05000425/LER-2005-002
| Vogtle Electric Generating Plant - Unit 2 | |
| Event date: | 07-21-2005 |
|---|---|
| Report date: | 09-19-2005 |
| 4252005002R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10 CFR 50.73 (a)(2)(i)(B). The unit operated in a condition prohibited by the Technical Specifications (TS) when the Loop 2 pressurizer pressure signal input to the overtemperature setpoint (OTSP) exceeded its allowable value for a period of time greater than allowed by the action requirements.
B. UNIT STATUS AT TIME OF EVENT
At the time of the discovery of this event, Unit 2 was in Mode 1 (Power Operation) at 100% of 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
On February 26, 2005, control room operators noticed that Reactor Coolant System (RCS) Loop 2 Overtemperature Delta-T (OTDT) reactor trip setpoint indicator 2TDI-0421C in the RCS Loop 2 OTDT reactor trip setpoint instrument channel 2T-421 (Process Control System) was reading about 5% higher than the other 3 channels. I&C technicians removed instrument channel 2T-421 from service. Initial calibration checks showed that the output of the summing amplifier 2TY-421L was reading higher than expected. I&C technicians replaced 2TY-421L and performed a calibration. Operators performed a channel check and instrument channel 2T-421 was restored to service.
On February 28, 2005, control room operators noted that 2TDI-0421C continued to trend up after the summing amplifier was replaced. A work order was written to investigate.
On March 1, 2005, non-intrusive voltage measurements were taken in the cabinet, which indicated that the pressurizer pressure summing amplifier 2PY-456H may be suspect. A work order was scheduled for March 3, 2005, to correspond with an already scheduled Channel Operational Test (COT) for channel 2T-421.
On March 3, 2005, voltage readings were again taken prior to removing the channel from service for the COT. These readings again indicated a problem with summing amplifier 2PY-456H, and I&C removed instrument channel 2T-421 from service. I&C also performed an as-found COT, confirming a problem with 2PY-456H. This summing amplifier was replaced and calibrations were performed on both 2PY- 456H and 2TY-421L, and the COT was completed. Instrument channel 2T-421 was restored to service.
2. DOCKET 6. LER NUMBER On June 23, 2005, the NRC resident inspector held a meeting with plant management to discuss concerns regarding the maintenance process and channel 2T-421 operability issues surrounding this event.
The delay between the March 3, 2005, discovery that summing amplifier 2PY-456H was outside of setpoint tolerance and the engineering evaluation which showed that this condition existed on February 26, 2005, was a result of an I&C technician determining that a procedure step was not applicable (N/A).
This procedure step would have required a Condition Report (CR) for as found conditions out of tolerance, and this CR would have initiated an engineering evaluation of the condition. The I&C technician erroneously determined the step to be N/A based on the fact that three previous CRs had been written on channel 2T-421.
On July 21, 2005, the Engineering Support Department determined that the amount of drift of the pressurizer pressure signal input to the OTSP exceeded the allowable values in TS Table 3.3.1-1, Note 1. The channel had been restored to service post-maintenance on February 26, 2005, with this condition present, and it was not corrected until March 3, 2005, which represents a period of time greater than the 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> allowed by the TS action requirements. Therefore, the unit operated in a condition prohibited by the TS.
D. CAUSE OF EVENT
The causes of this event were:
1. The event was initiated from summing amplifier 2PY-456H drifting beyond its allowable value.
2. Proper troubleshooting was not performed on February 26, 2005, per the work order, as needed to identify summing amplifier 2PY-456H as the source of the drifting OTSP signal. Prior experience led both Operations personnel and I&C technicians to limit the troubleshooting scope to only check for summing amplifier 2TY-421L as the failed component.
3. A COT was not performed to verify the condition of instrument channel 2T-421 prior to restoring it to service on February 26, 2005, because the procedure being utilized did not require a COT to be performed following maintenance activities. A contributing cause was the failure to generate a CR on March 3, 2005, for as found conditions out of tolerance.
4. An engineering evaluation of the event was not performed in a reasonable time frame.
E. ANALYSIS OF EVENT
The affected channel, RCS Loop 2 OTDT, provides a signal on a 2-out-of-4 logic to the OTDT Reactor Trip function. The other three channels were not out of service during the period from when 2TDI- 0421C was first found reading about 5% higher than the average of the three other Delta-T loops, to the time when the correct summing amplifier (2PY-456H) was replaced and the loop was recalibrated on March 3, 2005. This means the three other channels were still able to provide adequate indication and protection signals. Therefore, this condition represented only a degraded function and not a loss of system function. Based on this consideration, there was no adverse effect on plant safety, or on the health and safety of the public, as a result of the event.
This event does not represent a safety system functional failure.
F. CORRECTIVE ACTIONS
1. Summing amplifier 2PY-456H was replaced and calibrated. A channel COT was performed on March 3, 2005, to verify proper functionality. Testing found a failed potentiometer on the summing amplifier circuit card.
2. I&C personnel and their supervision have been briefed on the requirement to perform troubleshooting to determine the cause of problems, on expectations for asking for additional help on complex control loops or system problems, on the importance of noting non-obvious procedure "N/A's," and on the requirements to perform COTs after system repairs.
3. Procedures for performing RCS Temperature Loop COTs and channel calibrations have been revised to require a COT after corrective maintenance. In addition, procedure 29401-C, "Work Order Functional Tests," has also been revised to clarify the expectations for performing COTs following corrective maintenance.
4. Procedures for performing RCS Temperature Loop COTs and channel calibrations have been revised to require an engineering review for out-of-calibration conditions.
G. ADDITIONAL INFORMATION
1. Failed Components:
Summing Amplifier (NSA circuit card) manufactured by Westinghouse Electric Corporation.
Part / Model # 2837A14G02.
2. Previous Similar Events:
366A) There have been no previous similar LERs in the last three years.
3. Energy Industry Identification System Code:
Process Control System — JA Reactor Coolant System - AB