Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Synthesis of precious metals
Each kilogram of the fission products of U will contain 63.44 grams of ruthenium isotopes with halflives longer than a day. Since a typical used nuclear fuel contains about 3% fission products, one ton of used fuel will contain about 1.9 kg of ruthenium. The Ru and Ru will render the fission ruthenium very radioactive. If the fission occurs in an instant then the ruthenium thus formed will have an activity due to Ru of 109 TBq g and Ru of 1.52 TBq g. Ru has a half-life of about 39 days meaning that within 390 days it will have effectively decayed to the only stable isotope of rhodium, Rh, well before any reprocessing is likely to occur. Ru has a half-life of about 373 days, meaning that if the fuel is left to cool for 5 years before reprocessing only about 3% of the original quantity will remain; the rest will have decayed.