By Jonny Lupsha, Wondrium Staff Writer
Enriching uranium is a detailed and complex chemical process. Uranium is found naturally in the ground in two different forms, one of which is mostly useless. Where does depleted uranium come from?
All nuclear power currently starts with uranium. Like coal, uranium needs to be mined, but it must undergo an extensive and elaborate process in order to be used for nuclear power. This process, known as enriching, leads to waste byproducts, including what is known as depleted uranium. How does the process happen and what is the difference between depleted and enriched uranium?
Depleted uranium returned to the headlines recently when the British government defended its action of supplying the Armed Forces of Ukraine with ammunition that contained depleted uranium. In his video series The Science of Energy: Resources and Power Explained, Dr. Michael Wysession, Professor of Earth and Planetary Sciences at Washington University in St. Louis, explains where depleted uranium comes from.
How Is Uranium Found?
“We don’t find lumps of pure uranium; we find it in an oxidized state in the form of minerals such as uraninite—also called pitchblende, autunite, or tobernite,” Dr. Wysession said. “Different minerals result from different ratios of the uranium and oxygen atoms. Different minerals also result from other elements included. For example, crystals of the pretty yellow uranium mineral autunite also include atoms of calcium and phosphorous.”
Since uranium is constantly giving off gamma rays, it can be easily detected with a Geiger counter. Geiger counters contain tubes that hold inert gases like helium and argon. When helium and argon come in contact with gamma rays, they becomes ionized and, for a brief moment, that creates an electrical current. The amount of electrical current that is present indicates the amount of radiation that is present.
The electrical current is heard through a speaker on the Geiger counter as a clicking sound, which most of us are familiar with.
“Now you can fly over land in a plane equipped with a gamma ray spectrometer,” Dr. Wysession said. “It’s like a digital camera, but it records gamma rays instead of visible light rays, and you can map out regions of high uranium densities with it.”
Where Does Depleted Uranium Come From?
Once uranium has been found and mined, the next step is to concentrate uranium ore into a substance known as yellowcake. This is done with “leaching,” or using sulfuric acid to dissolve and remove uranium oxide from other rocks. Once it’s filtered and dried, you have yellowcake. Yellowcake is abundant with U3O8, uranium dioxide, and uranium trioxide.
If the uranium is intended for use in a heavy-water nuclear reactor, the uranium doesn’t need to be converted to Uranium-235. If a light-water reactor is being used, the uranium needs to be combined with fluorine to make uranium hexafluoride, or UF6. This process is known as enriching.
“There are several ways to separate the uranium isotopes to enrich the fuel in Uranium-235,” Dr. Wysession said. “The most common method uses high-speed centrifuges. You take the uranium hexafluoride gas, and put it in the centrifuge, and spin it really fast—100,000 times a minute.”
Eventually, the Uranium-238 will separate from the Uranium-235. Why bother? Nuclear power requires a “fissile” material, meaning that an atom will split apart when bombarded with a neutron. Uranium-235 is fissile, while Uranium-238 is not. After the enriching is done, the uranium hexafluoride gas is mixed with calcium, making a calcium fluoride salt, leaving the Uranium-235 behind.
“The outcome of enrichment is pellets enriched in Uranium-235, which are very radioactive and have to be handled very carefully,” Dr. Wysession said.
The other byproduct of enrichment, the leftover Uranium-238, is depleted uranium.
The Science of Energy: Resources and Power Explained is now available to stream on Wondrium.