Nuclear reactor physics
Modern deposits of uranium contain only up to ~0.7% U (and ~99.3% U), which is not enough to sustain a chain reaction moderated by ordinary water. But U has a much shorter half-life (700 million years) than U (4.5 billion years), so in the distant past the percentage of U was much higher. About two billion years ago, a water-saturated uranium deposit (in what is now the Oklo mine in Gabon, West Africa) underwent a naturally occurring chain reaction that was moderated by groundwater and, presumably, controlled by the negative void coefficient as the water boiled from the heat of the reaction. Uranium from the Oklo mine is about 50% depleted compared to other locations: it is only about 0.3% to 0.7% U; and the ore contains traces of stable daughters of long-decayed fission products.Fermi age theory
Isotopes of polonium
Polonium (Po) has 42 isotopes, all of which are radioactive, with between 186 and 227 nucleons. Po with a half-life of 138.376 days has the longest half-life of naturally occurring polonium. Po with a half-life of 125 years has the longest half-life of all isotopes of polonium. Po and Po (half-life 2.9 years) can be made through proton bombardment of bismuth in a cyclotron.