But the '' Castle Bravo '' accident of 1954 , in which a thermonuclear weapon produced a large amount of fallout which was dispersed among human populations , suggested that this was not what was actually being used in modern thermonuclear weapons , which derive around half of their yield from a final fission stage of the fast fissioning of the uranium tamper of the secondary . While some proposed producing " clean " weapons , other theorists noted that one could make a nuclear weapon intentionally " dirty " by " salting " it with a material , which would generate large amounts of long-lasting fallout when irradiated by the weapon core . These are known as salted bombs ; a specific subtype often noted is a cobalt bomb
Castle Bravo
Castle Bravo
Dirty bomb