At 0209
CST, on January 29, 2017, while the plant was in MODE 4 for a refueling outage, the main control room crew removed the AC/DC inverter in the Division 1, 120 VAC electrical distribution system from service due to an equipment malfunction. Removing the inverter from service caused a loss of the associated 120 VAC instrument buss. This instrument buss loss caused a trip of the Division 1 Control Building Chill Water and Ventilation system. The Division 2 Control Building Chill Water and Ventilation System was locked out for surveillance testing at the time of the equipment failure. This condition rendered both divisions of Control Building Chill Water and Ventilation Systems unable to perform the support function for cooling Division 1 and 2
AC and
DC power distribution systems. These systems are required to support the operability of two required divisions of
shutdown cooling. Division 2
Shutdown Cooling System was in service and remained in service through out the event.
The Division 2 Control Building Chill Water and Ventilation System was returned to service at 0220 CST on January 29, 2017.
Division 1 Control Building Chill Water remains inoperable pending restoration with the installed backup Division 1 DC/AC inverter. Actions are ongoing to place this component in service and restore the associated 120 VAC instrument buss.
The equipment malfunction was limited to the Division 1 inverter. The investigation of the inverter failure is ongoing. This condition is being reported in accordance with 10 CFR 50.72(b)(3)(v)(B).
The NRC Senior Resident Inspector has been notified.
