A bomb packed with both explosives and highly radioactive material. Although there isn’t enough uranium or plutonium to initiate a full-blown detonation an air burst would ensure that the nasty stuff is spread over a wide area, causing sickness, death, panic and house prices to fall.
Although not yet used it has the potential to form a cheap man’s weapon of mass destruction, making a little plutonium go a long way. Often thought of as a modern idea, Churchill was worried enough towards the end of WW2 to have plans drawn up for the evacuation of London should the Nazis use such a device. It was thought (rightly) that the Germans didn’t have enough material for a proper nuke, but more than enough for a dirty bomb or two. Blimey!
A bomb that needs a bit of a clean. I recommend Pledge for the job, but just don’t rub too hard, okay?