05000390/LER-2017-016
| Watts Bar Nuclear Plant. Unit 1 | |
| Event date: | 12-20-2017 |
|---|---|
| Report date: | 02-20-2018 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3902017006R00 - NRC Website | |
I. PLANT OPERATING CONDITIONS BEFORE THE EVENT
Watts Bar Nuclear Plant (WBN) Unit 1 and Unit 2 were at 100 percent rated thermal power (RTP) .
II. DESCRIPTION OF EVENT
A. Event Summary On December 20, 2017, at 1040 Eastern Standard Time (EST), the Watts Bar Nuclear Plant (WBN) 1B-B 6.9kV Shutdown Board (SDBD) {EIIS:EB} normal feeder breaker {EIIS:BKR} opened.
The loss of voltage to the 1B-B SDBD resulted in the start of Auxiliary Feedwater {EllS:BA} system 1B-B Motor Driven Auxiliary Feedwater (MDAFW) pump {EIIS:P}, the Unit 1 Turbine Driven Auxiliary Feedwater (TDAFW) pump, and the start of all four Emergency Diesel Generators (EDGs){EllS:DG}. Power was restored to the 1B-B 6.9 kV SDBD when it loaded on to its associated EDG.
Following initial investigation, the 1B-B 6.9 kV SDBD was transferred to its alternate offsite power source, Common Station Service Transformer (CSST) C at 1217 EST. A second loss of voltage occurred at 1230 EST when the 1B-B 6.9 kV SDBD alternate feeder breaker opened. The loss of voltage to the 1B-B SDBD did not result in the restart of the 1B MDAFW pump, the Unit 1 TDAFW pump, or EDGs; as this equipment remained running from the earlier event. Power was restored to the 1B-B 6.9 kV SDBD when it loaded on to the associated EDG. Restoration of normal offsite power to the 1B-B 6.9 kV SDBD was completed at 1654 EST.
During the event, the Unit 1 upper containment temperature dropped below the Technical Specification (TS) lower temperature limit. This TS was exited when upper containment heater elements were reenergized following restoration of normal power to the 1B-B 6.9 kV SDBD.
Other than several common Unit TS Limiting Condition for Operation (LCO) Action Statements being entered, Unit 2 was not operationally impacted by the loss of power to the 1B-B Shutdown Board.
This event is being reported to the Nuclear Regulatory Commission (NRC) under 10 CFR 50.73(a)(2)(iv)(A) as a valid safety system actuation and under 10 CFR 50.73(a)(2)(v)(C) and (D) as an event or condition that could have prevented fulfillment of a safety function needed to control the release of radioactive material and mitigate the consequences of an accident.
B. Inoperable Structures, Components, or Systems that Contributed to the Event No inoperable equipment contributed to this event.
C. Dates and Approximate Times of Occurrences Events described in the table below occurred on December 20, 2017.
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission. Washington. DC 20555-0001, or by e-mail to NEOB-10202. (3150-0104), Office of Management and Budget. Washington, DC 20503. If a means 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.
Time Event 1040 The normal feeder breaker from CSST D to the 1B-B 6.9kV SDBD opened, resulting in multiple alarms. All four EDGs and the 1B-B MDAFP and the TDAFP started as designed.
1040 Entered 0-A01-43.02, Loss of U1 Train B Shutdown Boards.
1040 Entered the following TS LCOs for Unit 1 and Unit 2:
TS 3.8.1 AC Sources - Operating, Condition A and D Two required offsite circuits inoperable TS 3.8.9 Distribution Systems - Operating, Condition A One or more AC electrical power distribution subsystems inoperable.
1048 Entered Unit 1 TS 3.1.8, Rod Position Indication not met due to more than one Analog Rod Position Indication (ARPI) outside of +/-12 steps from group demand position due to increased containment temperatures. Entered LCO 3.0.3 due to no condition existing for more than one ARPI outside of limits.
1114 Entered Unit 1 TS 3.6.5 Containment Air Temperature, Condition A due to upper containment air temperature being less than 87 degrees Fahrenheit.
1154 Exited TS 3.1.8, Rod Position Indication Unit 1 and LCO 3.0.3 for Unit 1 due to all ARPIs being restored to within 12 steps.
1217 1B-B SDBD energized from alternate feed from C CSST. Exited Unit 1 and Unit 2 TS 3.8.9 Condition A at 1226.
