[JackMcMack]

Interesting read. Though it says the nuclear fission happens in the crust and mantle, not the core.

[idlewords]

Here's a survey paper with a deeper treatment of the topic, for Earth and other solar system bodies:

https://link.springer.com/article/10.1007/s11214-020-00656-z

"On the Distribution and Variation of Radioactive Heat Producing Elements Within Meteorites, the Earth, and Planets"