ST. PAUL, Minn. — A broken pipe leading to a chlorine bleach tank caused a chemical spill Thursday at Xcel Energy's Prairie Island nuclear plant on the Mississippi River near Red Wing, but officials said no radiological material was released.

A worker at the plant discovered the break shortly before 4 a.m. About 500 gallons of chlorine bleach, used to clean river water needed to cool the plant, spilled inside the building that holds the tank. Dennis Koehl, Xcel Energy's chief nuclear officer, said none of the bleach leaked into the river.

"The health and safety of the public was not at risk at any point," Koehl said at a briefing in St. Paul with the state's director of homeland security. Koehl said no plant workers were hurt.

Koehl said it was not immediately known what caused the pipe to break. He said Xcel would conduct an investigation to determine what happened. The Nuclear Regulatory Commission said it sent two inspectors to the site.

The incident triggered an alert, the second-lowest in a four-level federal NRC system for nuclear plant emergencies, which plant operators lifted Thursday afternoon. Koehl said it was the first time the plant, in operation since 1973, has registered an alert on the federal system. The plant, about six miles northwest of Red Wing, has two reactors that generate enough power for nearly 1 million homes.

The alert activated emergency communications systems in Minnesota and Wisconsin, directly across the river from the plant. Minnesota Homeland Security Chief Kris Eide said officials never warned the public to take precautions, but that two Wisconsin school districts — Ellsworth and Prescott — delayed classes by two hours as a precaution. Prescott Superintendent Roger Hulne said he didn't have all the information about the leak by 6 a.m. when buses were supposed to start rolling.

Scott Burnell, spokesman for the Nuclear Regulatory Commission in Rockville, Md., said chlorine bleach — or sodium hypochlorite — is hazardous in its undiluted state and can cause respiratory problems.

"The plant has procedures in place to keep staff safe," Burnell said. A berm was used to contain the spill until hazardous materials handlers could begin cleaning it up.

Koehl said Xcel expected to finish cleaning up the spilled material by 6 p.m. Thursday.