05000425/LER-2013-003
| Vogtle Electric Generating Plant -Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4252013003R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10CFR 50.73(a)(2)(iv)(A) due to an unplanned manual actuation of the Reactor Protection System (RPS).
B. UNIT STATUS AT TIME OF EVENT
At the time of the event, Unit 2 was in Mode 1 at 25 percent rated thermal power.
C. DESCRIPTION OF EVENT
A manual Unit 2 reactor trip occurred at approximately 25 percent reactor power on October 22' 2013 at 11:44 due to lowering of condenser vacuum. Maintenance had lifted the Unit 2 main feed pump (MFP) B steam chest one inch in preparation for work to repair a galled valve stem on one of the control valves. During this evolution, the MFP B steam exhaust isolation valve to condenser, allowed enough system in-leakage to overwhelm the in-service Steam Jet Air Ejector (SJAE) and the two mechanical vacuum pumps that were running to maintain condenser vacuum. Prior to work commencing the possibility of impacting condenser vacuum due to maintenance activities was discussed and operational limits were established. As condenser vacuum approached the pre- determined operational limit of 23 inches of mercury vacuum, the Shift Supervisor directed a manual reactor trip of the Unit 2 reactor. Unit 2 was stabilized in Mode 3 on Auxiliary Feedwater with the Main Condenser available for decay heat removal. All systems responded as expected.
D. CAUSE OF EVENT
The cause of the event was valve leak-by from the MFP turbine exhaust valve causing an inadequate isolation boundary for the steam and vacuum environment.
E. SAFETY ASSESSMENT
When the reactor tripped, all rods fully inserted. As a result of the reactor trip, the Feedwater System isolated and the Auxiliary Feedwater System was placed in service. The unit was stabilized in Mode 3 at nominal operating temperature and pressure. The plant responded as designed and there were no complications with the plant shutdown, there was no adverse effect on plant safety or the safety and health of the public.
F. CORRECTIVE ACTION
Revise operating procedures to specify that MFP maintenance activities with the potential to impact condenser vacuum must be completed prior to entering Mode 1. In addition, procedures will require a condenser in-leakage calculation, an evaluation of system dynamics to ensure vacuum margin, and existence of adequate margin to account for potential new in-leakage from the work activity,
G. ADDITIONAL INFORMATION
1) Failed Components:
Component: ISV-IsolatIon Valve Manufacturer: M449-Mosser Industries Inc.
2) Previous Similar Events:
A review of Licensee Event Reports did not reveal another instance whereby a manual reactor trip occurred following the lowering of condenser vacuum due to in-leakage from use of unsuitable valve seat material.
3) Energy Industry Identification System Code:
Condenser Vacuum System - SH