05000425/LER-2011-001
| Vogtle Electric Generating Plant- Unit 2 | |
| Event date: | 10-10-2011 |
|---|---|
| Report date: | 12-15-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4252011001R00 - NRC Website | |
This report is required per 10 CFR 50,73(a)(2)(i)(B) since Unit 2 operated in a condition prohibited by Technical Specifications (TS). TS 3.3.4, "Remote Shutdown System" is applicable in Modes 1, 2 and 3 and requires two channels of Core Exit Thermocouples (CETC) [IM] to be operable. During startup from refueling outage 2R15, one of the CETC channels credited for satisfying this requirement was inoperable but was not recognized as being inoperable until the unit was in Mode 1. Since Mode changes were made with only one of the two required CETC channels Operable the unit was operated in a condition contrary to TS LCO 3.0.4.
B. UNIT STATUS AT TIME OF EVENT
At the time it was discovered that the quadrant 1 8 train CETC indicator was inoperable, the unit was in Mode 1 at 100 percent rated thermal power. Other than that described herein, there was no other inoperable equipment that contributed to this event.
C. DESCRIPTION OF EVENT
On October 10, 2011 at approximately 0002 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> Eastern Daylight Time (EDT), Unit 2 entered Mode 3 as part of the planned startup from refueling outage 2R15. Subsequently, on October 27, 2011 with Unit 2 in Mode 1 the B train quadrant 1 CETC indicator was found to be reading low during a scheduled surveillance to satisfy TS SR 3.3.4.1. Upon discovery of the condition, TS 3.3.4 function 5 condition A was immediately entered on October 27, 2011 at 0215 hours0.00249 days <br />0.0597 hours <br />3.554894e-4 weeks <br />8.18075e-5 months <br /> EDT.
The plant design includes eight B train CETCs in quadrant 1. The signals from the CETCs provide input into a remote processing unit (RPU). The signals are then sent to a data processing unit (DPU). The DPU processes the input from the CETCs and sends a signal to the CETC indicator on the B train remote shutdown panel. The algorithm in the DPU is set up such that it verifies there are at least four valid inputs from the B train quadrant 1 CETCs and averages the valid inputs after discarding the high and low reading. If there are less than four valid B train CETCs in quadrant 1, the output to the CETC indicator on the B train remote shutdown panel is set to 50 degrees F.
Since the bottom of scale reading on the B train quadrant 1 remote shutdown panel CETC indicator is 200 degrees, it would indicate approximately 200 degrees F.
Once it was discovered that the B train quadrant 1 CETC indicator was inoperable, maintenance and engineering personnel began troubleshooting activities. It was determined that the cause for the indicator reading low was due to the fact that there were only three CETCs in B train quadrant 1 that were providing a valid signal. Subsequent troubleshooting determined that one of the B train quadrant 1 CETCs that was not providing a valid signal was due to the leads being reversed. Since it could not be conclusively determined where the leads were reversed and it would have required entry into the Unit 2 Containment building to determine, a Temporary Modification was prepared Vogtle Electric Generating Plant —Unit 2 05000425 Containment Building. Once the Temporary Modification was implemented the B train quadrant 1 CETC indicator was verified to be indicating correctly and the LCO was exited on November 8, 2011 at approximately 2154 hours0.0249 days <br />0.598 hours <br />0.00356 weeks <br />8.19597e-4 months <br /> Eastern Standard Time.
Further investigation revealed that prior to entering Mode 3 on October 10, 2011 and up until the time the condition was corrected on November 8, 2011 there were only three B train quadrant 1 CETCs providing a valid signal. As such, the quadrant 1 B train CETC indicator on the B train remote shutdown panel which requires at least four valid CETCs could not have been providing accurate indication during this time period. The B train quadrant 1 CETC indicator was returned to service on November 8, 2011 which was within 30 days from the time the unit had initially entered Mode 3. However, since the unit had changed Modes with one of the two required CETC channels inoperable on the B train remote shutdown panel, this represented a condition that was prohibited by LCO 3.0.4.
D. CAUSE OF EVENT
The direct cause of this event was that five of the eight B train quadrant 1 CETCs were not providing a valid input into the DPU. One of the five nonfunctioning CETCs was not providing a valid input due to its leads being reversed. The cause for the other four CETCs not providing a valid input to the DPU is indeterminate since subsequent troubleshooting will require that the reactor be shut down in order to allow access due to radiation dose considerations.
A contributing cause was that Operations personnel performed a channel check to satisfy TS SR 3.3.4.1 prior to entering Mode 3 and erroneously concluded that the B train quadrant 1 CETC indicator was providing proper indication. The B train quadrant 1 CETC indicator was compared to the CETC indication on the Plant Safety Monitoring System (PSMS) display. Since the unit was in Mode 4 and RCS temperature was less than 350 degrees when the surveillance was performed, the temperature difference between the B train quadrant I CETC indicator, which was at or near the bottom of scale and the PSMS indication met the channel check criteria. Had Operations personnel recognized that the indicator was not providing a representative display of core temperature the channel would have been declared inoperable and entry into Mode 3 would not have been made until the cause for the inoperability had been determined and the channel returned to operable status.
Another contributing cause was that prior to entering Mode 3 plant personnel were aware that there were only three functioning B train CETCs in quadrant 1. However, plant personnel did not recognize at that time that the B train CETC indicator for quadrant 1 on the remote shutdown panel required at least four B train quadrant 1 CETCs to be functional to provide a valid indication. Based upon this incorrect assumption, plant management elected to start up the unit with what was believed to be the minimum number of CETCs required to be functional in B train in quadrant 1..
The Remote Shutdown System provides the operator with sufficient instrumentation and controls to place and maintain the unit in a safe shutdown condition from a location other than the control room. This capability is necessary to protect against the possibility that the control room becomes inaccessible. During the time the quadrant 1 B train CETC indicator was inoperable, the quadrant 4 B train CETC remained Operable. As such, the operator's capability to place and maintain the unit in a safe shutdown condition was not adversely affected by this condition. Based upon these considerations there was no adverse affect on plant safety or on the health and safety of the public.
Additionally, since the quadrant 4 B train CETC indicator remained operable during the time the quadrant 1 B train CETC was inoperable, the condition did not result in a loss of safety function.
F. CORRECTIVE ACTION
The corrective actions taken or planned to be taken include:
1. A Temporary Modification was implemented to reverse the leads on one of the quadrant 1 B train CETCs. Once the leads were corrected, the CETC provided valid indication.
2. A Work Order was generated to repair the non functioning CETCs. Since the repair of the CETCs will require the unit to be in a refueling outage, the expected completion date for this activity is May 1, 2013.
3. The surveillance procedure that performs the channel check of the CETC indicators on the B train remote shutdown panel will be revised to provide more definitive criteria for performing the channel check surveillance. The expected completion date for this activity is February 24, 2012.
4. The TS Bases 3.3.4 function 5 will be revised to clearly define the minimum number of CETCs required for the B train quadrant 1 CETC indicator to be operable. The expected completion date for this activity is March 16, 2012.
G. ADDITIONAL INFORMATION
1) Failed Components:
Five of the eight quadrant 1 CETCs were not providing indication. The thermocouples were manufactured by Westinghouse.
2) Previous Similar Events:
A review of Licensee Event Reports over the past three years did not identify a similar instance involving CETCs nor a case, where a Mode change was made, that was prohibited by the Technical Specifications.
3) Energy Industry Identification System Code:
IM-Temperature Monitoring System