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 protactinium
Protactinium-234m is a member of the uranium series with a half-life of 1.17 minutes. It was discovered in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring, who named it brevium for its short half-life. About 99.8% of decays of Th produce this isomer instead of the ground state (t=6.70 hours).