Just to tell you, though i think that you know. Roaches are able to survive the 4 times larger amount of radiation thats able to kill us... this is going to be full of mistakes because i dont really remember it now but roaches are able to survive more then 200 rads... which with this amount we humans would shine for a hundred years (our dead bodies i mean... we would die in 5 minuetes)
It is proved that insectoids, (scorpions, roaches, ants many other bugs) are VERY resistant to radiation. Sure, radiation kills them but but the amount required to kill them is MUCH higher then ours.
And death claws were created BEFORE the great war, if im correct. The US wanted to use them as weapons or something but then the war started and anfter a few years the inhabited the whole continent