CHICAGO - A failed electrical insulator blamed for a power loss to a nuclear reactor in northern Illinois was replaced Tuesday, and the U.S. Nuclear Regulatory Commission began a special inspection into how some equipment responded to the outage, officials said.

Exelon Energy began preparations to re-start the Unit 2 reactor at the Byron Generating Station about 95 miles northwest of Chicago, though it was unclear how soon it could return to service, spokesman Paul Dempsey said.

The insulator, a piece of protective equipment that helps regulate the flow of electricity in the plant's switchyard, failed Monday morning and fell off the metal structure to which it was attached. That interrupted power and caused the reactor to automatically shut down as a precaution.

It was not immediately clear what caused the insulator to fail, but the part will be sent to a lab for analysis, Dempsey said.

Meanwhile, the NRC began an inspection of water pumps that help cool the reactor, commission spokeswoman Viktoria Mitlyng said.

Some pumps are designed to switch off after a set period of time after detecting an undervoltage to prevent damage, then must be manually restarted. But some of those pumps shut down and restarted on their own after Monday's power outage, she said.

She said there was no danger because the plant has multiple backup pumps, but the NRC wants all pumps to perform properly.

"We are asking if all pumps, whether they have a built-in mechanism or not, functioned property and responded as expected and if there were any unexpected problems with equipment," Mitlyng said.

During the shut-down, steam was released to cool the reactor, but was being vented from the part of the plant where turbines produce electricity, not from within the nuclear reactor itself, officials said. The steam contains low levels of tritium, a radioactive form of hydrogen, but federal and plant officials insisted the levels were safe for workers and the public.

Releasing steam helped to "take away some of that energy still being produced by nuclear reaction but that doesn't have anywhere to go now," that the turbines are shut down, Mitlyng said. Even though the turbine is not producing electricity, she said, "you still need to cool the equipment."

Diesel generators were supplying the reactor with electricity, though it hasn't been generating power during the investigation. Another reactor at the plant was operating normally.

The NRC declared the incident an "unusual event," the lowest of four levels of emergency.

Mitlyng said officials can't yet calculate how much tritium was released. They know the amounts were small because monitors around the plant didn't show increased levels of radiation, she said.

Tritium molecules are so microscopic that small amounts are able to pass from radioactive steam that originates in the reactor through tubing and into the water used to cool turbines and other equipment outside the reactor, Mitlyng said. The steam that was being released was coming from the turbine side.

Tritium is relatively short-lived and penetrates the body weakly through the air compared to other radioactive contaminants. It can cause cell damage if it enters the body, but the amounts released from the Byron station under normal operating conditions and within the steam that's being vented now is not a health concern, said David Lochbaum, a nuclear engineer and director of the nuclear safety project for the Union of Concerned Scientists.

Illinois Emergency Management Agency workers on Tuesday began collecting water and vegetation samples to check for tritium around the plant, although it didn't expect to find any problems, said agency director Jonathon Monken. He said it was the agency's public duty to verify current levels of tritium in the area, and results should be available within a few days.

In March 2008, federal officials said they were investigating a problem with electrical transformers at the plant after outside power to a unit was interrupted.

In an unrelated issue last April, the commission said it was conducting special inspections of backup water pumps at the Byron and Braidwood generating stations after the agency's inspectors raised concerns about whether the pumps would be able to cool the reactors if the normal system wasn't working. The plants' operator, Exelon Corp., initially said the pumps would work but later concluded they wouldn't.