Radioactive Man's backstory is like a lot of classic super villains. He's a scientist that "flew too close to the sun" so to speak.

 

Dr. Wesley Roberts was searching for the ultimate cure for cancer but lost funding to his research projects. He took matters in his own hands and created a project that he thought would prove his research correct when really it turned him into a monster when the radioactive material he was working with overloaded and caused the lab to explode.

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Since the explosion Dr. Roberts has the ability to shift his exterior self to reflect the illustrated concept that is shown to the left.  He shoots out radioactive sludge from his hands that can disable his foes. If he covers his foes completely in sludge they can turn into hideous monsters that become under his control to do his diabolical bidding.