05000425/LER-2009-001
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 12-23-2009 |
|---|---|
| Report date: | 02-12-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4252009001R00 - NRC Website | |
A. REQUIREMENT FOR REPORT
This report is required per 10CFR50.73(a)(2)(iv)(A) due to a manual actuation of the Reactor Protection System (RPS) and automatic actuation of the Auxiliary Feedwater (AFW) System.
B. UNIT STATUS AT TIME OF EVENT
At the time of this event, Unit 2 was in Mode 1 (Power Operation) at 100% rated thermal power.
C. DESCRIPTION OF EVENT
On December 23, 2009 System Operators were dispatched to the Turbine Building to return an instrument air dryer to service which had been tagged out for maintenance. To return the instrument air dryer to service required a specific section of the system operating procedure to be performed. Once the System Operators arrived at the Turbine Building to return the instrument air dryer to service, they performed the incorrect section of the system operating procedure. This resulted in isolating instrument air to the Unit 2 Turbine Building, the Unit 2 Control Building and Unit 2 Containment. In response to the loss of instrument air pressure, several valves assumed their fail-safe position. In particular, several air operated valves on the secondary side repositioned which ultimately resulted in a decrease in suction pressure to the B main feedwater pump. The B main feedwater pump tripped on low suction pressure in accordance with plant design. Once the B main feedwater pump tripped, the control room operators, in anticipation of loss of main feedwater flow to the steam generators, manually tripped the reactor in accordance with the Abnormal Operating Procedure (AOP) for a main feedwater malfunction. Subsequently, when water level in one of the steam generators (SG) reached the lo-lo SG water level setpoint, the motor driven auxiliary feedwater pumps automatically started in accordance with plant design. All control rods fully inserted and all safety systems functioned in accordance with plant design. The plant was stabilized in Mode 3.
D. CAUSE OF EVENT
The direct cause of the reactor trip was the inappropriate isolation of the turbine building instrument air header while restoring an instrument air dryer to service following maintenance.
Both the system operator tasked with performing the procedure and the system operator who was performing a peer check were first time performers of the activity. Following a pre-job briefing for the clearance restoration, the system operators notified the operator at the controls (OATC) in the Unit 2 Control Room of the pending restoration of the instrument air dryer. Once the system operators arrived in the Turbine building, they discussed the task further prior to beginning.
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- ■■1111 U.S. NUCLEAR REGULATORY COMMISSIONNRC FORM 366A LICENSEE EVENT REPORT (LER)(9-2007) CONTINUATION SHEET 2009 - 001� 0 However, due to a failure to conduct an adequate pre-job brief of the system restoration per the system operating procedure, inadequate supervisory oversight, and a failure to apply fundamental system knowledge of the instrument air system by the system operators, they performed the incorrect section of the procedure. This resulted in isolating instrument air to the Unit 2 Turbine building, Unit 2 Control Building, and Unit 2 Containment.
The procedure had weaknesses as it did not contain adequate cautions and warnings to prevent the inadvertent operation of the instrument air header valves. Furthermore, the instrument air header valves were not labeled as potential reactor trip hazards. The isolation of instrument air to the Turbine building ultimately resulted in several secondary side air operated valves in the main feedwater and condensate system assuming their fail-safe position. This condition ultimately led to the decrease in suction pressure to the B main feedwater pump. Once the main feedwater pump suction pressure reached the low pressure setpoint, the B main feedwater pump automatically tripped. The reactor was subsequently manually tripped in anticipation of the loss of feedwater flow to the steam generators in accordance with procedure guidance.
E. ANALYSIS OF EVENT
When instrument air was isolated to the Unit 2 Turbine building, Unit 2 Control Building and Unit 2 Containment, pressure in the instrument air header in these buildings began to decrease.
As pressure in the instrument air header decreased, pneumatically operated valves began to assume their fail-safe position. The instrument air system at Vogtle Electric Generating Plant (VEGP) is required for normal operation of the plant but is not essential for safe shutdown of the plant. Pneumatically operated valves in the plant which are essential for safe shutdown and accident mitigation are designed to assume a fail-safe position upon loss of instrument air pressure. Therefore, instrument air is not essential following a design basis event or for safe shutdown of the plant. Once the main feedwater pump tripped, the control room operators acted appropriately to trip the reactor and prevent a challenge to the automatic trip actuation circuitry.
Since all safety related components responded in accordance with plant design, all control rods fully inserted, and the motor driven auxiliary feedwater pumps automatically started on lo-lo steam generator water level, this event did not result in a safety system functional failure. Based on these considerations, there was no adverse affect on plant safety or on the health and safety of the public as a result of this event.
F. CORRECTIVE ACTIONS
1. Instrument air isolation valves on both units had a restraint device installed to prevent inadvertent operation and labels were affixed to alert the operator of potential consequences (e.g. reactor trip) if the valves were closed while the unit was on line.
2. Revised the system operating procedure to include appropriate cautions and warnings to alert the user of potential consequences (e.g. reactor trip) if certain sections of the procedure were performed with the unit on line.
3. Briefed site personnel regarding rigor and formality in the use of the human performance error prevention tools.
G. ADDITIONAL INFORMATION
1. Failed Components:
None 2. Previous Similar Event: A review of VEGP events over the past three years was performed.
Licensee Event Report 1-2009-002 (Docket Number 05000 424) documents a reactor trip that occurred on Unit 1 on December 7, 2009 due to inadvertent contact with a handswitch by a control room operator. Although that event was due to a human performance error also, the causes associated with this event were different. Therefore the corrective actions put in place in response to the reactor trip that occurred on December 7, 2009 (LER 1-2009-002) did not address the failed barriers that caused this event.
3. Energy Industry Identification System Codes:
Instrument Air System-LD Main Feedwater System-SJ Auxiliary Feedwater-BA