Ceres is as old as Earth, so its Uranium should be as lowly enriched in U-235 as here, which is today too low for natural reactions. Additionally it would be highly unusual for Ceres to have concentrated Uranium ore deposits to a similar degree as Earth, for mineralogical reasons.
My crazy theory: you think it was a coincidence that, just as our spacecraft approaches Ceres, a defense satellite in Low Earth orbit 'mysteriously' explodes? Pah! That's just what they want you to think!
http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor