Many fictional characters in the last 60-70 years, be it a superhero, supervillain, monster, or something else are often the result of some kind of nuclear accident, testing, bombing, waste, etc.
How many can we name?
I start with an easy one: Godzilla.
(Is it a bad sign that we’re already meta?)
Firestorm, the Nuclear Man
Doctor Manhattan
Does the Incredible Hulk Count?
Absolutely.
Mothra
Juliet Burke (LOST)
Captain Atom
Sun Boy (powers granted by an accident with a power generator)
Socialist Red Guardsman (Not sure how he GOT his powers, but he is radioactive because of them)
Cyclotron, and by extension Al ‘Atom’ Pratt, and Albert Rothstein (AKA Nuklon, and Atom-Smasher) - Al being an enemy of Cyclotron, who gained superpowers from dealing with him regularly, and Albert being Cyclotron’s grandson.
Nobody mentioned Spider-Man yet?
Also Daredevil.
Legion of Super-Heroes Sun Boy and Wildfire.
Fission Boy (a somewhat flakey superhero who tends to go to pieces when under pressure).
There’s also Marvel’s Radioactive Man.
Firehawk and Multiplex from the Firestorm comics.
Atomicus from Astro city.
The Lord of Demons, Orcus of Empire of the East; a nuclear detonation transformed into a demon.
The Missing Link of Marvel.
Nuke of Marvel.
The Devil, in Hell-Fire by Asimov.
Duke Nukem from Captain Planet.
Quantum and Woody, kind of. Link to their tvtropes page because their wikipedia page is terrible.
At least one of Iron Man’s suits of armor was powered by a small nuclear reactor.
Lex Luthor got cancer from prolonged exposure to radiation from the kryptonite ring that he wore.
She-Hulk received a transfusion of Bruce Banner’s radioactive blood.
The first Spider-Woman’s first origin story had her getting radiation poisoning as a child, with the resultant attempt to cure her resulting in her development.
Nuclear Man from Superman IV.
Suzy Fusion, as well as much of her family
(we’ll see who else recognizes that one).
And oddly enough, neither Nuklear Man nor Atomik Lad is radiation-based (they’re both more metaphysical).
The Nuclear Family from the original Outsiders comic book