A fire that briefly knocked out the cooling system for used fuel at an idled Nebraska nuclear plant last June represented a serious safety threat, federal regulators said in a report released Monday.

The Fort Calhoun plant north of Omaha was shut down at the time of the fire, which started in an ill-fitting electrical breaker, and temperatures never exceeded safe levels, according to the Nuclear Regulatory Commission's preliminary findings. But the commission said the fire is considered a major concern because it could have happened any time and because workers didn't fully investigate an unusual smell in the area three days earlier, which could have led them to discover the problem and prevent the fire.

A serious threat finding typically could mean additional oversight for a nuclear plant, but Fort Calhoun already is under the NRC's strictest oversight level because of a prolonged shutdown that began last spring and several other reported problems _ including the failure of a key electrical part during a test and flood planning deficiencies, both found in 2010.

Fort Calhoun initially was shut down for refueling maintenance last spring, but major flooding along the Missouri River forced it to remain closed. The Omaha Public Power District is working to repair any flood damage and double-check all the plant's systems before restarting.

"I think with the NRC's presence they are making doubly sure this is done in a cautious engineering manner," said University of Nebraska-Lincoln professor and nuclear engineer Dennis Alexander.

Utility spokesman Jeff Hanson said OPPD doesn't plan to contest the severity of the NRC's finding on the fire.

The fire started in an electrical breaker that had been replaced about 18 months earlier. NRC spokeswoman Lara Uselding said the new breaker had to be modified to fit the existing switches, and the breaker didn't line up properly. That allowed grease to accumulate on the components, which allowed enough heat to build up to start the fire.

During the fire, smoke and soot spread into Fort Calhoun's backup electrical system and knocked that out as well. Uselding said the utility is working to redesign the system to prevent a fire from being able to knock out both power systems again.

The NRC said in December that Omaha Public Power District officials also were too slow to notify state emergency response officials about the fire when it happened.

Hanson said the faulty breaker already has been replaced, and was successfully tested last week. He said the utility is making progress toward restarting Fort Calhoun sometime this spring, but won't rush the process.

"The primary job is to make sure it is safe. That's our job, and the NRC is to make sure we do that job," Hanson said.

The utility submitted a detailed improvement plan to the NRC that regulators approved last fall and the NRC will have to sign off on all repairs before any restart.

While preparing for restart, OPPD discovered a problem with Fort Calhoun's emergency sirens used to warn area residents about any issue at Fort Calhoun. The utility said there also are backup notification plans.

Hanson said the utility lost communication with some or all of the sirens a couple times before replacing a faulty power supply. The NRC will review the siren problem and determine whether it should also be considered a safety issue for Fort Calhoun.

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Online:

Nuclear Regulatory Commission: www.nrc.gov

OPPD Fort Calhoun news: http://www.oppd.com/Nuclear/22_007432