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.
Peak uranium
The IAEA estimates that using only known reserves at the current rate of demand and assuming a once-through nuclear cycle that there is enough uranium for at least 100 years. However, if all primary known reserves, secondary reserves, undiscovered and unconventional sources of uranium are used, uranium will be depleted in 47,000 years.