I mean, it's possible that at some point evolution produced such nuclear reactored individual, but who got "naturally selected" rather quickly for obvious reasons
If you read some of the Dune supplementary materials, the sandworms are biological nuclear reactors. It's kinda handwavey but it's a cool concept - how do you keep a creature the size of a freight train moving constantly in a very thick medium... how about harvesting fissionables?
See the Isaac Asimov story "Pâté de Foie Gras" from the September 1956 issue of Astounding Science Fiction, available here [1] on the Internet Archive.