That’s because the U.S. can’t track or fully account for 5,900 pounds of “weapons usable” nuclear material that it once shipped overseas. Instead, U.S. officials have to rely on foreign governments’ assurances that the potentially cataclysmic stuff is safe. And when those officials occasionally visit the sites holding the nuclear material, nearly half the places “did not meet International Atomic Energy Agency security guidelines,” according to the GAO, Congress’ investigative arm.

“It’s amazing how completely cavalier the Department of Energy has been at tracking this. They’ve got nobody who worries about this on a day-to-day basis,” says Jeffrey Lewis, a nuclear weapons analyst at the Monterey Institute of International Studies (and occasional contributor to this blog)…

Starting in the Eisenhower administration, the U.S. sold 17,500 kilograms, or 38,5000 pounds, of fissile material overseas, mostly to help with civilian nuclear energy programs. Those sales came with conditions, however: countries had to keep the dangerous material safe; they couldn’t use it for weapons; and the U.S. had the option of taking back the radioactive stuff — someday, somehow.

But 12,400 of those 17,500 kilograms can’t be returned. It’s mostly in the hands — and reactors — of close allies like Germany, France, and Japan. 1,160 kilograms have been accounted for, and another 1,240 kg have been secured by the Energy Department’s “Global Threat Reduction Initiative,” an effort to covert nuclear power facilities from highly-enriched to low-enriched uranium, which is far less dangerous.