OMAHA, Neb. (AP) – Federal regulators have confirmed their preliminary finding that a fire last June at an idled Nebraska nuclear plant presented a serious safety threat. The Nuclear Regulatory Commission released its final determination on Tuesday, saying the small fire at the Fort Calhoun plant 19 miles north of Omaha merited the NRC’s most serious “high safety significance.” The finding mirrored the commission’s preliminary conclusion announced on March 12.  The fire briefly knocked out the cooling system for used fuel. The NRC says the plant was shut down at the time and that temperatures never exceeded safe levels.  But it also says the fire was a major concern because it could have happened when the plant was operational and that workers didn’t fully investigate an unusual smell in the area three days earlier.