Natural nuclear reactors and ionizing radiation in the Precambrian

238U/235U measurement in single-zircon crystals: implications for the Hadean environment, magmatic differentiation and geochronology

“…A scenario of weathering-transportsedimentation and tidal sorting of uraninite has, however, been proposed as a way to produce U-rich sediments capable of reaching criticality. 14,42 Even in this framework, the fact that the range of d 238 U values measured in early Earth zircons is centered on the chondritic value argues against any contribution from sources with high 238 U/ 235 U (e.g., nuclear reactor material or mineralized ore sediments). As such, our data provides no evidence for the widespread existence of Oklo-type reactors in the Hadean/Archean, at least within the basin sampled by the Jack Hills conglomerate.…”

Ionizing Radiation and Life

“…Close to the reactor core, the radiation field would have been a complex mix of fission fragments, energetic electrons, neutrons, and gamma rays (Draganic et al, 1983). It has been estimated by in-pile dosimetry that, operating at a kilowattlevel power, the Oklo natural nuclear reactors would have generated radiation dose rates of over 1 kGy/h within their cores.…”

“…With the thermal gradient in the clays surrounding the core calculated to be 100°C/m (GauthierLafaye et al, 1996), microbial life several meters away from the cores may have been able to survive the thermal hazard and been exposed to only the more penetrating neutron and gamma radiation from the fission reactions. Furthermore, 14% of the fission products have half-lives longer than one year and so would have created an elevated radiation environment for subterranean life during periods of reactor shutdown and also migrated outward from the core into the surrounding environment (Draganic et al, 1983).…”

Chemical Evolution: An Approach From Radiation Chemistry

“…Chemical reactions induced by ionizing radiation proceed via free radicals; in aqueous solutions, the radiation creates radical-rich pathways in the water, in which chemical reactions occur. The use of this source is also substantiated by calculations of the energy available from the decay of radioactive elements like potassium-40, uranium-235, uranium-238 and thorium-232 [17], all of which have half-lives on the order of 109 years. Several other naturally radioactive isotopes of long half-life, like rubidium-87 and indium-115, may have contributed, but to a minor extent.…”