They would be mixed at random evenly throughout the interior.
Uranium, thorium, plutonium, and lead would collect in the core, eventually triggering a nuclear explosion.
The interior would consist almost entirely of carbon, with a small iron core.
An onion-like set of layers would form, with the heaviest elements contained in shells within progressively lighter ones.
The elements are arranged as a uniform admixture of hydrogen and helium, with a coating of iron.