While other elements have been used in nuclear reactors to generate energy, the primary fuel for nuclear power reactors is uranium. It is a silvery-white metal. Uranium is a chemical element that is symbolized by the letter “U.” Uranium has ninety two protons in its nucleus giving it an atomic number of 92. Uranium is the highest element on the periodic table that is naturally produced. All transuranics with higher atomic numbers are produced by man.
Uranium is present in the crust of Earth in a ratio of from two to four parts per million. Uranium has six isotopes, all of which are radioactive and unstable. The half lives of the isotopes vary from seventy years to four and a half billion years. The most common isotope of uranium is U-238 with ninety two protons and one hundred forty six neutrons. Over ninety nine percent of the uranium on Earth is U-238. The next isotope in abundance is U-235 with ninety two protons and one hundred forty three neutrons. It amounts to about seven tenths of one percent of the uranium on Earth.
U-235 is the only naturally occurring fissile element that can sustain a nuclear fission reaction when sufficiently concentrated. Uranium is mined and refined to produce a mixture with a few percent U-235 to make the fuel for most nuclear power reactors. Uranium can combine with many different elements in many compounds and is found in deposits all over the world.
The dominant compound of uranium in many deposits is the crystalline mineral called uraninite which is a mixture of UO2 and U3O8. Up to this point it was believed that uranium deposits were created by natural chemical processes where uranium in groundwater underwent reduction by elements such as sulfur. It had been suggested that some uranium deposits may have been laid down by biological processes but this was seen as a rare exception to the inorganic origin of most deposits.
Professor Thomas Borch and Dr Amrita Bhattacharyya of Colorado State University have proposed an alternate theory for the origin of many uranium deposits. They recently published a report of their research in the journal Nature Communications. The two scientists were investigating uranium deposits in Wyoming where they drilled six hundred and fifty foot cores in the deposits. Bosch thought that the deposits might have a biological origin so he used analysis techniques that were only developed recently.
It turned out that about ninety percent of the uranium in the Wyoming deposits was combined with either organic material or carbonates and were not the crystalline form. They concluded that bacteria had created the deposits. Instead of using atmospheric oxygen to drive chemical reactions, the bacteria that created the deposits derived energy from reduction of uranium oxidation states.
The findings of the two researchers overturn decades of accepted theory about the origin of uranium deposits. This will have an effect on mining because mining licenses often include a requirement that that mining site be returned to its original state after the mine is closed. It turns out that the original state of many deposits may not be what it was assumed to be.
Non-crystalline uranium in deposits can be more easily converted to a water-soluble form than the crystalline form. This means that it is easy for uranium from bacterial deposits to enter groundwater and migrate through the Earth to threaten drinking water or the environment.
Uraninite crystal from Maine:
