The Sandia scientists found that although there are millions of licensed radiation sources in the U.S., only about 1,000 domestic irradiation devices in universities, hospitals, and laboratories are of concern in an RDD scenario. Even here, the sources that are the easiest to transport and conceal are the least threatening. The ones that are the most threatening are also the most difficult to move, let alone steal.
Radio active materials loose but don't worry.
This story in the Greensboro News and Record would cause some concern but not me. Here is why you really don't have to worry that much about RDDs (Radio-active Dispersal Devices or more commonly 'dirty bombs'). From the Washington Post blog