A Nebraska environmental group on Monday questioned the track record of the company hired to run the Fort Calhoun nuclear power plant because of concerns raised by federal regulators.

The Clean Nebraska group said it was troubled by the questions the Nuclear Regulatory Commission had raised about some of the calculations Exelon made for its other nuclear plants. The NRC said last week it was looking into whether Chicago-based Exelon intentionally understated the cost of decommissioning 23 of its nuclear reactors.

Exelon spokesman David Tillman said the company denies intentionally providing incorrect data and "disputes the findings." Tillman said the calculations are complicated and Exelon looks forward to meeting with the NRC to discuss the issue.

The Omaha Public Power District signed a 20-year contract with Exelon last year because of its experience and safe track record operating 17 nuclear reactors at 10 different power plants. Fort Calhoun has been idle since 2011.

Clean Nebraska spokesman Mike Ryan, who has called for Fort Calhoun to be permanently shut down, said it's troubling to see these questions raised about the company that was hired to rehabilitate Fort Calhoun.

"It appears that they've been less than honest with the NRC," Ryan said. "It just kind of makes us concerned about what's going on at Fort Calhoun."

The power plant that sits about 20 miles north of Omaha initially shut down for refueling maintenance in April 2011. But flooding along the Missouri River throughout the summer of 2011 and a series of safety violations forced it to stay closed.

The violations include the failure of a key electrical part during a 2010 test, a small electrical fire in June 2011, several security-related violations and deficiencies in flood planning that were discovered a year before the extended flooding.

And OPPD officials are evaluating the structural supports inside the building that houses the reactor because an engineer discovered last spring that some of the supports aren't strong enough to safely support the building under extreme circumstances.

But OPPD said significant progress has been made at Fort Calhoun, and the utility remains optimistic the plant will be able to restart this spring.

OPPD spokesman Jeff Hanson said the utility isn't concerned about the concerns the NRC raised about Exelon.

"We are satisfied with their performance under our contract to safely and efficiently restart Fort Calhoun Station and achieve sustained excellence at the plant," Hanson said.

Inspectors from the NRC are at Fort Calhoun now checking over the repairs and upgrades that OPPD has made. Regulators will have to sign off on all the work at the plant before it will be allowed to restart.

NRC officials have said repeatedly that they won't allow Fort Calhoun to restart until they are confident it can operate safely.