Heat from residual fission and self-perpetuating reactors are two different things. Yes, there are radioactive isotopes of various elements in the mantle. They do decay, and from that energy some heat is created. There will be no sustained reaction.
The missing ingredient here is critical mass. You have to put so much of a single radioactive isotope such as U235 into such and such a volume to set up a nuclear chain reaction. Careful measurement of both concentration of isotope and amount used are essential to success in controlling the reaction.
Thank you. The key phrase then is "sustained reaction." Thanks again.