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.
Accelerator-driven subcritical reactor
Unlike uranium-235, thorium is not fissile—it essentially does not split on its own, exhibiting a half-life of 14.05 billion years (20 times that of U-235). The fission process stops when the proton beam stops, as when power is lost, thus the reactor is subcritical. Only microscopic quantities of plutonium are produced and can be burned in the same reactor.