From a recent interview.
'GODZILLA MINUS ONE' Director Details Godzilla's Mutation Within a recent interview with HJWeb, 'Godzilla Minus One' Director answered various spotlight questions in regards to Godzilla and the film.
Q: "The juvenile's body on Odo Island, had not yet been affected by radiation, right?"
Director Takashi Yamazaki: "Correct. Godzilla has been affected by radiation since Operation Crossroads in Bikini Atoll, but before that, Godzilla was unaffected by radiation and his regenerative ability was unrelated to radiation. They[Godzilla's kind] are ordinary creatures, or rather monsters, that have nothing to do with radiation, and have extremely strong regenerative abilities. However, during Operation Crossroads, his skin was burnt by nuclear weapons and turned into a mess, but when he was regenerating it, he went out of control and became gigantic. [bitter smile]"