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An inspection team from the Nuclear Regulatory Commission (NRC) arrived at the Millstone nuclear plant in Waterford, CT, to look into a May 25 shutdown which led to 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 ended up being uncomplicated.

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Unit 3, though, experienced a loss of non-safety-related instrument air, the NRC said.

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.

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Inspectors will also investigate the performance of the unit’s turbine-driven auxiliary feedwater pump.

Both reactors started back up, with unit 2 restarting May 27 and unit 3 restarting Monday.

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