrease in heat removal from the secondary system. As a result, pressurizer pressure increased, and approximately 5 seconds later, the reactor tripped on high pressurizer pressure and the pressurizer PORVs opened as designed on the high pressurizer pressure trip signal. Following the high pressurizer pressure reactor trip, a turbine trip signal was generated, which closed the turbine control valves. Operation of the PORVs prevented a challenge to the pressurizer safety valves.
The RCS pressure remained below the maximum value of 2717 psia at the bottom of the reactor vessel predicted for the FSAR Section 14.2.1 Loss of External Load Event. Since the opening of the condenser dump valves was delayed due to the delayed turbine trip signal, the main steam safety valves were challenged. However, the Main Steam System pressure remained below the maximum value of 1086 psia predicted for the FSAR Section 14.2.1 Loss of External Load Event. Therefore, plant operation remained within the bounds of the FSAR Chapter 14 Safety Analysis. Based on the above, there was no adverse effect on the health and safety of the public.
4. Corrective Action Since the most probable cause of this event was that the 24VDC power supply to the EHC of the turbine valves did not tolerate the July 3, 2009 grid disturbance, a temporary modification was implemented prior to plant startup which augments the existing VR-11 power supply for the EHC Cabinet with an Uninterruptable Power Supply (UPS) [UJX].
The UPS is designed to maintain voltage during any short duration loss of VR-11.
Other long term corrective actions are being addressed in accordance with the Millstone Corrective Action Program.
5. Previous Occurrences On May 22, 2008 at 1359 with the Millstone Power Station Unit 2 at 100% power in Mode 1, the reactor automatically shutdown following a loss of load and subsequent turbine trip. Investigation determined that a lightning strike on a transmission line created an over current condition to which the unit responded. The direct cause of the switchyard breakers opening and remaining open was a mechanical failure of a connecting wire lug in the pilot wire circuitry causing an open circuit.
On June 28, 2008, at 1146 with the Millstone Power Station Unit 2 in Mode 1 at 100% power, operators manually tripped the reactor when both main feedwater pumps tripped. Main turbine combined intercept valve (CIV) testing was in progress. The testing of the #3 CIV had just been completed when the 2A feedwater heater level began oscillating. The perturbations in the feedwater heater levels caused a feedwater pump suction pressure to decrease to the pump trip setpoint tripping both pumps. The operators manually tripped the reactor prior to reaching the steam generator low level automatic trip setpoint. Ineffective configuration control of parts allowed parts to be installed in a feedwater level control valve causing it to operate incorrectly which caused divergent feedwater heater level oscillations and the main feedwater pumps to trip.
The May 22, 2008 event and the July 3, 2009 event were initiated by grid disturbances. However, the most probable causes for the reactor trips were different. There are no prior corrective actions from the May 22, 2008 event that could have prevented occurrence of the July 3, 2009 event.
The June 28, 2008 event showed anomalies related to the VR-11 power supply. Power to VR-11 was lost momentarily (approximately 3 seconds) subsequent to the reactor trip during fast transfer from the normal station service transformer (NSST) [XFMR], to the reserve station service transformer (RSST). Corrective actions are in place to implement a design change to install a UPS to VR-11. The implementation of this design change may have prevented occurrence of the July 3, 2009 event. Design and implementation efforts were originally targeted for a 2011 implementation date. The targeted implementation date has been changed to June 30, 2010 and efforts continue to improve on that target date.
Energy Industry Identification System (EDS) codes are identified in the text as [XX].