It’s my belief that uranium, concentrated in the Earth’s crust, gradually sinks towards the core-mantle boundary where it occasionally concentrates to the point where a fission chain reaction occurs.  At that point, increased heat and convection disperses it.

If this were not the case, volcanic plumes would not originate at the core-mantle boundary because there is no ongoing source of heat at the core.  The core itself is mostly iron in which uranium and it’s oxides don’t readily dissolve.  So the core does not have heat from ongoing radio-active decay or fission.

We know fission must be taking place because He³ is present in mid-ocean rift gases.  If the only source of heat were normal radio active decay and not a fission chain reaction, we would find only He⁴ since He³ is not produced in normal radioactive decay.