05000391/LER-2016-007
| Watts Bar Nuclear Plant, Unit 2 | |
| Event date: | 08-23-2016 |
|---|---|
| Report date: | 10-21-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3912016007R00 - NRC Website | |
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555.0001, or by internet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information Icollection.
3. LER NUMBER
2016 - 00
I. PLANT OPERATING CONDITIONS BEFORE THE EVENT
Watts Bar Nuclear Plant (WBN) Unit 2 was in Mode 1 at 48 percent Rated Thermal Power (RTP) .
II. DESCRIPTION OF EVENT
A. Event
On August 23, 2016, at 1356 Eastern Daylight Time (EDT), WBN Unit 2 reactor was manually tripped due to a loss of main feedwater {El IS:SJ} from the 2A Main Feedwater Pump (MFP) {El IS:P}. Concurrent with the reactor trip, the Auxiliary Feedwater (AFW) system actuated as designed. All control and shutdown rods fully inserted. All safety systems responded as designed.
This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(iv)(A) as a manual actuation of the reactor protection system and as an automatic actuation of the AFW system.
B. Inoperable Structures, Components, or Systems that Contributed to the Event No inoperable systems contributed to the event.
C. Dates and Approximate Times of Occurrences Date Time (EDT) Event 8/23/16 1352 2A MFP turbine speed decreases from 4543 to 3848 revolutions per minute (RPM) 1352 Steam Generator (SG) levels start to decrease from 49 percent narrow range 1356 Unit 2 manual reactor trip due to loss of Main Feedwater from 2A MFP when SG levels lowered to approximately 20 percent.
1357 Operations enters 2-E-0, Reactor Trip or Safety Injection 1402 Auxiliary operators report all AFW pumps started and are within expected parameters.
1406 Operations transition to 2-ES-0.1, Reactor Trip Response D. Manufacturer and Model Number of Components that Failed A fitting associated with the 2A MFP turbine governor valve leaked excessively, resulting in the plant trip. The MFP and turbine are a Model HDR manufactured by Byron Jackson.
E. Other Systems or Secondary Functions Affected
No other safety systems were affected by this event.
F. Method of discovery of each Component or System Failure or Procedural Error The failed hydraulic fitting was found during the post trip investigation of this event.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Infocollects.Resource@nrc,gov, and to the Desk Officer, Office of Information and Regulatory Affairs, used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
3. LER NUMBER
2016 - 00 G. Failure Mode and Effect of Each Failed Component The use of incompatible hydraulic fittings led to the event.
H. Operator Actions
Operations personnel identified the reduced main feedwater flow and set a manual trip trigger point of 20 percent SG water level. When this limit was approached, operations personnel manually tripped the plant. The subsequent recovery and response to the trip were uncomplicated.
I. Automatically and Manually Initiated Safety System Responses Operations personnel manually initiated the reactor protection system. Concurrent with the reactor trip, the AFW system automatically actuated as designed.
III. CAUSE OF THE EVENT
A. The cause of each component or system failure or personnel error, if known.
This event was the result of an incompatible fitting being used in the hydraulic controls for the 2A MFP turbine governor valve.
B. The cause(s) and circumstances for each human performance related root cause.
The cause was determined to be a human performance error during the assembly of the 2A MFP control hose connections. The hose connection was assembled incorrectly during the Nuclear Construction work to bring the 2A MFP turbine from an unused layup condition to a condition that was ready for operation as a part of WBN Unit 2 construction. This connection is not visible because it is inside the MFP turbine's oil return system. Therefore this misconfigured connection could not have been identified by the system turnover processes. The connection also functioned correctly during Preoperational testing and for a short time during Power Ascension testing so that there were no adverse indications as precursors to the event.
IV. ANALYSIS OF THE EVENT
WBN Unit 2 was operating at approximately 48 percent power based on power range instrumentation when the 2A MFP governor valve started to close. This was caused by an incorrect hydraulic fitting installation that used a female fitting with a 37-degree flare seating service connected to a standard pipe nipple which is not intended to mate with a 37-degree seating surface. Over time the fitting loosened and the fitting began to leak excessively. Operations personnel identified the degrading main feedwater flow and established a trigger value for a manual trip of 20 percent SG water level. Following the manual trip, operations personnel progressed promptly through their trip response procedures and stabilized the plant.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The loss of the 2A MFP led to a situation where operations personnel manually tripped the plant. The trip response and recovery were uncomplicated, and all safety systems operated as expected. The safety significance of this event was determined to be low.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event No safety systems failed during this event.
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 Not applicable.
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 Not applicable.
VI. CORRECTIVE ACTIONS
This event was entered into the Tennessee Valley Authority (TVA) Corrective Action Program and is being tracked under condition report (CR) 1206191.
A. Immediate Corrective Actions
The incompatible fittings were replaced with the correct fittings for the application.
B. Corrective Actions to Prevent Recurrence
Similar connections of both MFPs were inspected to confirm that no similar conditions existed. This event is being evaluated for inclusion in maintenance training related to pressure retaining connections.
VII. ADDITIONAL INFORMATION
A. Previous similar events at the same plant In LER 391-2016-005-00, Watts Bar reported a trip of the 2B MFP when the 2B MFP turbine condenser lost vacuum. The loss of the 2B MFP led to an automatic reactor trip on low SG water level. While this earlier event involved a MFP trip, it was caused by operator error when draining the 2A MFP turbine condenser, which is unrelated to this event.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
3. LER NUMBER
2016 - 00 007
B. Additional Information
None.
C. Safety System Functional Failure Consideration
This condition did not result in a safety system functional failure.
D. Scrams with Complications Consideration This reactor trip was determined to be uncomplicated.
VIII. COMMITMENTS
None.