05000296/LER-2013-002
| Browns Ferry Nuclear Plant, Unit 3 | |
| Event date: | 02-11-2013 |
|---|---|
| Report date: | 06-07-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation |
| 2962013002R01 - NRC Website | |
I. Plant Operating Conditions Before the Event
At the time of the event, Browns Ferry Nuclear Plant (BFN), Unit 3, was in Mode 3 at zero percent rated thermal power during a planned reactor shutdown.
II. Description of Events
A. Event:
On February 11, 2013, at 0613 hours0.00709 days <br />0.17 hours <br />0.00101 weeks <br />2.332465e-4 months <br /> Central Standard Time (CST), the Reactor Core Isolation Cooling (RCIC) system [BN] was manually started during a planned BFN, Unit 3, reactor shutdown. A Reactor Feedwater [SJ] recirculation piping through wall leak resulted in the loss of condenser vacuum and subsequent unavailability of the Main Turbine Bypass Valves [V] [JI]. The RCIC system was manually started to control reactor water level in anticipation of loss of Reactor Feedwater Pumps (RFPs) [P] tripping on low vacuum. Safety Relief Valves (SRVs) [SB] were manually operated to maintain reactor pressure. No Emergency Core Cooling System (ECCS) [BJ][BO][BM] or RCIC system reactor water level initiation set points were reached. The RCIC system was removed from service on February 11, 2013, at 1449 CST.
B. Status of structures, components, or systems that were inoperable at the start
of the event and that contributed to the event:
There were no inoperable structures, components, or systems that contributed to this event.
C. Dates and approximate times of occurrences:
February 11, 2013 at 0424 CST February 11, 2013 at 0613 CST February 11, 2013 at 1449 CST Operations initiated a planned reactor manual scram.
The RCIC system was manually started to control reactor water level.
The RCIC system was removed from service.
D. Manufacturer and model number (or other identification) of each component that failed during the event:
Model number 6023WE Feedwater Long Cycle (FWLC) flow control valves [FCV] 3-FCV-003-0071, 3-FCV-003-0072, and 3-FCV-003-0073, manufactured by the William Powell Company, were leaking. The leakage past these valves caused flow induced erosion and excessive vibration in the long cycle return line piping to the miscellaneous drain header resulting in a through wall leak in the long cycle return line piping, BFN-3-MISC-003.
E. Other systems or secondary functions affected:
The loss of condenser vacuum resulted in the RFPs tripping on low vacuum.
F. Method of discovery of each component or system failure or procedural error:
Operations received indication of decreasing condenser vacuum in the reactor feedwater recirculation system [AD].
G. The failure mode, mechanism, and effect of each failed component, if known:
Flow control valves 3-FCV-003-0071, 3-FCV-003-0072, and 3-FCV-003-0073, were leaking. These valves are solid disc gate valves, which are not suited for the high differential pressure conditions to which they were subjected.
The leakage past these valves caused flow induced erosion and excessive vibration in the long cycle return line piping to the miscellaneous drain header resulting in a through wall leak in the long cycle return line piping, BFN-3-MISC-003. This condition resulted is a loss of condenser vacuum.
H. Operator actions:
Operations manually operated the RCIC system and SRVs to control reactor water level and pressure during the planned BFN, Unit 3, reactor shutdown.
I. Automatically and manually initiated safety system responses:
Operations manually operated the RCIC system and SRVs to control reactor water level and pressure during the planned BFN, Unit 3, reactor shutdown.
Ill. Cause of the event A. The cause of each component or system failure or personnel error, if known:
This event has two root causes. First, the design used for valves 3-FCV-03-0071, 3-FCV-003-0072, and 3-FCV-003-0073 is not suited for the required operating conditions to which the valves were subjected. Second, BFN personnel did not consistently consider risk when making decisions in that work orders to replace the BFN, Unit 3, FWLC valves were cancelled with little or no justification.
B. The cause(s) and circumstances for each human performance related root cause:
Flow control valves 3-FCV-003-0071, 3-FCV-003-0072, and 3-FCV-003-0073, were leaking. The leakage past these valves caused flow induced erosion and excessive vibration in the long cycle return line piping to the miscellaneous drain header resulting in a through wall leak in the long cycle return line piping, BFN-3-MISC-003.
IV. Analysis of the event:
The Tennessee Valley Authority (TVA) is submitting this report in accordance with Title 10 of the Code of Federal Regulations (10 CFR) 50.73(a)(2)(iv)(A) as any event or condition that resulted in manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B), which includes the RCIC system.
In late December 2012, BFN, Unit 3, experienced an unusual rise in water in-leakage to the Radwaste Building [NE], concurrent with a higher than normal river water level. The amount of in-leakage Challenged the capacity of the Radwaste system [WD]. The source of in-leakage was determined to be the BFN, Unit 3, Condenser Circulating Water (CCW) [SG] conduit and the decision was made to conduct a planned outage in February 2013, to allow repairs to the CCW conduit. A manual scram was initiated on February 11, 2013 at 0424 CST to start the planned outage.
