WASHINGTON -- Exelon Corp., owner of the largest U.S. nuclear fleet, may have violated five federal rules with a slow and inadequate response to spills of radioactive water at its Braidwood nuclear reactor, the Nuclear Regulatory Commission said.

The spills didn't harm public health and released only an "insignificant" amount of radiation, the commission said in a letter to Exelon dated Thursday. Further investigation will occur because Exelon failed to adequately and swiftly assess the spills, the NRC said.

"We mishandled the tritium spills back in the 1990s when they took place," Exelon spokesman Craig Nesbit said. "We've taken full responsibility, and today we're doing the things to clean it up."

Exelon said Wednesday it had settled a lawsuit filed by Illinois Attorney General Lisa Madigan alleging that six of eight spills resulted from inadequate maintenance and operations at the Braidwood plant, located about 50 miles southwest of Chicago. Tritium is a radioactive isotope that in high concentrations can cause cancer.

The Illinois Environmental Protection Agency began investigating tritium contamination of nearby wells in March 2005. There was a spill estimated at 3 million gallons in 1998 and another estimated at 3 million gallons in 2000. There were other leaks between 1996 and 2004, according to the federal report.

Exelon will begin pumping out tritium-laced groundwater in about two weeks under the agreement approved by a state judge, the company said. Extraction of all the radioactive water from under neighboring property may take a year, Nesbit said.

Exelon hasn't released a cost estimate for the cleanup. Once neighboring property is cleaned, the company will begin work to extract an underground pool of tritium-contaminated water under the plant site, Nesbit said.

The plant should have diluted the radioactive water to safe levels before discharging it into a river for further dilution, Nesbitt said. Instead, the water was dumped on the ground.

The lapses probably will make the cleanup more expensive and difficult, the NRC said.

The NRC said it will investigate apparent violations of its rules such as failure to measure radioactivity that resulted from the spills until last year and failure to maintain records.