05000387/LER-2016-007

LER-2016-007, Inoperability of Swing Bus Transfer Switch Due to Deformed Bolt on Linkage

Susquehanna Steam Electric Station Unit 1
Event date:	03-05-2016
Report date:	05-0-2016
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
3872016007R00 - NRC Website

LER 16-007-00 for Susquehanna, Unit 1, Regarding Inoperability of Swing Bus Transfer Switch Due to Deformed Bolt on Linkage

ML16125A027
Person / Time
Site:	Susquehanna
Issue date:	05/03/2016
From:	Franke J A Susquehanna, Talen Energy
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
PLA-7465 LER 16-007-00
Download: ML16125A027 (6)

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 Int ocollects.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.

Susquehanna Steam Electric Station, Unit 1 05000387

CONDITIONS PRIOR TO EVENT

Unit 1 — Mode 1, approximately 87 percent Rated Thermal Power Based upon the failure mode of the transfer switch, the switch is considered to have been inoperable since the last time it was tested on February 5, 2016.

EVENT DESCRIPTION

Background Information/System Description:

Two redundant Class 1E 480 V Isolation Swing Buses (1B219 and 1B229) [EIIS System/Function Identifier:

ED/BU] provide power for the Residual Heat Removal (RHR) Injection Valves [EIIS System/Function Identifier: BO/INV], RHR Minimum Flow Valves [EIIS System/Function Identifier: BO/V], Recirculation Loop Bypass Valves [EIIS System/Function Identifier: AD/V], and Recirculation Discharge Valves [EIIS System/Function Identifier: AD/V]. The preferred power source to the isolation swing bus is from the A and B 480 VAC ESS Bus Load Groups via motor-generator set [EIIS Function Identifier: MG], and the alternate power is supplied directly from the C and D 480 VAC ESS Bus Load Groups, respectively. The transfer will occur automatically upon reduction or loss of voltage from the preferred source.

Event Timeline:

On March 5, 2016 during surveillance testing, Automatic Transfer Switch (ATS) [EIIS Function Identifier:

ASU], 1ATS229, failed to close in on the alternate supply resulting in a loss of the Division 2 Class 1 E ESS 480V MCC 1B229 and entry into Technical Specifications 3.3.3.1, 3.5.1, and 3.6.1.3.

The detailed timeline is as follows:

February 5, 2016 12:00 — 1ATS229 was successfully tested.

February 8, 2016 01:45 — Technical Specification 3.5.1 was entered for a Division 1 RHR system outage window. Work associated with 1ATS219 was performed during this window.

February 10, 2016 17:43 — The RHR system outage window was completed and Technical Specification 3.5.1 was exited.

March 5, 2016 21:25 - 1ATS229 failed to close in on the alternate supply. Testing was stopped and 1B229 was de-energized.

21:30 — The Field Unit Supervisor (FUS) reported that the ATS transfer motor was very hot to the touch and had an acrid odor.

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.

Susquehanna Steam Electric Station, Unit 1 05000387 March 6, 2016 00:45 - Electrical Maintenance personnel arrived to assess the status of 1ATS229.

04:05 - The MG set output breaker was closed and power was restored to 1B229 04:06 — Operations exited Technical Specifications 3.3.3.1 and 3.6.1.3 with valve power and indication restored. Technical Specification 3.5.1 for RHR Low Pressure Coolant Injection (LPCI) mode was maintained due to 1ATS229 still being inoperable to transfer to alternate power.

March 7, 2016 03:33 — Operations entered Technical Specifications 3.3.3.1 and 3.6.1.3 due to de-energizing 113229 for troubleshooting.

15:27 — Troubleshooting, repairs, and swing bus surveillance were satisfactorily completed.

16:57 — Operations exited Technical Specifications 3.3.3.1, 3.5.1, and 3.6.1.3.

