05000410/LER-2006-001
| Docket Numbersequential Revmokth Day Year Year Month Day Yearnumber No. | |
| Event date: | 03-09-2006 |
|---|---|
| Report date: | 05-05-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4102006001R00 - NRC Website | |
I. Description of Event
On March 9, 2006, at approximately 15:00 hours, control room operators received a common high level alarm (Control Room annunciator) for the Turbine Generator Gland Seal and Exhaust System (TME) 1A/B Clean Steam Reboilers (CSR). The offline 'B' CSR level was indicating high and the online 'A' CSR level was normal. The operators lowered the offline 'B' CSR level to normal per the annunciator response procedure (ARP). However, the CSR 1A/B Water Level High alarm did not clear. Upon checking the Plant Process computer, the operators concluded the high alarm was associated with the 'A' CSR, which was supplying sealing steam to the main turbine gland seals. Initially, the in service 'A' CSR level indicator indicated normal level; however, the indicator was subsequently observed to be slowly oscillating between normal and high level over a period of several hours.
Based on indication of normal water level in the 'A' CSR, operators concluded that the CSR 1A/B Water Level High alarm was faulty. Coincident with this alarm, other abnormal self-revealing plant indications were received, such as an increase in off-gas flow. From approximately 15:00 to 22:00 hours operations and engineering continued to diagnose other plant indications thought to be associated with the performance of condenser maintenance. Initial troubleshooting and actions by operations and engineering appeared to have resolved the abnormal indications.
At 22:08 hours, 'A' CSR high-high level alarm actuated, 'A' CSR steam outlet valve closed, and operators attempted to place the redundant 'B' CSR in service, however TME steam pressure rapidly degraded and the TME Emergency Main Steam Backup System (EMS) automatically initiated on low TME steam pressure. The mechanical linkage for the pressure indicating controller became disconnected causing the associated pressure regulating valve to fail closed, isolating main steam from the TME. Without the ability to supply sealing steam to the Main Turbine's gland seals, ondenser vacuum rapidly degraded. At 22:13 hours, a low vacuum alarm was received on all three condensers. At 2:14 hours, the Main Turbine tripped on low condenser vacuum, immediately followed by a reactor scram.
1 Following the scram operators started the condenser vacuum pumps restoring condenser vacuum, which prevented loss of the normal heat removal function.
II. Cause of Event
T e underlying cause of the event was the mechanical linkage for the Pressure Indicating Controller of the EMS, which b came disconnected. This caused the pressure regulating valve to fail closed, isolating main steam from the TME. The chanical linkage became disconnected due to either not being properly reassembled during previous maintenance, or d e to the pressure transient within the EMS resulting from its automatic actuation.
A contributing cause of the event was failure of the pressure regulating valve that supplies air to the 'A' CSR level transmitter which caused the 'A' CSR water level to be higher than indicated in the Control Room.
Another contributing cause of the event was the less than adequate organizational performance in addressing equipment issues and abnormal plant indications. Performance deficiencies were identified in the areas of adequate application of problem-solving tools and communications.
NRC I*ORM 366A (1-2001)
III. Analysis of Event
This event is reportable in accordance with 10 CFR 50.73(aX2)(iv)(A) because of the automatic reactor scram while critical.
There were no actual safety consequences associated with this event. There were no automatic initiations of safety systems, and operators effectively stabilized reactor parameters. The conditional core damage probability for the event was calculated as 7.0E-7.
Based on the above, the event did not pose a threat to the health and safety of the public or plant personnel.
IV. Corrective Actions
NOTE: There are no NRC regulatory commitments in this Licensee Event Report.
The following corrective actions have been developed such that, when completed, will prevent a similar event.
- Improve maintenance procedures for overhauling/calibrating pneumatic controllers by including checks to ensure linkages and other critical components are properly reassembled following maintenance.
- Correct the Emergency Steam Supply Backup System design deficiency to reduce the effects of a pressure transient.
- Conduct training to enhance problem solving skills for operations and engineering.
- Inspected and replaced air regulating valves for critical pneumatic controllers, where appropriate.
V. Additional Information
A. Failed Components:
Mechanical linkage between the pressure indicating controller and the process pointer of the Emergency Steam Supply Backup System to the Turbine Generator Gland Seal and Exhaust System (TME).
Pressure regulating valve which supplies air to the TME 'A' Clean Steam Reboiler level transmitter.
B. Previous similar events:
None C. Identification of components referred to in this Licensee Event Report:
Components IEEE 805 System ID� IEEE 803.A Function Reactor Protection System� JC� N/A Turbine Generator Gland Seal and Exhaust� TC� N/A System Condenser� SG� COND Main Turbine� TA� TRB Pressure Indicating Controller� TC� PIC Clean Steam Reboiler� TC� BLR Pressure Regulating Valve� TC� PCV Level Transmitter� TC� LT Mechanical Linkage� TC� CON