At 2002 EST on January 17, 2018, annulus differential pressure exceeded its pressure limit. At that time, the Shield Building was declared inoperable requiring entry into Technical Specification 3.6.15, Conditions A and B. Action was taken by field operators to swap annulus vacuum control dampers to restore annulus differential pressure. At 2024 EST, annulus differential pressure was restored to required limits, the Shield Building was declared operable, and LCO 3.6.15, Conditions A and B were exited.
The temporary loss of the Shield Building resulted from a failure of the annulus vacuum control system to maintain the required differential pressure. Manual swap-over of pressure control to the backup damper restored differential pressure to required limits allowing exit from TS LCO 3.6.15 and restoration of the Shield Building safety function.
The Shield Building ensures that the release of radioactive material from the containment atmosphere is restricted to those leakage paths and associated leakage rates assumed in the accident analysis during a Loss of Coolant Accident (LOCA). The Emergency Gas Treatment System (EGTS) would have automatically started and performed its design function to maintain annulus vacuum within required limits.
This event is being reported pursuant to 10 CFR 50.72(b)(3)(v)(D).
LER 17-008-00 for Watts Bar, Unit 1, Regarding Shield Building Inoperability and Potential Loss of Safety Function Resulting from Spurious Equipment Operation Shield Building Inoperability and Potential Loss of Safety Function Resulting from Spurious Equipment Operation
On June 15, 2017, at 1219 Eastern Daylight Time (EDT), Technical Specification (TS) Limiting Condition for Operation (LCO) 3.6.15 Condition B was entered for Watts Bar Nuclear Plant (WBN) Unit 1 annulus pressure not within limits, resulting in Shield Building inoperability. At 1221 EDT, the WBN Unit 1 annulus pressure returned to normal, the Shield Building was declared operable, and LCO 3.6.15 Condition B was exited. Because the shield building is a non-redundant safety system, operation outside of TS allowable limits represents an event that could have prevented fulfillment of a safety function.
The temporary loss of the Shield Building resulted from a loss of pressure control in the Auxiliary Building caused by a loss of Auxiliary Building General Ventilation due to a spurious cross zone fire alarm. The Auxiliary Building Gas Treatment System was started to maintain Auxiliary Building pressure within limits and the non-safety related Annulus Auxiliary Building ventilation supply fans were replaced.