Nuclear power reactors

The vast majority of all nuclear power reactors require 'enriched' uranium fuel in which the proportion of the uranium-235 isotope has been raised from the natural level of 0.7% to about 3.5% to 5%.  The enrichment process needs to have the uranium in gaseous form, so on the way from the mine it goes through a conversion plant which turns the uranium oxide into uranium hexafluoride.

About 27 tonnes of fresh fuel is required each year by a 1000 MWe nuclear reactor.

Uranium Oxide.

 The most common forms of uranium oxide are U3O8 and UO2. Both oxide forms are solids that have a low solubility in water and are relatively stable over a wide range of environmental conditions. U3O8 is the most stable form of uranium and is the form found in nature. The most common form of U3O8 is “yellow cake,” a solid named for its characteristic color that is produced during the uranium mining and milling process. UO2 is a solid ceramic material and is the form in which uranium is most commonly used as a nuclear reactor fuel.  At ambient temperatures, UO2 will gradually convert to U3O8.  Uranium oxides are extremely stable in the environment and are thus generally considered the preferred chemical form for storage or disposal.

 Isotope Separation

 Natural uranium is a mixture of 0.711% 235U and 92.89% of 238U. The enrichment process enriches the 235U content in natural uranium to the desired percentage. Low-enriched uranium which is typically used in nuclear reactors has 3-4% percent of 235U, while the highly enriched uranium has more than 50% of 235U and is typically used in nuclear weapons.

 Uranium is made from uraninite, which is a mixture of UO2, UO3, oxides of lead, thorium and rare earth elements. Uraninite is calcined to evaporate some impurities, then agglomerated and crushed. U3O8 is then put in a kiln with hydrogen:

 U3O8 + 2H2 =⇒ 3UO2 + 2H2O,                  Heat = −109kJ/mole       (1)

UO3 + H2 =⇒ UO2 + H2O                              Heat = −109kJ/mole       (2)

 The uranium dioxide is then treated with hydrogen fluoride in another kiln.

 UO2 + 4HF =⇒ UF4 + 2H2O,                        Heat = −176kJ/mole       (3)

 Finally the tetrafluoride is fed into a fluidized bed reactor with fluorine to produce the uranium hexafluoride (238UF6, 235 UF6) that is used in the separation process.

 UF4 + F2 =⇒ UF6                                                                                             (4)

 After the separation of molecules the UF6 is vaporized in autoclaves with steam and reacted with hydrogen at 700°C:

UF6 + 2H2O + H2 =⇒ U3O8 + 6HF                                                              (5)

 The final product is the so-called yellow cake, which is the basic raw material for nuclear fuel fabrication.

 Uranium hexafluoride is used because it has great storage properties. It can be used as a solid, liquid or gas, with minimum variations in pressure or temperature. It is usually stored as a solid, when in use it can be turned into liquid which is ideal for pumping. For the actual separation process it is used as a gas.

Uranium Hexafluoride.

 UF6  is the chemical form of uranium that is used during the uranium enrichment process. Within a reasonable range of temperature and pressure, it can be a solid, liquid, or gas. Solid UF6 is a white, dense, crystalline material that resembles rock salt.  UF6 does not react with oxygen, nitrogen, carbon dioxide, or dry air, but it does react with water or water vapor (including humidity in the air). When UF6 comes into contact with water, such as water vapor in the air, the UF6 and water react, forming corrosive hydrogen fluoride (HF) and a uranium-fluoride compound called uranyl fluoride (UO2F2). For this reason, UF6 is always handled in leak-tight containers and processing equipment. Although very convenient for processing, UF6 is not considered a preferred form for long-term storage or disposal because of its relative instability.

Enrichment processes require uranium to be in a gaseous form at relatively low temperature, hence uranium oxide from the mine is converted to uranium hexafluoride in a preliminary process, at a separate conversion plant.

Enriched UF₆ is transported to a fuel fabrication plant where it is converted to uranium dioxide powder. This powder is then pressed to form small fuel pellets, which are then heated to make a hard ceramic material. The pellets are then inserted into thin tubes to form fuel rods. These fuel rods are then grouped together to form fuel assemblies, which are several meters long. 

The number of fuel rods used to make each fuel assembly depends on the type of reactor. A pressurized water reactor may use between 121-193 fuel assemblies, each consisting of between 179-264 fuel rods.

There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion (referred to as first generation) and gas centrifuge (second generation), which consumes only 2% to 2.5%^[9] as much energy as gaseous diffusion, with centrifuges being at least a "factor of 20" more efficient.

The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. Each cylinder's rotation creates a strong centripetal force so that the heavier gas molecules containing ²³⁸U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in ²³⁵U collect closer to the center.

Today, 5% U-235 is the maximum level of enrichment for fuel used in normal power reactors. 

Uranium Metal.

 Uranium metal is heavy, silvery white, malleable, ductile, and softer than steel. It is one of the densest materials known (19 g/cm3), being 1.6 times more dense than lead. Uranium metal is not as stable as U3O8 or UO2 because it is subject to surface oxidation. It tarnishes in air, with the oxide film preventing further oxidation of bulk metal at room temperature. Water attacks uranium metal slowly at room temperature and rapidly at higher temperatures. Uranium metal powder or chips will ignite spontaneously in air at ambient temperature.