05000458/LER-2006-004

06-14-2006

On April 15, 2006, at 4:15 p.m., an automatic reactor scram occurred while the plant was operating at 100% power. Prior to the scram, both reactor recirculation pumps shifted from high to slow speed due to a failed optical isolator card in the reactor protection system. With the reactor control rod pattern set for full poWer operation, the decrease in reactor recirculation flow,caused the average power range monitors (APRM) to trip on a simulated high thermal power signal. The APRM trip signal caused the reactor scram approximately twelve seconds after the recirculation pumps downshifted. This event is being reported in accordance with 10CFR50.73(a)(2)(iv) as a valid actuation of the reactor protection system. There were no safety-related systems out of service at the time of the event. Plant systems responded to the scram as designed, and reactor pressure and water level were promptly stabilized. Both optical isolator cards in the EOC- RPT circuitry were replaced during the forced outage. A downshift of both reactor recirculation pumps while operating at full power is an event analyzed in the River Bend Updated Safety Analysis Report. As such, this event was of minimal safety significance. NRC FORM 366 (6-2004) PRINTED ON RECYCLED PAPER

LER-2006-004, Automatic Reactor Scram Following Recirculation Pump Downshift Due to Failed Optical Isolator Card
Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber Na 05000
Event date:	04-15-2006
Report date:	06-14-2006
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation
4582006004R00 - NRC Website
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05000458/LER-2006-004

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