Radiogenic heating occurs as a result of the release of heat energy from radioactive decay  during the production of radiogenic nuclides.  Along with heat from the Primordial Heat (resulting from planetary accretion), radiogenic heating occurring in the mantle and crust make up the two main sources of heat in the Earth's interior.  Most of the radiogenic heating in the Earth results from the decay of the daughter nuclei in the decay chains of uranium-238 and thorium-232, and potassium-40.