05000455/LER-2011-002
| Byron Station, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| 4552011002R00 - NRC Website | |
Background
Technical Specifications (TS) 3.6.4, "Containment Pressure," applicable in Modes 1 through 4, requires containment pressure to be greater than or equal to 0.1 psig and less than or equal to 1.0 psig. If containment pressure is not within limits, then Condition A requires it to be restored to within limits within one hour. If Condition A is not met, then Condition B requires the Unit to be in Mode 3 in six hours and Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Surveillance Requirement 3.6.4.1 requires verification of containment pressure to be within limits every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. This is accomplished by an Operator recording and verifying pressure is within limits from a narrow range Containment Ventilation (VP) [BK] system differential pressure indicator (Le., 2PDI-VP236) located in the Main Control Room. Pressure Indicator 2PDI-VP231, also displays in the Main Control Room, is fed from the same pressure transmitters as the 2PDI-VP236 indicator. Other containment pressure indications exist, such as the four Safety Injection (SI) [BQ] system containment pressure indicators 2SI-0934, 0935, 0936 and 0937, however each are wide range instruments and their control board indicators are not conducive to accurately measuring pressures in the normal containment pressure range.
Unit 2 had just completed a refueling outage on October 9, 2011.
A. Plant Operating Conditions Before the Event:
Unit 2 was in Mode 1 — Power Operations at approximately 95% power and ramping to full power.
Reactor Coolant System (RC) [AB]: Normal operating temperature and pressure.
B. Description of Event
On October 12, 2011, during standard Main Control Room control board instrument monitoring, an Operator identified that the four wide range SI containment pressure indicators displayed containment pressure ranging from approximately 1.5 to 2.0 psig while the narrow range 2PDI-VP236 indicated pressure was within limits at value of 0.5 psig. 2PDI-VP231 also indicated containment pressure was within limits. This condition was entered into the Corrective Action Program to investigate and resolve this discrepancy. On October 13, 2011, initial investigations indicated the four SI containment pressure indicators appeared to be working properly. Consequently, at approximately 1400, a containment release was initiated to reduce containment pressure, as sensed by the SI pressure instruments, to below the one psig limit. During the release, the four SI pressure channels correctly indicated the expected pressure reduction trend while the 2PDI-VP236 indicated no change in containment pressure. As a result, the 2PDI- VP236 was declared non-functional and at 1430, Condition A for TS 3.6.4 was entered. All four SI containment pressure channels were below the 1.0 psig limit by 1512, and Condition A was exited.
An investigation determined that during the calibration of the inside containment pressure transmitter for the 2PDI-VP236 instrument loop on September 28, 2011, an instrument technician inappropriately installed a foreign material exclusion (FME) cap over the brass fitting that exits the transmitter housing. This fitting is connected to the pressure sensing unit within the transmitter's housing. With this FME cap installed, Byron Station, Unit 2 05000455 Description of Event (cont'd) containment atmosphere was isolated from the pressure sensing unit and therefore, rendered it unresponsive to containment pressure changes.
A historical review from the process computer of the SI containment pressure indicates that on October 9, 2011, at approximately 1600, containment pressure high limit of 1.0 psig was exceeded and remained above the limit until the containment release on October 13, 2011. Consequently, at approximately 2300 on October 9, 2011, TS 3.6.4 was unknowingly violated by not complying with TS 3.6,4 Conditions A and B. This is a condition prohibited by TS and reportable to the NRC in accordance with 10 CFR 50.73 (a)(2)(i)(B).
On October 14, 2011, the FME cap was removed and the transmitter restored to functional status.
C. Cause/s of the Event The instrument technicians conducting the calibration of the inside containment transmitter on September 28, 2011, failed to correctly use the Maintenance Alteration Process.
The calibration of the inside containment pressure transmitter was initiated by one instrument maintenance crew on September 23, 2011, and due to delays, later finished by a second crew on September 28, 2011.
The second crew believed a FME cap was necessary for the open fitting after the calibration was completed as a FME control device; however, a FMS cap should not have been installed unless the Maintenance Alteration log indicated a FME cap had been previously removed from the fitting. The second crew assumed fhe FME cap was removed previously by the first crew and they had failed to document it on the Maintenance Alteration log.
Contributing causes include the instrument technician's unfamiliarity with the purpose of the fitting on the pressure transmitter's housing and operator's over reliance on one indicator to verify containment pressure I was within limits when other indicators indicated containment pressure limits were exceeded.
D. Safety Significance
There were minimal adverse safety consequences f rpm this event. The instrument Ipop for 2PDI-VP236 only provides the Main Control Room indication of containment pressure and provides input to the "Containment Pressure High" annunciator.
During this timeframe the inside containment pressure transmitter's pressure sensing fitting was capped, the highest containment pressure indication, rom the SI pressure indicators was 1.91 psig. Had a design basis loss of coolant accident occurred during the brief time period containment pressure exceeded the TS limit, the peak containment pressure would have reached approximately 40 psig. This is approximately 1.0 psig higher than currently analyzed. Howver, ample 'margin still exists to the conlain'mect design pressure of 50 psig.
Though the containment high pressure annunciator was disabled, standard operating monitoring of control room indicators would have detected and prevented containment pressure from inadvertently reaching the SI Containment Pressure High setpoint of 3.4 psig. This is not a safety system functional failure.
E. Corrective Actions
The requirement to use the Maintenance Alteration Process will be reinforced with Maintenance personnel.
The VP pressure transmitters will be labeled and their calibration procedures revised to contain a warning that the external fitting on the transmitter's housing must remain open during plant operations.
An extent of condition review will be conducted to identify other pressure instrumentation that must be open to the environment.
The Instrument Maintenance department will evaluate the need for additional training to technicians regarding pressure instrumentation.
Other daily/shiftly readings by Operations will be reviewed and enhanced based on lessons learned of this event.
F. Previous Occurrences
No previous events were identified at Byron Station in previous three years.
NRC