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
Xenon-135
Iodine-135 is a fission product of uranium with a yield of about 6% (counting also the iodine-135 produced almost immediately from decay of fission-produced tellurium-135). This I decays with a 6.57 hour half-life to Xe. Thus, in an operating nuclear reactor, Xe is being continuously produced. Xe has a very large neutron absorption cross-section, so in the high neutron flux environment of a nuclear reactor core, the Xe soon absorbs a neutron and becomes almost-stable Xe. Thus, in about 50 hours, the Xe concentration reaches equilibrium where its creation by I decay is balanced with its destruction by neutron absorption.