05000424/LER-2011-003
| Vogtle Electric Generating Plant - Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4242011003R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10 CFR 50.73(a)(2)(i)(B) since the unit operated in a condition prohibited and Unit 2 control room radioactive gas monitors [IL] is stopped rendering them inoperable. TS 3.3.7 Condition P requires that one Control Room Exhaust Filtration System (CREFS) [VI], in each unit, be placed in the emergency mode within one hour. Since it was not recognized that the damper had closed until after the one hour completion time had elapsed and a CREFS train in each unit had not been placed in the emergency mode, the unit operated in a condition prohibited by TS 3.3.7.
B. UNIT STATUS AT TIME OF EVENT
At the time the damper was found closed, Unit 1 was in Mode 1 at 100% of rated thermal power.
Unit 2 was in Mode 6 (refueling) at 0% 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 September 22, 2011 at approximately 0155 hours0.00179 days <br />0.0431 hours <br />2.562831e-4 weeks <br />5.89775e-5 months <br /> Eastern Daylight Time (EDT) a System Operator entered the control room and noticed that the control room doors did not open as he had become accustomed to due to decreased differential pressure across the door. He immediately informed the Unit 1 Operator who investigated the condition and found that the Control Room Normal HVAC (Heating, Ventilation, and Air Conditioning) Outside Air Damper, AHV-12153 was closed. The Unit 1 Operator verified that there was no valid reason for the damper to be closed and then opened the damper on September 22, 2011 at approximately 0214 hours0.00248 days <br />0.0594 hours <br />3.53836e-4 weeks <br />8.1427e-5 months <br /> EDT, thereby restoring air flow through the control room radioactive gas monitors returning them to operable status.
Subsequent investigation found that the damper had closed on September 19, 2011 at approximately 2029 hours0.0235 days <br />0.564 hours <br />0.00335 weeks <br />7.720345e-4 months <br /> EDT based upon indication from the Integrated Plant Computer (IPC) [ID]. TS 3.3.7 Condition P requires one CREFS train in each unit to be placed in the emergency mode within one hour if all four control room radioactive gas monitor channels are inoperable.
Since the damper closed on September 19, 2011 at 2029 hours0.0235 days <br />0.564 hours <br />0.00335 weeks <br />7.720345e-4 months <br /> EDT but was not discovered until September 22, 2011 at 0155 hours0.00179 days <br />0.0431 hours <br />2.562831e-4 weeks <br />5.89775e-5 months <br /> EDT, the one hour completion time to place one CREFS train in each unit in the emergency mode was not met.
D. CAUSE OF EVENT
An investigation into the cause for the damper going closed was inconclusive. A review of operating logs, tagging orders and outage work did not find any activity which should have resulted in the damper going closed. Additionally, an inspection of the damper and solenoid valves did not find any condition which would have caused the damper to close unexpectedly.
E. SAFETY ASSESSMENT
During this event, the damper was closed while Unit 1 was in Mode 1 and Unit 2 was in Mode 6 with no fuel movement or core alterations in progress. The CREFS acts to terminate the supply of unfiltered outside air to the control room, initiate filtration, and pressurize the control room. This ensures the control room is kept habitable for operators stationed there during accident recovery and post accident operations by minimizing the radiation exposure of control room personnel. In Modes 1, 2, 3, and 4, the radioactive gas monitor actuation of the CREFS is a backup for the Safety Injection (SI) signal actuation. During movement of irradiated fuel assemblies and core alterations, the radioactive gas monitor actuation is the primary means to ensure control room habitability in the event of a fuel handling accident. During the time the damper was closed, the SI actuation for the Unit 1 CREFS remained operable. Since there was no movement of irradiated fuel assemblies or core alterations on Unit 2 during the time the damper was closed, the radioactive gas monitors were not required to be operable for Unit 2. Since the SI actuation for the Unit 1 CREFS remained operable during the time the damper was closed, the event does not represent a safety system functional failure. Also, based upon these considerations, there was no adverse affect on plant safety or on the health and safety of the public.
F. CORRECTIVE ACTION
A design change package will be developed to provide a control room annunciation to alert the operator of the condition so that necessary actions can be taken. The expected implementation date for the design change is November 15, 2014.
A review of Licensee Event Reports (LER) over the past seven years finds that a similar event occurred in 2004 as documented in LER 2004-002-00. In that event, the damper was found closed also. However, the cause for the damper closing was traced back to switchgear cleaning that caused two of the solenoid valves that are required to be energized to maintain the damper open, to lose power. The switchgear cleaning procedure was revised to alert personnel that this activity would cause the damper to close. The closure of the damper on September 19, 2011 could not be tied to any maintenance activity, including switchgear cleaning.
3) Energy Industry Identification System Code:
Control Room Emergency Filtration System — VI Radiation Monitoring System — IL Integrated Plant Computer System — ID