What are the scientific objections to the Deep Earth Reactor theory?

I have read in multiple sources that the heat produced in the interior of the Earth is the result of "natural radioactivity". Specifically from the decay of Uranium, Thorium, and Potium isotopes. So my question is this: What other naturally occurring processes are there that convert radioactivity into heat OTHER than fission? Because if fission is the only known process that can do this, isn't it likely that fission does indeed take place deep inside the Earth? If not the core than perhaps in the lower mantle.