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
MOX fuel
Plutonium-241 is fissile while the isotopes of plutonium with even mass numbers are not (in general thermal neutrons will usually fission isotopes with an odd number of neutrons, but rarely those with an even number), so decay of plutonium-241 to americium-241 leaves plutonium with a lower proportion of isotopes usable as fuel, and a higher proportion of isotopes that simply capture neutrons (though they may become fissile isotopes after one or more captures). The decay of plutonium-238 to uranium-234, and subsequent removal of this uranium, would have the opposite effect, but plutonium-238 both has a longer half-life (87.7 years vs. 14.3) and is a smaller proportion of the spent nuclear fuel. Plutonium-239, -240, and -242 all have much longer half-lives so that decay is negligible. Plutonium-244 has an even longer half-life, but is unlikely to be formed by successive neutron capture because plutonium-243 quickly decays with a half-life of 5 hours giving americium-243.