Uranium: Definition & Uses

Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Mention of the word uranium most likely conjures up images of mushroom clouds or evacuation zones around nuclear power plants. But uranium itself is not inherently dangerous. You'll learn about this interesting element in this lesson.

Uranium Chemistry

Uranium is the heaviest (most massive) naturally-occurring known element. Every atom of uranium contains 92 protons (particles with positive electrical charge) in its nucleus (that is an element's atomic number). On the periodic table of elements, the symbol for uranium is a capital U.

While all uranium atoms have 92 protons, the number of neutrons (neutral particles with about the same mass as a proton) in the nucleus can vary. That leads to uranium atoms that have different atomic mass. Such atoms are called isotopes.

There are more than two dozen isotopes of uranium. But only three occur naturally (the rest are produced in nuclear reactors or colliders). The isotopes U-238 and U-235 are created in the cores of exploding supernovae. Uranium-234 forms in the decay series of U-238. About 99.27% of uranium atoms are U-238; another 0.72% are U-235; and 0.005% are U-234.


All isotopes of uranium are radioactive, meaning that the nuclei of the atoms are not stable and will spontaneously release energy, mostly in the form of alpha particles (two protons plus two neutrons). This process, known as radioactive decay, results in the uranium atom becoming a new element with two fewer protons.

The radioactive decay series of U-238. Half-lives are labeled in years(a), days(d), minutes(m) or seconds(s). Source: U.S. Geological Survey
U-238 decay series

We describe the process of radioactive decay of an element in terms of the half-life, the time required for half of a large original number of atoms to decay. In the case of U-238, the half-life is about 4.5 billion years. The other uranium isotopes all have different half-lives. For U-234 it is about 250,000 years, while U-235's half-life is over 700 million years.

Since the decay of any single atom is not predictable, if you started with only ten U-238 atoms, you couldn't say with certainty that after 4.5 billion years there would only be five left. There might be any number left. But if you started with a gazillion atoms, you'd be on fairly solid ground estimating that after 4.5 billion years you'd have only half a gazillion U-238 atoms left.

The fact that the half-life of the common uranium isotopes are so long means that they are not decaying all that quickly. So from a radioactivity point of view, uranium is not all that dangerous to be around. There is a big difference between radioactivity and fission (the process of splitting a nucleus to release energy).

Discovery of Uranium

Uraninite, also known as pitchblende, is one common ore of uranium. Source: Wikimedia user Geomartin, GNU Free Documentation License

Uranium was discovered in 1789 by a German chemist, who dissolved pitchblende in nitric acid, then neutralized the solution, which produced a yellow solid. After further testing that solid, he isolated what he though was a new element (although what he probably found was a uranium oxide) and he named it after the recently-discovered planet Uranus.

Antoine Becquerel discovered the radioactivity of uranium in 1896 when he happened to place a uranium compound he was using atop an unexposed photographic plate. The plate was foggy-looking when he later developed it and he realized that it was something in the uranium compound that caused that effect.

Abundance and Occurrence of Uranium

Despite its notoriety, uranium ranks 51st on the list of the natural abundance of elements on Earth, which qualifies it as a trace element. On the other hand, it is more abundant than silver!

Almost all soil and rock on Earth contain at least a small amount of uranium. Sedimentary rocks, including sandstone, shale, phosphate rocks, limestone and coal, contain uranium, as do many igneous and metamorphic rocks. Most of the mined uranium ore deposits in the world are in sandstone or shale beds, where the minerals were deposited by uranium-bearing groundwater.

Carnotite, distinguished by its bright yellow color, is the most common uranium ore in North America. Source: U.S. Geological Survey

It doesn't take a very high concentration of uranium oxide to make a rock mineable. Low grade ore has less than 0.25% uranium oxide, but some high grade ore can reach nearly 100 times that amount. The Athabasca sandstone deposits in Saskatchewan, Canada are the world's richest, containing about 23% uranium oxides (by weight).

In the United States, the major uranium ore deposits are found in the western states, from the Colorado Plateau northward through Utah and Wyoming. The United States has the world's fifth-largest reserves of uranium ore (7% of the total), but the ore tends to be low grade, and is not economical to mine when the price drops.

Most U.S. uranium ore is found in the western states, but tends to be low-grade and not economical to mine. Source: U.S. Department of Energy
U.S. uranium

Uses of Uranium

The United States is the world's largest consumer of uranium (although China is likely to move into that position soon), but produces only about 5% of what it uses. Russia provides about half of the uranium used in the U.S., followed by Canada and Australia.

Ceramic Glaze

The addition of uranium oxide to glazes was once a very common method of creating brilliantly-colored ceramics and pottery. Bright yellow or reddish-orange were the most common colors. During World War II the government snatched up all the uranium, and after the war, concerns about radioactivity led to the method being discontinued.

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