GC: n

CT: Before its use in a reactor, uranium must undergo four processing steps to convert it from an ore to solid ceramic fuel pellets. These processes are: mining and milling, conversion, enrichment and fabrication.
Mining and Milling
Uranium miners use several techniques to obtain uranium: surface (open pit), underground mining and in-situ recovery. Uranium also is a byproduct of other mineral processing operations. Solvents remove the uranium from mined ore or in-situ leaching, and the resulting uranium oxide—called yellowcake—undergoes filtering and drying.
Uranium Conversion
The yellowcake then goes to a conversion plant, where chemical processes convert it to uranium hexafluoride. The uranium hexafluoride is heated to become a gas and loaded into cylinders. When it cools, it condenses into a solid.
Uranium Enrichment
Uranium hexafluoride contains two types of uranium, U-238 and U-235. The percentage of U-235, which is the type of uranium that fissions easily, is less than 1 percent. To make the uranium usable as a fuel, its U-235 content is increased to between 3 percent and 5 percent. This process is called enrichment. The concentration of U-235 is so low in enriched uranium that an explosion is impossible.

S: http://www.nei.org/Knowledge-Center/Nuclear-Fuel-Processes (last access: 20 December 2015)

N: 1. enriched (adj): from enrich (v). Late 14c., “to make wealthy,” from Old French enrichir “enrich, enlarge,” from en- “make, put in”+ riche “rich”. Figurative sense “supply with abundance of something desirable” is from 1590s. Meaning “to fertilize” is from c. 1600. Scientific sense of “to increase the abundance of a particular isotope in some material” is first attested 1945.
uranium (n): rare metallic element, 1797, named 1789 in Modern Latin by its discoverer, German chemist and mineralogist Martin Heinrich Klaproth (1743-1817), for the recently found planet Uranus.
2. Uranium and enriched uranium: Uranium was discovered by Martin Klaproth, a German chemist, in 1789 in the mineral pitchblende, and was named after the planet Uranus. It occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth’s crust as tin, tungsten and molybdenum and about 40 times as common as silver. Being relatively soluble (in contrast to thorium), it is also found in the oceans, at an average concentration of 3 parts per billion. There are a number of locations in different parts of the world where it occurs in economically-recoverable concentrations. When mined, it yields a mixed uranium oxide product, U3O8. Uraninite, or pitchblende, is the most common uranium mineral.
In the past, uranium was also used to colour glass (from as early as 79 AD) and deposits were once mined in order to obtain its decay product, radium. This element was used in luminous paint, particularly on the dials of watches and aircraft instruments up to the 1950s, and in medicine for the treatment of disease.
For many years from the 1940s, virtually all of the uranium that was mined was used in the production of nuclear weapons, but this ceased to be the case in the 1970s. Today the only substantial use for uranium is as fuel in nuclear reactors, mostly for electricity generation. Uranium-235 is the only naturally-occurring material which can sustain a fission chain reaction, releasing large amounts of energy.
Natural uranium contains 0.7% of the U-235 isotope. The remaining 99.3% is mostly the U-238 isotope which does not contribute directly to the fission process (though it does so indirectly by the formation of fissile isotopes of plutonium). Isotope separation is a physical process to concentrate (‘enrich’) one isotope relative to others. Most reactors are light water reactors (of two types – PWR and BWR) and require uranium to be enriched from 0.7% to 3-5% U-235 in their fuel. There is some interest in taking enrichment levels to about 7%, and even close to 20% for certain special power reactor fuels.
3. Uranium enrichment is the process by which a sample of uranium has its proportion of U-235 increased.
The first people to figure out how to do this were the scientists of the Manhattan Project during World War II. They came up with four methods to separate the U-235 from uranium ore: gaseous diffusion, electromagnetic separation, liquid thermal diffusion and centrifugation, though at the time they deemed centrifugation not practical for large-scale enrichment.
The most common methods for enriching uranium today are centrifugation (decades of development have made this method more efficient than it was during WWII) and gaseous diffusion. And other methods are being developed, including several based on laser techniques.
Highly enriched uranium, the type used in bombs, is expensive and difficult to create, which is why it remains a barrier, though not an insurmountable one, for countries wishing to develop nuclear weapons. And once a nation develops the capability for enriching uranium beyond reactor grade (Iran has reportedly begun to produce uranium enriched up to 20 percent), the path to weapons-grade uranium is significantly sped up.
4. With reduced demand for enriched uranium following the Fukushima accident, enrichment plants have continued running, since it is costly to shut down and re-start centrifuges. The surplus SWU output can be sold, or the plants can be underfed so that the enricher ends up with excess uranium for sale, or with enriched product for its own inventory and later sale. The inertia of the enrichment process thus exacerbates over-supply in the uranium market and depresses SWU prices (from $160/SWU in 2010, the spot price early in April 2015 was $79).
Obsolete diffusion plants have been retired, the last being some belated activity at Paducah in 2013.
Natural uranium is usually shipped to enrichment plants in type 48Y cylinders, each holding about 12.5 tonnes of uranium hexafluoride (8.4 tU). These cylinders are then used for long-term storage of DU, typically at the enrichment site. Enriched uranium is shipped in type 30B cylinders, each holding 2.27 t UF6 (1.54 tU).

S: 1. OED (last access: 18 December 2015). 2. WNA – http://www.world-nuclear.org/info/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment/ (last access: 18 December 2015). 3. Smithsonian – http://www.smithsonianmag.com/science-nature/what-is-enriched-uranium-17091828/?no-ist (last access: 18 December 2015). 4. WNA – http://www.world-nuclear.org/info/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment/ (last access: 20 December 2015).

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CR: depleted uranium, natural uranium, nuclear energy, nuclear power plant, uranium .