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Is uranium radioactive?

All isotopes of uranium are radioactive, with most having extremely long half-lives. Half-life is a measure of the time it takes for one half of the atoms of a particular radionuclide to disintegrate (or decay) into another nuclear form. Each radionuclide has a characteristic half-life. Half-lives vary from millionths of a second to billions of years. Because radioactivity is a measure of the rate at which a radionuclide decays (for example, decays per second), the longer the half-life of a radionuclide, the less radioactive it is for a given mass. The half-life of uranium-238 is about 4.5 billion years, uranium-235 about 700 million years, and uranium-234 about 25 thousand years.

Uranium atoms decay into other atoms, or radionuclides, that are also radioactive and commonly called "decay products." Uranium and its decay products primarily emit alpha radiation, however, lower levels of both beta and gamma radiation are also emitted. The total activity level of uranium depends on the isotopic composition and processing history. A sample of natural uranium (as mined) is composed of 99.3% uranium-238, 0.7% uranium-235, and a negligible amount of uranium-234 (by weight), as well as a number of radioactive decay products.

In general, uranium-235 and uranium-234 pose a greater radiological health risk than uranium-238 because they have much shorter half-lives, decay more quickly, and are thus "more radioactive." Because all uranium isotopes are primarily alpha emitters, they are only hazardous if ingested or inhaled. However, because several of the radioactive uranium decay products are gamma emitters, workers in the vicinity of large quantities of uranium in storage or in a processing facility can also be exposed to low levels of external radiation.

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