1230 Received multiple alarms, the 1B-B 6.9 KV SDBD alternate feeder breaker has opened, all 4 EDGs remained running with the 1B EDG now energizing the 1B-B 6.9 KV SDBD. Entered 0-A01-43.02, Loss of Unit 1 Train B Shutdown Boards.
1230 Entered TS 3.8.9 Distribution Systems - Operating, Condition A not met for Units 1 and 2 1300 Authorized 1B-B 6.9kV SDBD troubleshooting and repairs.
1312 1A EDG shutdown 1333 2A EDG shutdown 1335 2B EDG shutdown 1510 Conducted crew brief for performance of work order to restore D CSST to 1B-B 6.9kV
SDBD
1635 Entered TS 3.3.5 Loss of Power Diesel Generator Start Instrumentation Actions A and B 1645 Energized 1B-B 6.9kV SDBD from CSST D 1654 TS 3.8.9 Distribution Systems - Operating now met. Exit TS 3.8.9 Condition A. 1B-B SDBD energized from CSST D. 1B Reactor Vent Board and 1B2-B C&A Vent board restored.
1700 Exited Unit 1 and Unit 2 TS 3.8.1 Conditions A and D 1730 Entered Unit 1 and Unit 2 TS LCO 3.3.5 C.1. This action requires declaring the 1B EDG inoperable in accordance with TS 3.8.1.
1744 Exited Unit 1 TS 3.6.5 Containment Air Temperature with Upper containment air temperature greater than 87 degrees Fahrenheit.
On December 21, 2017 at 1714, operations authorized completion of repairs to the Potential Transformer (PT) moveable secondary connectors for the PT drawer. Following completion of those repairs, the 1B EDG was returned to service and declared operable on December 22, 2017 at 0218.
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001. or by e-mail to Infocollects.Resource@nrc.gov. and to the Desk Officer, Office of Information and Regulatory Affairs.
NEOB-10202, (3150-0104). Office of Management and Budget, Washington, DC 20503. If a means 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
2017 - 00 016 D. Manufacturer and Model Number of Components that Failed During the Event The secondary PT contacts that did not have good contact were part of a General Electric Magne- Blast 6.9kV PT Drawer.
E. Other Systems or Secondary Functions Affected
With the 1B-B 6.9kV Shutdown board energized from the 1B EDG, the upper containment heaters could not be energized, which resulted in the upper containment average air temperature going below the TS limit of 85 degrees Fahrenheit (2 degrees assumed for instrument uncertainty, or 87 degree Fahrenheit operational limit).
F. Method of discovery of each Component or System Failure or Procedural Error The control room received multiple alarms due to the loss of the 1B-B 6.9kV SDBD.
G. Failure Mode and Effect of Each Failed Component The failure mode is a loss of contact of secondary PT connections in the protective relay drawer for the 1B-B SDBD.
H. Operator Actions
Operator actions to recover from the loss of power to the 1B-B SDBD are described in Section II.0 of this report.
I. Automatically and Manually Initiated Safety System Responses The loss of power to the 1B-B 6.9 kV SDBD resulted in the automatic start of the 1B MDAFP, the Unit 1 TDAFWP, and the four EDGs.
III. CAUSE OF THE EVENT
A. The cause of each component or system failure or personnel error, if known.
The trip of the 1B-B 6.9kV SDBD feeder breaker was caused by poor contact of the B and C phases of the protective relay PT drawer secondary connections.
B. The cause(s) and circumstances for each human performance related root cause.
No human performance causes are attributed to this event.
IV. ANALYSIS OF THE EVENT
The normal feeder breaker for the 1B-B 6.9kV SDBD unexpectedly tripped on December 20, 2017. All EDGs started and 6.9kV SDBD 1B-B was supplied by EDG 1B. Shortly after this the 1B-B 6.9kV SDBD was aligned to its alternate feeder breaker, the board tripped again and EDG 1B once again supplied comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001. or by e-mail to Infocollects.Resource@nrc.gov. and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202. (3150-0104), Office of Management and Budget, Washington. DC 20503. If a means 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
2017 - 00 016 power to the board. The 6.9kV SDBD 1B-B was able to be normally supplied with the isolation of the degraded and loss of voltage trip circuits. This isolation allowed for a transition from the 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> LCO of TS 3.8.9 to the LCOs of TS 3.8.1 B via TS 3.3.5 and allowed access to the protective relay PT drawer for troubleshooting. These protection circuits were reconnected upon completion of corrective actions.
