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.
Rum Jungle, Northern Territory
One of the main environmental impacts of uranium mining is the creation of large volumes of radioactive mine waste (tailings) which are left behind on the site. The major radioactive component of these tailings is uranium-238, an isotope with a halflife of 4.46 billion years. In 2003, a government survey of the tailings piles at Rum Jungle found that capping which was supposed to help contain this radioactive waste for at least 100 years, had failed in less than 20 years.