05000382/LER-2014-003
| Waterford 3 Steam Electric Station | |
| Event date: | 08-18-2014 |
|---|---|
| Report date: | 10-17-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3822014003R00 - NRC Website | |
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INITIAL CONDITIONS
On 8/18/2014 at 0820, Waterford Steam Electric Station Unit 3 (Waterford 3) was in Mode 1 at approximately 100% power. Component Cooling Water (CCW) [CC], Auxiliary Component Cooling Water (ACCW) [KE] and Ultimate Heat Sink (UHS) [BS] train A were operable. Planned system outage had CCW train B inoperable, which resulted in supported systems inoperable by cascading Technical Specification (TS) 3.7.3 Limiting Condition for Operation (LCO) for CCW and ACCW Systems, including the Ultimate Heat Sink (UHS) TS 3.7.4 LCO, 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> ACTION (a) to restore the inoperable train (A) UHS to operable.
EVENT DESCRIPTION
On 8/18/14, at approximately 0853, the A Train ACCW Wet Cooling Tower (WCT) [CTW] Fan [BLO] 6A tripped, rendering the redundant train A UHS inoperable, causing an unplanned entry into the 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> TS 3.7.4 ACTION (b), as the plant was already in TS 3.7.4 ACTION (a) for planned maintenance and cascading on CCW Train B that had already rendered the train B UHS inoperable. TS LCO 3.7.4 ACTION (b), which requires restoring at least one UHS train to OPERABLE status within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and COLD SHUTDOWN within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
At approximately 0948 CDT the WCT Fan 6A electric motor thermal overload relays were reset, the fan restarted, and operated properly; but, not yet declared operable. At 1016, CCW train B had been restored from the planned maintenance and was declared operable, exiting the 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 6 hour6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> TS LCO 3.7.4 ACTION (b), and re-entering the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> TS LCO 3.7.4 ACTION (a).
Waterford 3 maintenance determined that WCT Fan 6A motor amperage readings were within specification (-33 amps per phase, with nameplate 39 full load amps). The thermography technician found and recorded a 22 degrees Fahrenheit temperature delta at A phase thermal overload to motor termination, with a 0 degrees Fahrenheit reading delta on the other two phases, indicating a high resistance associated with A phase connection [CON] Maintenance checked for loose connections in the Motor Control Center [MCC] MCC-315A WCT fan 6A starter (ACCEOL315A-12M RELAY) [MSTR] and found the center phase A thermal overload-to-motor T-lead termination was loose inside the mechanical lug. Technicians noted the phase A mechanical lug was tight (galled) and could not be tightened further; but, the motor T-lead showed looseness when moved. The phase A mechanical lug was replaced. All three phases of thermal overloads were tested and found to be within tolerance. The thermal overloads for all three phases were replaced with spares that were tested successfully prior to installation.
Review of equipment database shows the mechanical lug is from original construction and has not been replaced since then, approximately 30 years.
Thermography was performed on the WCT train A associated Motor Control Center (MCC 315A) on 10/7/2014, which identified elevated temperatures on the load side of the starter at the mechanical lug and motor T-lead connections associated with 3 WCT fans (3A, 4A, & 8A), which were subsequently tightened over the next two days. Subsequently, thermography was performed on DCT A Fans 1-15, with readings satisfactory. (CR-WF3- 2014-5157) The timeline (below) indicates wiring connections were not loose when MCC-315A connections were checked during preventive maintenance on 4/24/14 based on thermography taken with motors running detected no anomalies when performed on 6/4/14. The condition of the loose wire terminal connection, which caused the thermal overload trip from the elevated temperature at A phase motor T-lead termination, was determined a recent development.
System Description:
The Wet Cooling Towers (WCTs) are located outside the Reactor Auxiliary Building on the east and west sides.
Two 100% capacity, evaporative, wet type cooling towers are provided to assist the CCW system whenever the heat rejection capacity of the CCW system is exceeded. Each tower is capable of dissipating the maximum heat load from the CCW Heat Exchanger plus an Essential Chiller.
