That's according to the International Atomic Energy Agency's Incident and Trafficking Database, which works with governments around the world to track such incidents. Many of the cases are never publicly reported -- the IAEA doesn't release details so that governments will be more willing to fess up when material goes missing. A group called the Nuclear Threat Initiative tracks open-source reports of these incidents. They don't catch everything, but they've recorded enough to show the alarming frequency with which dangerous nuclear materials are lost or go missing. Some of those cases from the past decade are mapped out below. If you weren't worried about loose nuclear material before, you will be after reading details of the incidents at the bottom of the map.
(Click the image to see it full-size, or click here.)
For more, read my interview with nuclear policy expert Mark Hibbs on why this happens and what makes it dangerous.