  We assess the likelihood and geochemical consequences of the presence of
nuclear georeactors in the core mantle boundary region (CMB) between Earths
silicate mantle and metallic core. Current geochemical models for the Earths
interior predict that U and Th in the CMB are concentrated exclusively in the
mineral calcium silicate perovskite (CaPv), leading to predicted concentration
levels of approximately 12 ppm combined U and Th, 4.5 Ga ago if CaPv is
distributed evenly throughout the CMB. Assuming a similar behaviour for
primordial 244Pu provides a considerable flux of neutrons from spontaneous
fission. We show that an additional concentration factor of only an order of
magnitude is required to both ignite and maintain self sustaining georeactors
based on fast fission. Continuously operating georeactors with a power of 5 TW
can explain the observed isotopic compositions of helium and xenon in the
Earths mantle. Our hypothesis requires the presence of elevated concentrations
of U and Th in the CMB, and is amenable to testing by direction sensitive
geoneutrino tomography.
