A "dirty bomb" or radiological dispersal device (RDD) is a conventional explosive or bomb containing radioactive material. The conventional bomb is used as a means to spread radioactive contamination. It is not a nuclear bomb and does not involve a nuclear explosion. Any type of radioactive material could be used in a dirty bomb, but in general these devices would be unlikely to cause serious health effects beyond those caused by the detonation of conventional explosives.
Issues in Ag-Security (May 2003), v.1, no. 5