The progression of nuclear fuel through a series of stages of production, use and disposal is referred to as the nuclear fuel cycle. When the spent fuel is used once and then disposed of, the cycle is called open or once-through. When the spent fuel is reprocessed and used again, the cycle is called closed.
Uranium is a very common element that is found in most rocks and soils as well as river and sea water. Granite comprises about sixty percent of the Earth's crust and it contains minute amounts of uranium.
Naturally occurring uranium contains about ninety nine percent U-238, a stable isotope and one percent U-235, a radioactive isotope. In order to be considered for exploitation, a deposit of uranium ore has to have a high enough percentage of uranium in a recoverable form at a low enough cost to be profitable.
Uranium ores contain uranium oxide in concentrations of less than one percent. Sources of uranium ore in the US are under three tenths of one percent with greater percentages found in ores in other countries. Open pit and underground techniques are the primary methods of uranium mining. Some uranium is mined by passing oxygenated groundwater through porous ore which dissolves the uranium oxide and allows it to be brought to the surface.
Uranium ore is ground to produce particles which are then subjected to a chemical process called leaching to recover the uranium. This process results in uranium oxide in the form of a dry powder known as yellowcake which is usually about eighty percent uranium. This processing is usually done near the mine.
Most yellowcake is converted to uranium hexafluoride in a process that involves nitric acid, ammonium and hydrogen. Uranium hexafluoride is a solid at room temperature but becomes a gas when heated above one hundred fourty degrees. It is highly corrosive and toxic. It reacts violently with water and must be transported in sealed containers.
In order to be utilized as a nuclear fuel, the uranium must be enriched in order to raise the percentage of U-235. Light water reactors use normal water for cooling and require a concentration of U-235 to about three and one half percent. Uranium is enriched by isotope separation. The uranium hexafluoride is heated to a gaseous state and spun in high speed centrifuges to separate the two isotopes which have slightly different weights. Most of the resulting depleted uranium byproduct is stored as uranium hexafluoride. Some of it is converted to metallic uranium and used in armor, artillery, radiation shielding and ballast.
Enriched uranium fluoride is processed into uranium dioxide powder which is then mixed with binders and formed into pellets. The pellets are fired in a sintering furnace to produce ceramic pellets. These pellets are then machined into a standard size and stacked into alloy tubes to produce fuel rods. The rods are combined into bundles and bundles are combined to create the reactor core. The size and shape of the rods and bundles depend on which type of reactor they are intended for.