> From what I gather, it's believed that there is significant but not extremely substantial heat generated from ongoing rasioactive decay.

This depends on where you are. I.e., this is broadly correct for the core specifically in that we don't consider there to be many radioactive elements in the core. However, in terms of the total internal heat budget, radioactive heat production accounts for roughly half of the heat budget, but this is primarily from elements in the mantle and crust.