The principal goals of federal regulations are to limit the seepage of radionuclides and heavy metals into groundwater and reduce emissions of radon-222 to the air.
Of the naturally-occuring uranium isotopes, only uranium-235 can sustain a chain reaction– a reaction in which each fission produces enough neutrons to trigger another, so that the fission process is maintained without any external source of neutrons.
If inhaled or ingested, however, its radioactivity poses increased risks of lung cancer and bone cancer.
Uranium is also chemically toxic at high concentrations and can cause damage to internal organs, notably the kidneys.
These three kinds of radiation have very different properties in some respects but are all ionizing radiation–each is energetic enough to break chemical bonds, thereby possessing the ability to damage or destroy living cells.
Uranium-238, the most prevalent isotope in uranium ore, has a half-life of about 4.5 billion years; that is, half the atoms in any sample will decay in that amount of time.