The inspection will begin at the plant, which is located in Waterford, Conn., and operated by Dominion Nuclear Connecticut Inc.

A five-member team will perform the inspection. Focus areas will include the effectiveness of Dominion’s response to the event, equipment performance and the company’s evaluations of what occurred during the dual-unit shutdown.

"There were developments during the course of the reactor shutdowns that were not expected and that therefore merit a closer look, with a particular attention on what happened at Millstone Unit 3," NRC Region I Administrator Bill Dean said. "Our inspectors will be tasked with, among other things, acquiring a clearer picture of how the event unfolded."

Just after 7 a.m. on May 25, the Millstone 2 and 3 reactors shut down due to a loss of off-site power. (Nuclear power plants not only send power out to the grid, they also take power from it for operational purposes.) The units safely shut down and there were no impacts on plant workers or on public health and safety. The cause of the off-site power loss was subsequently determined to be an electrical fault that interrupted the flow of energy from a high-voltage power line, one of three off-site lines that provide power to the site.

An "Unusual Event" — the lowest of four levels of emergency classification used by the NRC — was declared by Millstone at 7:15 a.m. on May 25 in response to the loss of off-site power. The declaration was terminated at 2:14 p.m. following the restoration of off-site power, which occurred at 12:56 p.m.

Millstone Unit 2 did not experience any significant complications as it was shutting down. However, several complications occurred at Millstone Unit 3. Specifically, there was an unexpected loss of non-safety instrument air at Unit 3 following the loss of off-site power. This led to the inability to establish a normal reactor coolant system drain flow path and resulted in the opening of the tank’s rupture disk. Also, a relief valve on the unit’s volume control tank lifted, causing the primary drains transfer tank to overflow inside the auxiliary building.

In addition to these problems, the NRC team will review the operation of the unit’s turbine-driven auxiliary feedwater pump during the event.

The inspection team will document its findings and conclusions in a report to be issued within 45 days after the end of the review.