Operators of the Prairie Island nuclear power plant in Red Wing shut down one of two units Thursday after an early morning fire alarm went off, plant owner Xcel Energy said.

The Minneapolis-based utility said it appears to have been a false alarm.

Xcel said operators have no direct indication of a fire, and no radiation was released. Unit 2 was shut down and remained offline while the matter was investigated.

The Nuclear Regulatory Commission said its resident inspectors are closely following the plant's actions from the control room. In an "unusual event" report to the NRC, the utility said that a disk ruptured in a pressure tank, triggering the alarm and affecting the cooling and ventilation systems, leading to the shutdown.

It was not clear how long Unit 2 will be out of service. Unit 1 was not affected, Xcel said.

Staff report