A cobalt bomb is a thermonuclear weapon with an external cobalt casing. When the weapon detonates, the neutron flux from the deuterium-tritium reaction converts the ordinary cobalt-59 to the intensely radioactive isotope cobalt-60, via a reaction known as neutron activation. This greatly increases the radioactive fallout, and the duration of lethal radiation levels in the fallout zone.
Other reactive casings for thermonuclear weapons have been proposed, and even tested, such as compound fission-fusion-fission weapons. However, the cobalt device seems best adapted to use as an enhanced radiation weapon as cobalt 60's 5 year half-life gives a good compromise between duration and radioactive intensity.
Dr Leo Slizard has suggested that a large enough quantity of cobalt bombs could destroy all life on Earth, and several novels and films, such as On the Beach and Doctor Strangelove, Or How I Learned to Stop Worrying and Love the Bomb, have used this possibility as a major plot point. Later films such as Return to the Planet of the Apes sometimes misrepresented the cobalt bomb as a weapon with a greatly enhanced blast strength. Although this idea has remained popular, it is untrue.