Gender: Male Location: Training for the Fanboy Olympics
he's nowhere near immune to kryptonite. If you didn't notice after he tried that stunt all of his powers were sapped for the duration of his fight between him and Lex.
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In general it's a "sapping" effect. He basically has radiation poisoning when he's exposed to it. Nowadays (at least until DC's big retcon), he has some small resistance to it, but it's still dangerous in large quantities.