The proto-Earth geo-reactor: Reassessing the hypotheses

• Geo-reactor past occurrence is reviewed on the basis of gas isotope signatures.
• Even if actinides segregated during the proto-Earth period their ignition is unlikely.
• Present neutrino investigations show a priori absence of criticality.

Recent high-precision isotope analysis data could be used to revisit the hypothetical past occurrence of a geo-reactor. Specific noble gas isotope signatures that could be generated by binary and ternary fissions were identified in volcano emanations or as soluble/associated species in crystalline rocks and semi-quantitatively quantified as isotopic ratio or estimated amounts. Presently if it would have been hypothetically stated that according to the actinide inventory on the Earth, local potential criticality of the geo-system, if locally concentrated, may have been reached, several questions remain such as why, where and when did any geo-reactor have been operational? Even if the hypothesis of a geo-reactor operation in the proto-Earth period would have been plausible, it is likely that a geo-reactor is not operating today. This has been recently tested by reconstructing the occurrence of actinides by antineutrino detection and tomography through the Earth.The present paper focuses on the geo-reactor hypothetical conditions including history, spatial extension and regimes. The discussion based on recent calculations involves investigations on the limits in term of fissile inventory, size and power, based on coupling of geochemical reactions and stratification through the gravitational field considering behavior through the inner mantle, the boundary with the core and the core. The reconstruction allows to formulating that from the history point of view it would have been possible that the geo-reactor reached criticality in a proto-Earth period as a reactor triggered by 235-uranium and that thorium may have worked as an absorber if the actinide concentration was locally large enough. Without actinide separation the initiation of the criticality is unlikely. However did the segregation of actinides occur in any Earth layer?