See wikipedia's hydrogen bomb or nuclear weapon - "Another variant uses Cobalt in the shell, and the neutrons convert the Cobalt into Cobalt 60, a powerful long-term emitter of Gamma rays. The primary purpose of this weapon is to create extremely radioactive fallout to permanently deny a region to an advancing army, a sort of wind-deployed mine-field. It was actually tested by the British in Central Australia, in areas that remain uninhabitable to this day." The fears over destroying the atmosphere were applied to the first atomic tests.