Consequently, if you don't want to treat it as a separate elemental sub-type, why not handle it as a poison/disease? In RL, fallout from radiation causes sickness, which can degenerate the body enough to kill. I would think about making some sort of disease (with appropriate saving throws, effects, etc.) which creatures must attempt to resist when they enter nuclear areas. The primary difference from other "diseases" is that the effect of radiation should have some permenance i.e. remove disease cannot completely cure you, etc.
Another side effect of the "disease" is that there is a percentile chance that the radiation instead causes some sort of mutation. The mutations can be as appropriately random as you wish. Either all negative (disfiguration (-2 to Charisma?), weakness (creature has less hp or lowered Constitution), altered metabolism (lower Strength or occassional effects similar to slow)) or positive (increased Strength, resistance to certain energy effects, increased speed, etc.). Those would be your perogative.
Just my thoughts on it. As far as messing with the actual Faerunian weave, that is a hefty task which would have several game balance considerations.