Indeed. When you do that with Uranium, we call it a "bomb". If that's what happened to all the uranium, the planet would have been vaporized.
Do you have figures or a reference handy? It would help to understand the amount of uranium in the earth and the amount of heat, etc. released by accelerated decay. This would help generate boundary conditions for any modeling of accelerated decay.
I would note in passing that at least one creation model includes an expansion of space (similar to that involved in 'big bang' models) at the time of the Kataklusmos, that would have the effect of cooling off matter within the expanded space. I regard this as extremely speculative, but it may be an answer to this objection.