Radiological dispersal devices (RDD), also known as "dirty bombs," consist of radioactive material combined with conventional explosives. They are designed to use explosive force to disperse the radioactive material over a large area, such as multiple city-blocks. Around the world, there are many sources of radioactive material that are not secure or not accounted for. Rogue nations and/or terrorist groups can obtain these materials for dirty bombs. These explosive weapons may initially kill a few people in the immediate area of the blast but are used primarily to produce psychological rather than physical harm by inducing panic and terror in the target population. Their use would also result in costly cleanup for decontamination [from OSHA website].
OSHA - US Occupational Safety & health Administration - covers all aspects of protecting First Responders from the effects of Dirty Bombs.
U.S. Navy photo by Photographer’s Mate Second Class Michael A. Damron, taken as part of his official duties. As a work of the US federal government the image is in the public domain.