An inspection team from the Nuclear Regulatory Commission arrived at the Millstone nuclear plant Monday to investigate a May 25 shutdown and an overflow of coolant at unit 3.A fault in the Connecticut plant's switchyard cut power from a high-voltage line serving Millstone's two reactors. Both tripped, backup diesel generators started and the plant issued an unusual event declaration. Both units shut down safely, and the trip at unit 2 was uncomplicated.Unit 3, though, experienced a loss of non-safety-related instrument air, according to a press release and other NRC documents. This affected the normal reactor coolant system drain flow path, causing a pressurizer relief tank rupture disk to burst. While restoring normal letdown, a relief valve lifted on a volume control tank, which led to an overflow from the primary drains transfer tank in the auxiliary building. Inspectors will also investigate the performance of the unit's turbine-driven auxiliary feedwater pump. Both reactors have reentered service, with unit 2 restarting May 27 and unit 3 restarting Monday.
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