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.
Burnup
Unprocessed used fuel from current light-water reactors consists of 5% fission products and 95% actinides, and is dangerously radiotoxic, requiring special custody, for 300,000 years. Most of the long-term radiotoxic elements are transuranic, and therefore could be recycled as fuel. 70% of fission products are either stable or have half lives less than one year. Another six percent (I and Tc) can be transmuted to elements with extremely short half lives (I—12.36 hours—and Tc—15.46 seconds). Zr, having a very long half life, constitutes 5% of fission products, but can be alloyed with uranium and transuranics during fuel recycling, or used in cladding, where its radioactivity is irrelevant. The remaining 20% of fission products, or 1% of unprocessed fuel, for which the longest-lived isotopes are Cs and Sr, require special custody for only 300 years. Therefore, the mass of material needing special custody is 1% of the mass of unprocessed used fuel.