A fire at the Palo Verde Nuclear Generating Station Monday was controlled without any release of hazardous material and without threat to public safety, the plant's operator said.

Arizona Public Service Co. declared an "unusual event" when insulation behind a pump caught fire in a turbine area of Unit 2. An "unusual event" is the lowest of four emergency levels for nuclear power plants.

The fire did not interrupt power production at the plant 50 miles west of downtown Phoenix, and APS said the plant was running normally Tuesday.

APS told the federal Nuclear Regulatory Commission that there was no release of radioactive material and no threat to public health or safety.

APS said Palo Verde firefighters responded when a security guard spotted smoke, and the fire was declared out 45 minutes later. The cause was under investigation.

Palo Verde is the nation's largest nuclear power plant, with three reactors and a capacity of 3,942 megawatts, according to the U.S. Energy Information Administration.