On February 11, 2013, at 0613 hours0.00709 days <br />0.17 hours <br />0.00101 weeks <br />2.332465e-4 months <br /> CST, the RCIC system was manually started to control reactor water level because a Reactor Feedwater recirculation piping through wall leak resulted in the loss of condenser vacuum and subsequent unavailability of the Main Turbine Bypass Valves. The RCIC system was manually started to control reactor water level in anticipation of loss of RFPs tripping on low vacuum. The SRVs were manually operated to maintain reactor pressure. No ECCS or RCIC system reactor water level initiation set points were reached. The RCIC system was removed from service on February 11, 2013, at 1449 CST.
During the investigation into this event, it was discovered that valves 3-FCV-003-0071, 3-FCV-003-0072, and 3-FCV-003-0073 were leaking. The leakage past these valves caused flow induced erosion and excessive vibration in the long cycle return line piping to the miscellaneous drain header. This resulted in a through wall leak in the long cycle return line piping.
A work order search of the maintenance history for valves 3-FCV-003-0071, 3-FCV-003-0072, and 3-FCV-003-0073 was conducted. The work order search showed a history of leakage past the valve seats starting in 1998. The valves were repaired multiple times until 2005 when a work order was written on each of these valves identifying them as chronic maintenance problems and stating a new valve design was needed for this application. The fixed disc gate valve design installed was not a suitable design for this application due to the susceptibility to leak and difficulty in performing repairs. The use of gates valves for this application is a root cause of this event.
The work orders generated in 2005 were cancelled in 2010 following a management directive to reduce maintenance work order back log. The justification for removal of these work orders in 2010 did not adequately address risk. Station personnel had not consistently considered risk when making this decision. These actions exposed BFN, Unit 3, to continued risk of failed piping downstream of the FWLC valves providing the potential for degraded, or loss of, condenser vacuum.
V. Assessment of Safety Consequences
The RCIC system is designed to operate either automatically or manually following reactor pressure vessel (RPV) isolation accompanied by a loss of coolant flow from the feedwater system to provide adequate core cooling and control of the RPV water level.
The RCIC system was manually initiated during this event to control reactor water level.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event:
This event resulted in a loss of reactor water level control due to the loss of reactor feedwater. The RCIC system was manually initiated to control reactor water level and the SRVs were manually operated to control reactor pressure. The RCIC system maintained reactor water level in the prescribed band during this event. In addition to the RCIC system, the ECCS and Automatic Depressurization System were operable and available to provide core cooling if needed.
B. For events that occurred when the reactor was shut down, availability of systems or components needed to shutdown the reactor and maintain safe shutdown conditions, remove residual heat, control the release of radioactive material, or mitigate the consequences of an accident:
All safety systems remained available during this event and were operated as required.
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from discovery of the failure until the train was returned to service:
There were no safety systems rendered inoperable as a result of this event.
Safe shutdown conditions were established and maintained. Therefore, NA concluded there was minimal safety significance for this event.
VI. Corrective Actions
Corrective Actions are being managed by TVA's corrective action program under Problem Evaluation Report (PER) 710206.
Immediate and Interim Corrective Actions A temporary patch was placed on the failed FWLC return line piping. Once the patch was in place, condenser vacuum was restored. Repairs to the failed FWLC return line piping were completed on February 21, 2013.
Corrective Actions to Prevent Recurrence
1. Replace valves 1, 2, 3-FCV-03-0071, -0072, -0073 with valves appropriately designed for the required operating conditions.
2. The BFN has implemented a Strategic Performance Management process to reinforce and institutionalize conservative decision making principles.
3. Establish initial and continuing training requirements, and develop and deliver training to provide expected behaviors for leaders and craft that support their roles and responsibilities for Operational Focus, Nuclear Safety, Risk Awareness/Focus, and Decision Making.
VII. Additional Information:
A. Previous similar events at the same plant:
A search of BFN LERs for Units 1, 2, and 3 for the last several years did not identify any similar events.
A search was performed on the BFN corrective action program. Similar PERs related to the condition which caused the event reported in this LER are PERs 41131 and 52947.
The PER 41131 was closed to PER 52947 which identified that excessive movement of the long cycle piping caused the weld to the miscellaneous drain header to fatigue. The piping movement was caused by flashing in the line due to leakage through one or more of the feedwater long cycle valves. The corrective actions did not address the risk of failing to repair and maintain the valves.
As a result of not addressing the actual failure mode and recognizing the risk associated with the FWLC return valves, BFN missed an opportunity to prevent this event.
Since 2003, when PER 52947 was written, BFN corrective action procedures have undergone revisions to improve the quality of causal evaluations.
B. Additional Information:
There is no additional information.
C. Safety System Functional Failure Consideration:
In accordance with Nuclear Energy Institute 99-02, this condition is not considered a safety system functional failure.
D. Scram with Complications Consideration:
This event did not result in an unplanned scram with complications.
VIII. COMMITMENTS
There are no commitments.