Washington — There have been at least twice as many serious accidents with US nuclear weapons than have previously been officially acknowledged, according to a Defense Department document obtained by Reuters. It lists 27 so-called "Broken Arrows," the military's code word for serious nuclear accidents. The Pentagon's official list admits to only 13. The 27 accidents are listed in an unpublished interim report prepared for the Defense Department, which is expected to make public its final report after it has been approved. Military sources said the figure of 27 serious accidents is conservative, and that there have been up to 10 others, too politically sensitive to discuss because of the countries in which they occurred.