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.
Thorium fuel cycle
Another challenge associated with the thorium fuel cycle is the comparatively long interval over which breeds to . The half-life of is about 27 days, which is an order of magnitude longer than the half-life of . As a result, substantial develops in thorium-based fuels. is a significant neutron absorber and, although it eventually breeds into fissile , this requires two more neutron absorptions, which degrades neutron economy and increases the likelihood of transuranic production.