Unexpected high levels Sunday of an ammonia-like chemical called hydrazine prompted only the third Notification of Unusual Event in 10 years at Prairie Island nuclear plant.

The Notification of Unusual Event - the lowest of four emergency classifications established by the U.S. Nuclear Regulatory Commission - was declared by plant operators at 3:52 a.m. Sunday. It ended at 10:22 p.m., less than 20 hours later.

Operators detected the elevated levels of hydrazine in the turbine building, which is on the non-nuclear side of the plant, while in the process of returning Prairie Island's Unit 1 to service.

The unit had been shut down at 8:17 p.m. Thursday when a component of the unit's control circuitry failed while crews conducted routine testing on a redundant circuit. The circuits are part of the plant's instrumentation system that controls and monitors the reactor's operation.

Repairs were made and operators commenced reactor startup Saturday afternoon.

Levels of hydrazine were detected that were higher than normal. However, officials said, the concentrations were less than levels that are considered dangerous to life and health. Ventilation in the affected part of the plant was increased to dissipate the chemical. No detectable hydrazine was found outside the plant.

Hydrazine is added to secondary feedwater at the plant to remove dissolved oxygen, which helps reduce corrosion in the piping. Hydrazine also has a number of common industrial uses, from rocket fuel to the making of Spandex.

According to Internet research, hydrazine (N2H4) is a reducing agent whose byproducts are typically nitrogen gas and water. It is a colorless liquid. The U.S. Department of Health and Human Services Web site says that hydrazines easily evaporate to the air, where they are broken down by reactions within minutes or hours.

There were no injuries and no radiological release as the result of the shutdown or of the hydrazine detection, Xcel Energy officials said.

Prairie Island Unit 2 continued to operate at 100 percent power throughout the event, and Unit 1 was back at 100 percent on Sunday.

A Notification of Unusual Event declaration indicates a potential reduction in the level of safety at the plant but no threat to public safety. The declaration's purpose is to have the plant's operating staff reach a state of readiness for emergency response if necessary.

The last two Notifications of Unusual Event before Sunday's event occurred on Aug. 3, 2001, when there was a fire in the non-nuclear side of the plant, and on Jan. 5, 1999, when there was a transformer fire.

Xcel Energy spokesmen said investigations are continuing into both the cause of the Unit 1 automatic shutdown on Thursday and the elevated levels of hydrazine.

R-E reporter Ruth Nerhaugen contributed to this story