Radiation lacks usable rules in 3.5e. Here is my take on the mystery of SCIENCE! and the most lethal threat we can't even see. Radiation uses rather simple rules that sacrifice a bit of realism for simplicity's sake, I attempted to make it lethal (not like D&D mundane fire). If you like your game less lethal, lowering damage caused by instant exposure by 5d6 and lowering every fortitude save by 3 points might do it.