05000390/LER-2014-003
| Watts Bar Nuclear Plant, Unit 1 | |
| Event date: | 07-13-2014 |
|---|---|
| Report date: | 09-11-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| LER closed by | |
| IR 05000390/2015000 (30 April 2015) | |
| 3902014003R00 - NRC Website | |
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Watts Bar Nuclear Plant, Unit 1 05000390
I. PLANT CONDITIONS
At the time of the event, Watts Bar Nuclear Plant (WBN) Unit 1 was in Mode 1 at 100 percent rated thermal power (RTP).
II. DESCRIPTION OF EVENT
A. Event
On July 13, 2014, at 1927 Eastern Daylight Time (EDT), both #7 Heater Drain Tank (HDT) pumps {EIIS:P} tripped off-line due to a failed relay {EIIS:RLY} associated with low level circuitry for the #7 Heater Drain Tank {EIIS:TK}. Operations personnel entered the abnormal operating instruction (procedure) for heater drain malfunction, however, HDT level continued to increase because the HDT level controller {El IS:LIC} did not transmit an open demand signal to the bypass valve which would have diverted excess inventory to the condenser. The #7 HDT level continued to increase and began filling the Low Pressure Feedwater Heaters, resulting in high level isolation signals that secured the three Feedwater Heater Strings {EIIS:SM}. Due to the impending loss of feedwater, operations personnel manually tripped the reactor before an automatic trip would occur on low-low steam generator level.
This event is reportable under 10 CFR 50.73(a)(2)(iv)(A).
B. Inoperable Structures, Components, or Systems that Contributed to the Event No inoperable structures, components, or systems contributed to this event. A non-safety related relay and level controller were found to have failed, which resulted in automatic feedwater heater isolation.
C. Dates and Approximate Times of Occurrences Date Time Event (EDT) 7/13/2014 1927 Both #7 Heater Drain Tank Pumps Tripped off-line 7/13/2014 1928 Operators entered procedure/-A0I-47, Heater Drains Malfunction 7/13/2014 1935 Operators initiated downpower to 92% at 1% per minute in accordance with 1-AOI-47.
7/13/2014 1937 All three Low Pressure Feedwater Heater Strings began isolating.
7/13/2014 1937 Operators initiated a manual Unit 1 Reactor and Turbine trip.
D. Manufacturer and Model Number of Components that Failed.
Masonneilan Series 12800 Liquid Level Controller, Model Number 12811 {EIIS:LIC}.
General Electric HFA 71-L7 relay, Model 12HFA51A41F {EIIS:RLY}.
E. Other Systems or Secondary Functions Affected
No other systems or secondary functions were affected by this event beyond the failures identified.
F. Method of discovery of each Component or System Failure or Procedural Error The failures associated with the HDT control scheme were identified as a result of this event .
G. Failure Mode and Effect of Each Failed Component There were two failures associated with this event. The first was the failure of a normally energized relay coil after more than 10 years of operation. This resulted in the trip of both HDT pumps. The second failure is attributed to age related degradation of "soft" parts associated with a level indicating controller. This failure resulted in the inability of the level indicating controller to send an adequate pneumatic signal that would result in the opening of 1-LCV-6-190B, the HDT bypass to the condenser. This second failure led to a high level isolation of the feedwater system.
H. Operator Actions
Upon receiving an annunciation that both Heater Drain Tank pumps had tripped, operators entered procedure 1-A0I-47, Heater Drains Malfunction. Based on this procedure, operators commenced a manual downpower of the unit. When the Low Pressure Feedwater heater strings began isolating on high level, operators manually tripped the reactor.
I. Automatically and Manually Initiated Safety System Responses Upon the loss of all three Low Pressure Feedwater Heater strings, operators manually initiated a Unit 1 Reactor and turbine trip. All safety systems responded as expected.
III. CAUSE OF THE EVENT
A. The cause of each component or system failure or personnel error, if known.
The cause for both component failures is attributed to no associated preventative maintenance (PM) tasks on these components .
B. The cause(s) and circumstances for each human performance related root cause.
Watts Bar has determined that the root cause was that replacement PMs did not exist for the failed components.
IV. ANALYSIS OF THE EVENT
All plant safety systems operated as planned in response to this event.
Watts Bar Unit 1 has three separate feedwater heater strings that support plant operation. The Low pressure plant feedwater heaters numbers 4, 5, 6 and 7 (A, B, and C heater strings) drain to the common #7 Heater Drain Tank. Drains collected in the #7 HDT are pumped forward to the condensate system between the #6 and #7 feedwater heaters using two Heater Drain pumps. The design of the system includes a Net Positive Suction Head (NPSH) protection feature of the Heater Drain pumps by providing a pump trip on low level in the #7 HDT. This protective feature is provided by a limit switch (LS-6-190B) which includes a normally energized auxiliary relay in the control circuit. At the commencement of this event, the normally energized auxiliary relay (GE HFA Relay) coil failed, resulting in the trip of the Heater Drain pumps.
With the trip of the Heater Drain pumps, level in the #7 HDT increased to the level setpoint where Level Indicating Controller (1-LIC-6-190B) should have caused bypass valve 1-LCV-6-190B to open, diverting drain flow to the condenser. This controller failed to provide an adequate opening pneumatic signal to 1-LCV-6-190B as a result of age-related degradation of a diaphragm and o-ring within the device. With the failure of 1-LCV-6-190B to open, level in the system continued to rise to the high level trip setpoint where all feedwater strings were automatically isolated. With the impending loss of normal feedwater, operations personnel manually tripped the unit prior to the receipt of an automatic reactor trip signal.
V. ASSESSMENT OF SAFETY CONSEQUENCES
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The failures that resulted in this plant trip were on plant secondary systems. No redundancy to these devices is provided by the design. All safety systems operated as designed and no abnormal responses were noted. The sequence of events associated with the trip were bounded by the safety analysis assumptions.
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 Systems and components required to maintain safe shutdown conditions were available during the event.
C. For failure that rendered a train of a safety system inoperable, an estimate of the elapsed time from the discovery of the failure until the train was returned to service There were no failures that rendered a safety system inoperable during this event.
VI. CORRECTIVE ACTIONS
This event was entered into the TVA Corrective Action Program (CAP) and is being tracked as problem evaluation report (PER) 909612.
A. Immediate Corrective Actions
The failed relay and level indicating controller were identified and replaced.
B. Corrective Actions to Prevent Recurrence
A root cause analysis determined that replacement preventative maintenance (PM) work orders did not exist for the failed components. Preventative maintenance work orders will be developed for the impacted components. In addition, replacement PMs will be developed for similar critical components of the Secondary Systems based on EPRI Guidance.
VII. ADDITIONAL INFORMATION
A. Previous similar events at the same plant
Watts Bar Unit 1 reported a manual reactor trip due to the start of feedwater heater isolation in {EIIS:LCV} to open, the cause of the valve's failure to open was different (air line to valve failed as a result of vibration induced fatigue as a result of improper installation).
B. Additional Information
Design changes have been implemented at Sequoyah and at Watts Bar Unit 2 to prevent similar single level switch vulnerabilities from tripping both HDT pumps.
C. Safety System Functional Failure Consideration
There were no safety system failures associated with this event.
D. Scrams with Complications Consideration There were no complications during the plant response to this scram.
VIII. COMMITMENTS
None.