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On 09/17/2005 at 1129 CDT, Browns Ferry Unit 3 reactor automatically scrammed due to the trip of the main generator turbine on low main condenser vacuum. Prior to the trip, the unit was operating at reduced power (74 percent) and maintenance was in progress to repair in place a secondary plant moisture separator level control valve. This maintenance activity resulted in the low condenser vacuum condition. Plant response to the scram was as expected. Post-scram transient RPV low water level caused a valid automatic initiation of primary containment isolation (PCIS) valves group 2 (containment sumps and shutdown cooling valves), group 3 (reactor water cleanup system), group 6 (ventilation systems, including initiation of standby gas treatment and control room emergency ventilation systems), and group 8 (traversing in-core probe system).

This event is reportable to the NRC via ENS within four hours under 10CFR50.72(b)(2)(iv)(B) (RPS actuation while critical), and within eight hours under 10CFR50.72(b)(3)(iv)(A) (RPS and containment isolation actuation). Browns Ferry Unit 2 was not affected. The reactor shutdown and all control rods fully inserted. Condenser vacuum has been restored and decay heat is being removed using the bypass valves.