Georeactor

The georeactor is a proposal by J. Marvin Herndon that a nuclear fission reactor may exist and operate at the core and serves as the energy source for the geomagnetic field. Herndon had earlier proposed the existence of fission reactors at the centers of large gaseous planets such as Jupiter and Saturn.
The georeactor hypothesis radically transforms the widely accepted dynamo theory. The georeactor hypothesis explains how convection can occur and power a dynamo, even though the author claims that the solid rock of the crust insulates the core and should stop the convection. Georeactors would, in his view, also explain apparent excess heat produced by the core of the Earth and certain gas giant planets. The argument for georeactors depends on unverified assumptions about heat loss and geologic helium emissions which are disputed on the basis of fundamental facts of thermodynamics, geochemistry, and other issues. Fission georeactors would depend upon the elemental abundance, concentration and dispersion of fissionable Uranium, in contradiction to the Goldschmidt classification, which gives reasons why uranium and other fissionable actinides accumulate in the Earth's crust, while being generally excluded from the core.
Apart from fundamental concerns over thermodynamics, elemental abundances at different depths within the earth, and others, the Georeactor hypothesis could be tested by observations of neutrino emissions. The experiment Borexino (2010) by observation of antineutrinos has set the upper limit on the possible power of the fission georeactor: P < 3 terawatt, which is 14 times less than total heat flow from the Earth (~42 TW).
The disputed georeactor hypothesis refers specifically to the formation of a self-sustaining nuclear chain reaction and not to the production of heat from the decay of radionuclides such as potassium-40, uranium-238 and thorium-232 which is more widely accepted.
Natural nuclear reactors
In the 1970s, geochemists documented the existence of naturally-occurring slow fission reactors in uranium-bearing geologic formations at Oklo in Gabon, Africa. The Oklo natural nuclear fission reactors operated approximately 1.5 to 2.0 billion years ago, when the natural occurrence of the uranium-235 isotope (required for the fission chain-reaction) was much higher. These are quite different conditions from those suggested for the georeactor.
The georeactor
Herndon's calculations also permitted the existence of a similar reactor at the Earth's core, depending on certain unconventional assumptions regarding the composition of the core, in particular the oxidation state of uranium and the likelihood of its precipitating to the center. He justifies these assumptions by comparison with the composition of enstatite chondrite meteorites, which do have the necessary highly reduced oxidation states and are, according to Herndon, the only chondrite meteorites which have sufficient iron metal-alloy to match the composition of the Earth with its massive core.
The Earth's magnetic field in relation to the reactor
According to Herndon, the energy produced by the reactor is what sustains the magnetic field of the Earth. He also claims that the field has weakened in recent years to indicate a possible polarity switch of our planet's poles. In his theory the switches in the field are caused by the reactor turning on and off.
Dynamo Theory and Georeactor hypothesis
In 2007, Herndon suggested a radical modification of dynamo theory, in which the electrically conducting operating fluid, and thus region of dynamo action, may be contained within a geocentric nuclear fission reactor, called the georeactor, in a fluid sub-shell of the solid core, rather than distributed throughout the liquid layer of the Earth's nickel-iron core. Herndon has claimed the following reasons why long-term stable convection would not be favorable within the Earth’s fluid core
The following is taken from a San Francisco Chronicle article by Keay Davidson describing that test:
:One of Herndon's leading critics is planetary scientist David Stevenson of the California Institute of Technology. He says in an e-mail: "Herndon is a solid and knowledgeable person when it comes to (nuclear) reactors. But the amount of attention this (georeactor) idea has received is out of proportion with its plausibility. ... It's not complete nonsense, but it's highly unlikely. There are many instances in science where this happens. This one has merely received more attention than most.
:"The idea is based on two very dubious propositions: (a) That uranium (or any heavy element) would naturally go to the center of the Earth. This is almost certainly untrue. It is a misunderstanding of chemistry and statistical physics at a very fundamental level. (b) That there is something about Earth's heat flow or helium that is so wildly discordant with our usual ideas that it requires an outrageous hypothesis to explain it. This is incorrect."
:"...If a scientist has any serious disagreement, he/she should publish it in the literature, with all supporting documentation, in circumstances under which I might respond with full documentation. No one, including David Stevenson, has attempted to do that. If Stevenson truly believes that I have made some misunderstanding, he should provide the documentation. He won’t, because he can’t. My work is on solid footing."
:"...Statistical physics is a fancy, important-sounding term that is completely irrelevant for the scientific considerations involved. A term more appropriate to the subject is metallurgical thermochemistry and that, I assure you, is a different animal entirely."

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