Troubleshooting/Investigation Engineering completed a troubleshooting plan and a Failure Mode Analysis. Prior to investigating the internals of the transfer switch, Electrical Maintenance attempted to swap the transfer switch in its alternate position and as expected the switch cycled continuously without swapping. All the hardware on the transfer switch was checked for tightness, but nothing abnormal was identified. The transfer switch linkage was observed while in the upward normal position, but no abnormalities were found. The transfer switch was then manually manipulated to the alternate (down) position and the linkage was observed again. It was determined that in this position the shaft of the motor had a great deal of downward force applied to it indicating that the upper linkage rod was too long. The bolt attaching the rod to the mounting plate was removed and it was discovered that the bolt was slightly deformed. The bolt was removed and the rod was pulled up to the proper position on the mounting plate, which revealed that the rod was approximately 1/4 inch too long resulting in the continuous cycling of the switch without latching in the alternate position. An adjustment was made to the rod to align it with the mounting plate. During adjustment it was noted that all nuts attaching the upper rod were tight eliminating the possible cause of a loose connection on the upper rod linkage. The rod was reinstalled with the deformed bolt and then the transfer switch was tested. It successfully transferred to the alternate position with the deformed bolt; however, since the bolt being deformed over time in addition to the rod being too long caused the failed transfer, the bolt was replaced to eliminate any concern. No other issues were found and all additional attempts to place the transfer switch in the alternate position were successful.

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 internal 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.

Susquehanna Steam Electric Station, Unit 1 05000387 Reportability Based on the cause of the failure, Susquehanna believes that there is firm evidence that the transfer switch would have failed on its next actuation following testing on February 5, 2016. Based on this conclusion, the condition existed for a period longer than allowed by Technical Specifications. As a result, this condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications. Since Division 1 was inoperable from February 8, 2016 at 01:45 through February 10, 2016 at 17:43 and this timeframe is enveloped by the time that Division 2 was unknowingly inoperable, this is also considered a condition that could have prevented fulfillment of a safety function and is also reportable in accordance with 10 CFR (a)(2)(v)(D).

CAUSE OF EVENT

The direct cause was determined to be a deformed bolt on the upper linkage.

The apparent cause was the upper linkage rod being too long causing the deformation of the bolt. The previous preventive maintenance (PM) activity inspected the linkage, but did not provide criteria for the inspection specifically with regard to linkage rod length.

ANALYSIS/SAFETY SIGNIFICANCE

Actual Consequence The actual consequence was failure of 1ATS229 to transfer to its alternate supply causing ESS Swing Bus 1B229 to de-energized and resulting in entry into Technical Specifications 3.3.3.1, 3.5.1, and 3.6.1.3.

Potential Consequence In the event of a Unit 1 LOCA, LPCI would be required to inject to the Reactor Pressure Vessel (RPV). If this had occurred during the time that 1ATS229 was affected (2/5/16 to 3/5/16), and concurrent with a loss of power to the primary source for 1ATS229, then transfer to the alternate power supply would have failed and bus 16229 would have lost power. Since the LPCI injection valve HV151F0156 receives power from 16229, it would not open and LPCI injection from Division 2 RHR would not have occurred as required.

Recirculation Pump B discharge and discharge bypass valves HV143F0316 and HV143F032B also receive power from 1B229 and are required to close on a LPCI injection signal to ensure the LPCI flow is directed into the RPV. Additionally, during the period when the Division 1 transfer switch (1ATS219) was out of service for scheduled maintenance, LPCI injection from Division 1 RHR would also not have occurred.

Since HV151F015B, HV151F0176 and HV151F007B are powered from 1B229 and are used to establish the necessary RHR system alignment for Shutdown Cooling (a non-safety mode of operation), this RHR mode of operation would have been prevented.

The RHR minimum Flow valve HV151F007B receives power from 16229 and would not have closed as required after a Division 2 RHR pump start when system flow is established. This would result in diversion of some RHR flow back to the suppression pool via the minimum flow line. While this would not prevent operation of Division 2 RHR in other modes, it could result in reduced flowrate due to the flow diversion.

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 intemet 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.

CORRECTIVE ACTIONS

Revise applicable PM activities associated with the transfer switch to include the following:

1. Inspect the upper and lower linkage for any abnormalities such as deformed bolts that connect the upper and lower rods to the motor or mounting plates and replace any deformed or damaged equipment as necessary.

2. Establish criteria for rod length and adjustment.

COMPONENT FAILURE INFORMATION

Automatic Transfer Switch Information

• Manufacturer — RUSSELECTRIC

• Model Number — RMT4004CEF

• Voltage Characteristics — 480VAC, 400A, 30

PREVIOUS SIMILAR EVENTS

Condition Report 2013-03361: On November 10, 2013, 1ATS229 failed to close on the alternate supply. The cause was vibrations during monthly testing that caused the transfer switch motor mounting bolts to loosen.