The WCTs are next to the DCTs in the floating chambers of the Nuclear Plant Island Structure. Each WCT consists of a Wet Tower Basin, ACCW spray headers and nozzles, and eight fans. Each wet tower is constructed of two adjacent cells. Each cell houses four of the eight fans. The basin is common to both cells.
Timeline:
04/24/14 MCC connections checked tight during preventive maintenance per ME-004-151, "480 VAC MOTOR CONTROL CENTER (MCC)" under WO- 52370262 during RF-19.
06/04/14 Semi-annual Thermography of MCC-315A cubicles performed under WO-52530534.
08/18/14 Declared Component Cooling Water (CCW) [CC] Train B Inoperable for maintenance to 0820 replace relay EG EREL2392-Q-RELAY ((TIMING REL) DGB SEQUENCER-SIX8 REL).
08/18/14 Received Annunciator B0509, Wet Cooling Tower "A" Fan Power Lost. PMC PID D46217, 0853 WET CLG TWR A FAN MTR OVERLD, indicated "detected".
08/18/14 Operations shift NAO's reset the thermal overloads for WCT Fan 6A. Fan was verified 0948 operating properly.
08/18/14 PME performed independent verifications for EG EREL2392-Q-RELAY and the relay was 1016 bench tested SAT under WO-371879. The shift crew exited TS 3.7.4.b due to declaring CCW Train B operable.
08/18/14 Swapped protected trains. The protected train is now Train B.
1402 08/18/14 PME Troubleshot Wet Cooling Tower fan 6A and replaced "A" phase T-lead mechanical lug.
1800 Thermography technician found and recorded a 22 degrees Fahrenheit temperature delta at "A" phase thermal overload to motor T-lead termination. Technicians noted the "A" phase mechanical lug was tight (galled) but the motor T-lead showed looseness when moved.
08/18/14 Completed OP-903-118 Attachment 10.16, Cooling Tower A Fans Operability, for retest of 1938 Wet Cooling Tower Fan 6A after completion of maintenance. Declared Wet Cooling Tower Fan 6A operable. Exited TS 3.7.4 action (a).
10/07/2014 EOC Thermography performed on MCC-315A finds Elevated temperature on three more WCT Fan motors (CR-WF3-2014-5157) 10/8/14 Maintenance technicians wiggled the suspect wire under WO-395657 for WCT Fan 8A starter T-lead connection and found "B" phase wire loose and connector was tightened 1/2 turn.
10/9/14 Maintenance technicians wiggled the suspect wire under WO-395629 for WCT Fan 3A starter T-lead connection and found "B" phase wire loose and connector was tightened 3/8 turn.
Also, the maintenance technicians wiggled the suspect wire under WO-395656 for WCT Fan 4A starter T-lead connection and found "C" phase wire loose and connector was tightened 3/8 turn. (CR-WF3-2014-5205)
CAUSAL FACTORS
The loose wire resulted in a trip of the WCT fan 6A associated thermal overload (time over current), which resulted in the loss of WCT Fan 6A and entry into one hour shutdown action per TS 3/4.7.4 ACTION (b) (due to a concurrent CCW Train B out of service), resulting in both trains of Ultimate Heat Sink [BS] to be inoperable.
Subsequently, WCT Fan 6A could not be restored operable status without reworking the loose wiring connection.
Thermography technician found and recorded a 22 degrees Fahrenheit temperature delta at A phase thermal overload to motor T-lead termination.
The direct cause to the WCT fan 6A trip was due to localized heating at the motor starter T-lead connection on the thermal overload because a galled connection on the mechanical lug resulted in degraded clamping force, over time, on the A phase conductor. Technicians noted that the A phase mechanical lug was tight (galled) but the motor T-lead showed looseness when moved. The cause was determined to be the procedure ME-004-151 contained insufficient detail on how to verify successful implementation for tightening a mechanical lug, and corrective action is planned to add the appropriate detail.
