“About 50% of the heat given off by the Earth is generated by the radioactive decay of elements such as uranium and thorium, and their decay products. That is the conclusion of an international team of physicists that has used the KamLAND detector in Japan to measure the flux of antineutrinos emanating from deep within the Earth. The result, which agrees with previous calculations of the radioactive heating, should help physicists to improve models of how heat is generated in the Earth.

Geophysicists believe that heat flows from Earth’s interior into space at a rate of about 44 × 1012 W (TW). What is not clear, however, is how much of this heat is primordial – left over from the formation of the Earth – and how much is generated by radioactive decay.  […]

One possibility that has been mooted in the past is that a natural nuclear reactor exists deep within the Earth and produces heat via a fission chain reaction. Data from KamLAND and Borexino do not rule out the possibility of such an underground reactor but place upper limits on how much heat could be produced by the reactor deep, if it exists. KamLAND sets this limit at about 5 TW, while Borexino puts it at about 3 TW.”