During the time period that the 1B-B 6.9 kV SDBD was powered from EDG 1B, certain 480V loads were prevented from loading on to EDG 1B by design to prevent overloading. These loads included the upper containment heaters used to maintain temperatures within accident analysis limits. Unit 1 upper containment average air temperature dropped to 84.9 degrees Fahrenheit (no allowance for instrument uncertainty).
Visual investigation of the protective relay PT drawer revealed that the plate housing the secondary finger connections was slightly bent. Since the connections are not visible nor accessible when the PT drawer is closed, a contact swipe was taken. This swipe revealed minimal contact engagement of B and C phase.
The micarta mounting block that houses the secondary pins was trimmed to offset the bent secondary finger plate. Post work contact swipe revealed significant improvement in the finger-pin engagement.
V. ASSESSMENT OF SAFETY CONSEQUENCES
From an accident standpoint, the most significant item was a decrease in upper containment average temperature below the accident analysis assumptions (lowest average indicated value of 84.9 degrees Fahrenheit). A review of the containment analysis indicates, for the normal 85 degree limit in upper containment, that the limiting pressure for a large break loss of coolant accident (LOCA) is 9.36 psig and limiting temperature is 235 degrees Fahrenheit. The small temperature decrease below that specified for upper containment temperature is not expected to have an impact on the containment safety function.
During the event, TS 3.0.3 was entered for multiple ARPI being outside limits, which is a condition not addressed by TS 3.1.8. TS 3.0.3 was exited in just over one hour, which is well within the limit of seven hours before Mode 3 must be entered. All ARPI were restored to within limits, and no rod movement occurred during this event. A risk assessment for this event concludes the additional risk from this short term equipment unavailability was negligible.
A. Availability of systems or components that could have performed the same function as the components and systems that failed during the event The 1A-A 6.9 kV Shutdown board remained operable 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 The 1B-B 6.9 kV Shutdown board was powered from the 1B EDG during the period of this event, so the 1B-B 6.9 kV SDBD could perform its safety function.
comments regarding burden estimate to the Information Services Branch (T-2 F43), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to NEOB-10202. (3150-0104), Office of Management and Budget Washington, DC 20503. If a means 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.
VI. CORRECTIVE ACTIONS
This event was entered into the Tennessee Valley Authority (TVA) Corrective Action Program and is being tracked under Condition Report (CR) 1370572.
A. Immediate Corrective Actions
Upon receiving alarms for a loss of the 1B-B 6.9 kV SDBD, efforts commenced to manage the event and restore power. The micarta mounting block that houses the secondary pins was able to be trimmed to offset the bent secondary finger plate. Post work contact swipe revealed significant improvement in the finger-pin engagement.
B. Corrective Actions to Prevent Recurrence or to Reduce Probability of Similar Events Occurring in the Future The procedure associated with inspection of the PT drawer will be revised to address lessons learned from this event. The mounting blocks that house the secondary pins for the 6.9 kV SDBDs 1A-A, 2A-A, and 2B-B PT drawers will be inspected and either replaced or adjustments made to ensure quality electrical connections in a manner consistent to what was completed on the 1B PT drawer.
VII. PREVIOUS SIMILAR EVENTS AT THE SAME SITE
On October 16, 2017, WBN Unit 1 reported a loss of voltage to the 1B-B 6.9kV SDBD in LER 390/2017- 010. On August 17, 2017, WBN lost power to the 1B-B 6.9kV SDBD. The cause of this event was determined to be mechanical vibration while closing a panel drawer resulting in actuation of protective relays that led to a loss of power. The CR associated with LER 390/2017-010 had created actions to inspect the PT drawer described in LER 390/2017-016, but these actions had not been completed at the time of this event.
On July 15, 2016, WBN Unit 1 reported a loss of voltage to the 1 B-B 6.9kV SDBD in LER 390/2016-008.
On May 17, 2016 while restoring from a plant modification, the feeder breakers for the 6.9kV SDBD 1B-B tripped resulting in a loss of bus voltage. The feeder breakers tripped due to actuation of the loss of voltage relays in the shutdown board protective relay trip logic circuit resulting in separation of offsite power from the 6.9kV SDBD 1B-B. The cause was attributed to an inadequate process due to the order of AC and DC fuse restoration.
The two events in 2017 (LER 390/2017-010 and LER 390/2017-016) are related due to the secondary contacts having poor finger-pin engagement leading to the loss of the 1B-B 6.9 kV SDBD.
VIII. ADDITIONAL INFORMATION
None.
IX. COMMITMENTS
None.