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On August 6, 2011, Reactor Protection System (RPS) power supply 1B failed resulting in a partial loss of power to Primary Containment Isolation System (PCIS) groups and an invalid actuation of those PCIS groups. PCIS groups 1 and 2 received partial isolation signals with no subsequent system isolations, as designed. PCIS group 3, 6, and 8 received partial isolation signals with resulting system isolations, also as designed. The combination of loss of RPS 1B and PCIS group 6 isolation resulted in the isolation of the Drywell Floor Drain Sump and the Drywell Continuous Atmospheric Monitor for both particulate and gaseous activity. Thus, both means of automatic monitoring of Reactor Coolant System leakage became inoperable. Unit 1 entered Technical Specification Limiting Condition for Operation (LCO) 3.4.5.D (all required leakage detection systems inoperable) and immediately entered LCO 3.0.3 as required. At the time of occurrence, RPS 1A was being supplied from its alternate source for scheduled maintenance. Thus, the alternate source was not available to RPS 1B. Unit 1 entered LCO 3.0.3 at 0524 (CDT), 'Initiate actions within one hour to place the unit in MODE 2 within 10 hours; MODE 3 within 13 hours; and MODE 4 within 37 hours.' At 0617, Unit 1 began reducing reactor power to comply with LCO 3.0.3. This event requires a 4 hour report IAW 50.72(b)(2)(i), 'The initiation of any nuclear plant shutdown required by the plant's Technical Specifications.' The PCIS isolations which occurred at 0524 CDT are also reportable within 8 hours per 10CFR 50.72(b)(3)(iv)(A) 'Any event or condition that results in a valid actuation of any of the systems listed in paragraph (b)(3)(iv)(B)(2), 'General Containment Isolation signals affecting containment isolation valves in more than one system or multiple main steam isolation valves (MSIVs)), except when the actuation results from and is part of a pre-planned sequence during testing or reactor operation.' This event also requires an LER within 60 days per 10CFR 50.73(a)(2)(iv)(A). The event time for the PCIS isolations is 0524 CDT. The NRC resident inspector has been notified. Service Request 412927 was initiated in the Corrective Action Program.

• * * UPDATE ON 08/06/2011 AT 1350 EDT FROM WILLIAM BAKER TO ERIC SIMPSON * * *

Browns Ferry restored power to the 1B Reactor Protection System power supply at 1208 CDT, reset all isolations and exited LCO 3.0.3. The licensee plans to return the unit to full power. The licensee notified the NRC Resident Inspector. Notified R2DO (Binoy Desai).