Uranium
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.468 years, roughly the age of the Earth. Uranium-235 has a half-life of about 7.13 years, and uranium-234 has a half-life of about 2.48 years.
For natural uranium, about 49% of its alpha rays are emitted by each of U atom, and also 49% by U (since the latter is formed from the former) and about 2.0% of them by the U. When the Earth was young, probably about one-fifth of its uranium was uranium-235, but the percentage of U was probably much lower than this.
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.