Uranium–uranium dating
U, with a half-life of about 4.5 billion years, decays to U through emission of an alpha particle to thorium-234 (Th), which is comparatively unstable with a half-life of just 24 days. Th then decays through beta particle emission to protactinium-234 (Pa)). This decays with a half-life of 6.7 hours, again through emission of a beta particle, to U. This isotope has a half-life of about 245,000 years. The next decay product, thorium-230 (Th), has a half-life of about 75,000 years and is used in the uranium-thorium technique. Although analytically simpler, in practice U/U requires knowledge of the ratio at the time the material under study was formed and is generally used only for samples older than the ca. 450,000 year upper limit of the Th/U technique. For those materials (principally marine carbonates) for which these conditions apply, it remains a superior technique.
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.