05000424/LER-2011-002
| Vogtle Electric Generating Plant - Unit 1 | |
| Event date: | 08-31-2011 |
|---|---|
| Report date: | 10-24-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4242011002R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10CFR50.73(a)(2)(iv)(A) due to an unplanned actuation of the Reactor Protection System (RPS) [JC] and an automatic actuation of the Auxiliary Feedwater System (AFW)[BA],
B. UNIT STATUS AT TIME OF EVENT
At the time of this event, Unit 1 was in Mode 1 (Power Operation) at 100 percent rated thermal power. Other than that described herein, there was no inoperable equipment that contributed to the occurrence of this event.
C. DESCRIPTION OF EVENT
Due to erratic behavior of feedwater flow to steam generator (S/G) 2 that was observed on August 26, 2011 a work order was initiated to replace the contact potentiometer on the main feedwater [SJ] regulating valve (MFRV) for S/G 2. The contact potentiometer is part of the control loop for the MFRV and has a chronic history of degradation as a result of localized vibration at the MFRV. On August 31, 2011 at approximately 0901 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.428305e-4 months <br />, the MFRV for S/G 2 was placed on an air gag to allow replacement of the contact potentiometer. Positioning of the MFRV during normal operation is the function of a three-element feedwater level control system [JB]. The three-element control system maintains feedwater flow equal to steam flow, and steam generator water level is used as an input to trim feedwater flow and maintain programmed water level. The air gag consists of a regulator, a pressure indicator, a three way valve and associated tubing, which is normally isolated from the output of the positioner for the MFRV. The purpose of the air gag is to bypass the positioner and supply air directly to the valve actuator at a given pressure to maintain the valve in position. However, it will not preclude the MFRV from closing in response to an Engineered Safety Features Actuation Signal (ESFAS). Once the air gag is installed, S/G level is controlled by the bypass feedwater regulating valve (BFRV). To place the MFRV on the air gag, operators are dispatched to the air gag panel located near the MFRV. The BFRV is then placed in position at approximately 50 percent demand and S/G level is verified to be stable. The operator records the air pressure from the pressure gauge on the output of the positioner to the MFRV. The operator then adjusts the pressure regulator associated with the air gag to this same pressure. Once these pressures are approximately equal, a three way valve is aligned to port the air directly from the output of the air gag regulator to the valve actuator while isolating air from the positioner to the MFRV.
This maintains the position of the MFRV. However, shortly after the air gag was installed, feedwater flow to S/G 2 increased abruptly. The control room operator responded by closing the BFRV in 10 percent increments until the BFRV was fully closed. With the BFRV fully closed, feedwater flow to S/G 2 was still greater than it was prior to installation of the air gag. As a result, water level in S/G 2 continued to rise.
When water level in S/G 2 reached the Hi-Hi level nominal trip setpoint (NTS), an automatic turbine trip and main feedwater isolation signal were initiated. The turbine trip, with power above the P-9 permissive setpoint, generated a reactor trip signal which then opened the reactor trip breakers. Additionally, the main feedwater isolation resulted in a trip of both main feedwater pumps and an automatic start of both motor driven auxiliary feedwater pumps in accordance with plant design. When turbine trip and reactor trip occurred, water level in all four steam generators went below the low-low level NTS causing an automatic start of the turbine driven auxiliary feedwater pump (TDAFWP). All required plant systems responded normal to the trip and no complications were observed.
D. CAUSE OF EVENT
A root cause team was formed to analyze this event. Troubleshooting directed by the root cause team determined that the pressure gauge associated with the air gag was indicating approximately 3 psig lower than the actual pressure applied. As a result, when the operators matched the indicated pressure from the air gag pressure gauge with the pressure gauge on the output of the positioner and aligned the three way valve to port air directly from the air gag regulator to the MFRV, the air pressure was approximately 3 psig higher than from the positioner. The increased air pressure to the MFRV actuator caused the valve to be further open than when it was controlled from the positioner. The root cause team determined that the abrupt increase in feedwater flow that was observed was consistent with the expected increase in valve travel with the 3 psig additional air pressure being applied. Since the increased feedwater flow through the MFRV was beyond the capability of the BFRV to control even with the BFRV fully closed, S/G 2 water level continued to rise until the Hi-Hi S/G level NTS was reached. This resulted in the reactor trip and subsequent AFW actuations.
E. SAFETY ASSESSMENT
As a result of the water level in S/G 2 exceeding the Hi-Hi NTS, a main feedwater isolation and turbine trip occurred in accordance with plant design. The turbine trip, with power above the P-9 permissive setpoint, generated a reactor trip signal which then opened the reactor trip breakers.
Additionally, the main feedwater isolation resulted in a trip of both main feedwater pumps and an automatic start of both motor driven auxiliary feedwater pumps in accordance with plant design.
The turbine trip and reactor trip caused water level in all four steam generators to go below the low- low level NTS which caused an automatic start of the TDAFWP. All rods fully inserted, the control room operators responded appropriately to control feedwater to the steam generators, and the plant was stabilized in Mode 3. Based upon these considerations, there was no adverse effect on plant safety or on the health and safety of the public as a result of this event. Additionally, since all safety systems functioned in accordance with plant design, this event does not involve a safety system functional failure.
F. CORRECTIVE ACTION
1. The procedure for placing the MFRV on the air gag was revised to require the use of calibrated pressure gauges to determine the pressure from the positioner and from the air gag regulator.
2. The procedure was also revised to require an operator to stay at the air gag panel as long as it is controlling the MFRV.
3. The long term corrective is to implement a Design Change Package (DCP) to replace the contact style potentiometer with one that is not as susceptible to vibration induced wear. The expected completion date for this corrective action on Unit 1 is November 1, 2012 and May 1, 2013 on Unit 2.
G. ADDITIONAL INFORMATION
1) Failed Components: None. The MFRV opened in response to the increase in air pressure applied to the actuator from the air gag. Since the increase in feedwater flow was beyond the capability of the BFRV to control, steam generator level increased until it reached the Hi-Hi level NTS. All safety systems responded in accordance with the plant design.
2) Previous Similar Events: A review of LER's over the past six years finds that there were three previous occasions (LER 2005-003-00, 2005-004-00 and 2006-001-00) where issues associated with the MFRVs resulted in a reactor trip. Two of the previous LER's involved a failed I/P transducer, while the third LER involved a circuit board failure. Since this reactor trip was not due to a circuit board failure or an I/P transducer failure, previous corrective actions would not have prevented this trip. However, due to the reliability issues associated with the control loop components for the MFRVs, design changes were implemented starting in 2006.
These design changes installed new digital valve positioners and the air gag on the MFRVs which allows the control loop components to be worked while the unit is at power. This was the first reactor trip that has occurred since installation of the design changes were implemented starting in 2006.
3) Energy Industry Identification System Code:
Main Feedwater System — SJ Auxiliary Feedwater System — BA Plant Protection System — JC Feedwater/Steam Generator Level Control System - JB