Uranium enrichment is a process that is necessary to create an effective nuclear fuel out of mined uranium by increasing the percentage of uranium-235 which undergoes fission with thermal neutrons. Although many reactors require enriched fuel, the Canadian-designed CANDU, the British Magnox reactor and the proposed Molten salt reactor can use natural uranium as their fuel.
Nuclear fuel is mined from naturally occurring uranium ore deposits, and then isolated through chemical reactions and separation processes. These chemical processes used to separate the uranium from the ore are not to be confused with the physical and chemical processes used to enrich the uranium. In its isolated form, the uranium is known as yellowcake and has the chemical formula U3O8. However, naturally occurring uranium does not have a high enough concentration of 235U at only about 0.72% with the remainder being 238U. Due to the fact that uranium-238 is fissionable and not fissile, the concentration of uranium-235 must be increased before it can be effectively used as a nuclear fuel. The purpose of uranium enrichment is to increase the percentage of the uranium-235 isotope with respect to others, with a necessary percentage of around 4% for light water reactors.
Enrichment requires uranium to be in a gaseous form, and the simplest way to achieve this is to convert it to a different chemical known as uranium hexafluoride. Uranium needs to be in a gaseous form for enrichment due to the varying chemical and physical properties the different isotopes (U-235 and U-238) have. These differences are most easily utilized and manipulated when uranium is in gaseous form.
The process of changing uranium oxide concentrate to uranium hexafluoride takes place at a conversion plant, the first step for uranium after it leaves a mine. The main conversion process used in Canada, France and Russia is known as the 'wet process' and involves multiple chemical conversion stages. Firstly, the uranium oxide concentrate is dissolved in nitric acid (HNO3), which creates uranyl nitrate (UO2(NO3)2). This uranyl nitrate is then purified, evaporated and finally thermally decomposed to form uranium trioxide powder (UO3). After which there are two kiln processes wherein the UO3 is converted to UO2, then reacted with hydrogen fluoride (HF), to produce uranium tetrafluoride (UF4). Finally the UF4 is fed into a fluidized bed reactor and reacted with gaseous fluorine to produce UF6. After the conversion process, the UF6 needs to be further refined due to the presence of impurities.
For many years the main process was gaseous diffusion. In order to physically separate the uranium the yellowcake uranium was first chemically transformed into uranium hexafluoride (UF6). This chemical is in its solid form under normal conditions, but transforms into a gas if the temperature is raised slightly or the pressure is lowered. Since the 235UF6 molecules are slightly lighter than the 238UF6 molecules, they move more quickly as a gas through diffusion. Thus if uranium hexafluoride is passed through a very long pipe, the gas that emerges at the far end of the pipe will have a slightly higher percentage of 235U. However, the pipe must be extremely long as the lighter 235UF6 diffuses only 0.43% faster than 238UF6. Because of this, the method of gaseous diffusion is not widely used anymore.
Today, enrichment is achieved using a gas centrifuge. The separation process here relies on the mass difference of the molecules (see gaseous diffusion above). Here, uranium hexafluoride is fed into an evacuated cylinder containing a rotor. When these rotors are spun at a high speed, the heavier 238UF6 collects near the walls of the cylinder while the slightly lighter 235UF6 collects near the central axis. The enriched product is then drawn off. This method is preferred over gaseous diffusion as it requires only about 3% of the power to separate the uranium. A centrifugal separation method is much more energy-efficient than diffusion, as it requires only about 50-60 kWh per SWU (separative work unit, which is the amount of separation done by an enrichment process). Additionally, these plants can be smaller as they don't require an extremely long pipe. For efficient separation to occur, these centrifuges must rotate quickly - generally at 50 000-70 000 rpm.
Although centrifuges hold less uranium than a diffusion stage, they are able to separate isotopes much more efficiently. Centrifuge stages generally are composed of a large number of centrifuges in parallel, forming a cascade.
The use of lasers in a separation process is still being developed. This separation technique requires lower energy input and other economic advantages. In this process, a laser with a very specific frequency interacts with a gas or vapour. Since the frequency has an associated energy, the interaction of the beam with the gas allows for the excitation or ionization of certain isotopes in the vapour. With this excitement, it may be possible to separate molecules containing a specific isotope to collect only the excited isotope.
Most enrichment processes involve only natural, long-lived radioactive materials. Uranium is only weakly radioactive, but its chemical toxicity is much more significant . Thus protective measures required for an enrichment plant are similar to those in other chemical industries. When exposed to moisture, uranium hexafluoride forms a very corrosive acid, hydrofluoric acid. Any leakage of this chemical is undesirable and to prevent this almost all areas of an enrichment plant keep the uranium hexafluoride gas below atmospheric pressure. This stops any outward leakage. Additionally, double containment is provided in areas where higher pressures are required and venting gases are collected and treated.
Enrichment accounts for around half of the cost of nuclear fuel in a light water reactor (a BWR or PWR) and 5% of the cost of the electricity generated. Previously enrichment has been the main source of greenhouse gases from the nuclear fuel cycle as electricity used for enrichment was generated using coal. Although there are associated greenhouse gas emissions, it is only about 0.1% of the emissions of an equivalent coal-fired power plant.