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.
Uranium-236
The ratio is less than 190 when the decay products of each are included. U-238's decay chain to uranium-234 and eventually lead-206 involves emission of eight alpha particles in a time (hundreds of thousands of years) short compared to the halflife of U-238, so that a sample of U-238 in equilibrium with its decay products (as in natural uranium ore) will have eight times the alpha activity of U-238 alone. Even purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of U-234 and therefore about twice the alpha activity of pure U-238. Enrichment to increase U-235 content will increase U-234 to an even greater degree, and roughly half of this U-234 will survive in the spent fuel. On the other hand, U-236 decays to thorium-232 which has a halflife of 14 billion years, equivalent to a decay rate only 31.4% as great as that of U-238.