There was a 1990s theory that the centre of the Earth's core was natural uranium nuclear reactor that provided a some of Earth's internal heat, possibly helped drive the Earth's magnetic field. This extraordinary theory was proposed by J. Marvin Herndon who published a fantastical paper in 1993 claiming that he inner core was was composed of natural uranium and it was a natural fission reactor, a georeactor.

The idea was supported by plausible, at the time, geochemistry, that appeared to suggest that uranium could differentiate from iron-nickel and form an inner uranium core at the center of the Earth. However, most at the time disagreed, said the uranium content wasn't high enough to form a uranium core and/or the chemistry at high pressures prevent the separation of uranium from other metals.

Also, futher, advances in geoneutrino detection, that's neutrinos emited by radioactive decay and hypothetical inner core fission, ruled out a fission reactor well below the energy output proposed by J. Marvin Herndon.

Didn't stop him promoting the idea however, of a weaker fission reactor, or one that was more active in the past when there was more fissionable 235U.

Also, there's other scientists, who proposes variations on the Core Fission Hypothesis. A recent claim proposed a uranium oxide fission reactor at the inner/outer core boundary. This was published in a reputable journal. They proposed a 30 TeraWatt georeactor. That's insane.

I must look into this more, at it seems some scientists seriously think this is possible.

Refs.:

Herndon, J.M., 1993. Feasibility of a nuclear fission reactor at the center of the Earth as the energy source for the geomagnetic field. Journal of geomagnetism and geoelectricity, 45(5), pp.423-437.

Rusov, V.D., Pavlovich, V.N., Vaschenko, V.N., Tarasov, V.A., Zelentsova, T.N., Bolshakov, V.N., Litvinov, D.A., Kosenko, S.I. and Byegunova, O.A., 2007. Geoantineutrino spectrum and slow nuclear burning on the boundary of the liquid and solid phases of the Earth's core. Journal of Geophysical Research: Solid Earth, 112(B9).