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.
Radioactive waste
The radioactivity of all radioactive waste weakens with time. All radionuclides contained in the waste have a half-life—the time it takes for half of the atoms to decay into another nuclide—and eventually, all radioactive waste decays into non-radioactive elements (i.e., stable nuclides). Certain radioactive elements (such as plutonium-239) will remain hazardous to humans and other creatures for hundreds of thousands of years. Other radionuclides remain hazardous for millions of years. Thus, these wastes must be shielded for centuries and isolated from the living environment for millennia. Since radioactive decay follows the half-life rule, the rate of decay is inversely proportional to the duration of decay. In other words, the radiation from a long-lived isotope like iodine-129 will be much less intense than that of a short-lived isotope like iodine-131. The two tables show some of the major radioisotopes, their half-lives, and their radiation yield as a proportion of the yield of fission of uranium-235.