The Energy Department has failed to assure safety at the four aging federal reactors that produce special nuclear material for the nation's atomic bomb program and "cannot . . . rule out" a severe accident, a government study said. The National Academy of Sciences report concluded that the Energy Department and private contractors operating the weapons material reactors do not fully understand how the reactors might behave during a severe accident or whether plant safety systems could cope with one. Energy Department officials acknowledged at a news conference many of the shortcomings cited in the report.