 After one to six months, the fission products from even a large-yield thermonuclear weapon decay to levels tolerable by humans.The large-yield two-stage (a fission trigger/primary with a fusion–fission secondary) thermonuclear weapon is thus automatically a weapon of radiological warfare, but its fallout decays much more rapidly than that of a cobalt bomb.A cobalt bomb's fallout on the other hand would render affected areas effectively stuck in this interim state for decades