CORRECTIVE ACTIONS
CCW [CC] (Component Cooling Water) train B had been restored from the planned maintenance and was declared operable, exiting TS LCO 3.7.3 and associated cascading TSs on Train B at 1016 on 8/18/2014.
The event was entered into the Waterford 3 corrective action program as Condition Report CR-WF3-2014-4430, in which contains the following corrective actions (CA):
Completed Actions:
- Operability of WCT Fan 6A was restored after maintenance replaced A phase T-lead mechanical lug and operations completed post maintenance testing, at 1938 on 8/18/2014.
- Thermography was performed on the WCT train A associated Motor Control Center (MCC 315A) on 10/7/2014, which identified elevated temperatures at connections associated with 3 WCT fans (3A, 4A, & 8A), which were subsequently tightened over the next two days. Subsequently, thermography was performed on DCT A Fans 1-15, with readings satisfactory. These actions were completed 10/9/2014 under work order (WO) 392074.
Planned Actions:
- Revise Electrical Maintenance procedure ME-004-151, 480-VAC Motor-Control Center (MCC), to add a step to verify successful implementation for tightening a motor T-lead mechanical lug connection (i.e. "Wiggle" wire at the motor starter T-lead connection to ensure connector and wire is tight.). This additional detailed step is expected to identify and prompt correction of loose connections resulting from the effects of those similar to galled threads by checking resistance to applied force on connection terminal lead. (CA-15)
- Perform, and track the performance of, thermography of the WCT A and WCT B fans MCC cubicles, WO 392074 and WO 392075. The performance of these work orders supports an extent of condition review. WO 392074 is associated with MCC-315A thermography related to WCT and DCT fan motor electrical connections on train A, which has been completed (see above). WO 392075 is associated with MCC-315B thermography related to WCT and DCT fan motor electrical connections on train B. (CA-10 and CA-08)
- Discuss condition report lessons learned for connector failure mode with Electrical Maintenance technicians to heighten awareness of connector issues during Motor Control Center (MCC) maintenance activities. (CA-11)
- Verify WCT A fan motor starter T-lead connection mechanical lugs are tight. Reference: WO-395960 (WCT Fan 1A), WO-395961 (WCT Fan 2A), WO-395962 (WCT Fan 5A) and WO-395963 (WCT Fan 7A). Note that these 4 referenced train A UHS (WCT and DCT) fans are those that had thermography results not indicating the need for their associated connections to be physically verified tight. (CA-16)
- Perform a needs analysis using the Systematic Approach to Training (SAT) process to determine training requirements related to Electrical Worker Practices and ME-004-151 Procedure contained insufficient detail on how to verify successful implementation for tightening a mechanical lug. Upon completion of the needs analysis, issue additional corrective actions as necessary to document completion of required training.
SAFETY SIGNIFICANCE
A review of the event determined that the safety significance is minimal for the following reasons:
- It was for a very short duration that the condition existed with the inoperability of both trains of UHS, approximately 83 minutes.
- The loss of one WCT fan is reasonably comparable to other conditions allowed by Technical Specification (TS), such as the loss of a single Dry Cooling Tower (DCT) fan, or less. Each train of the UHS system has 8 WCT fans and 15 DCT fans. With ambient temperatures at the time (0853 —1016 on 8/18/2014) not exceeding 87 degrees Fahrenheit, Waterford 3 TS 3.7.4 would have allowed up to 3 DCT fans to be inoperable without entering a restoration or shutdown action. At the time, none of the 15 DCT fans were inoperable. It is reasonable to conclude that the heat removal capacity from losing the single WCT fan (6A) is less than the heat removal capacity of the TS supported allowance for loss of 1, 2 or 3 DCT fans, at the given ambient temperatures for the timeframe, and therefore, having no safety significance.
SIMILAR EVENTS
There were no similar events at Waterford 3 identified.
ADDITIONAL INFORMATION
Energy industry identification system (EllS) codes and component function identifiers are identified in the text with brackets [ ].