The enriched product is then drawn off. For efficient separation to occur, these centrifuges must rotate quickly - generally at 50 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.
Additionally, double containment is provided in areas where higher pressures are required and venting gases are collected and treated.
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.
Fossil Fuels. Nuclear Fuels. Acid Rain. Climate Change. The difference in mass between U and U allows the isotopes to be separated and makes it possible to increase or "enrich" the percentage of U All present and historic enrichment processes, directly or indirectly, make use of this small mass difference. Some reactors, for example the Canadian-designed Candu and the British Magnox reactors, use natural uranium as their fuel.
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.
There is significant over-supply of enrichment capacity worldwide, much of which has been used to diminish uranium demand or supplement uranium supply. The ability of enrichment to substitute for uranium see description of underfeeding below has become more significant as centrifuge technology has taken over, since this means both lower SWU costs and the need to keep the centrifuges running, so capacity remains on line even as demand drops away.
These plus Germany, Netherlands and Japan provide toll enrichment services to the commercial market. Part of the motivation for international centres is to bring all new enrichment capacity, and perhaps eventually all enrichment, under international control as a non-proliferation measure.
Precisely what "control" means remains to be defined, and will not be uniform in all situations. But having ownership and operation shared among a number of countries at least means that there is a level of international scrutiny which is unlikely in a strictly government-owned and -operated national facility.
The centre is to provide assured supplies of low-enriched uranium for power reactors to new nuclear power states and those with small nuclear programs, giving them equity in the project, but without allowing them access to the enrichment technology.
The three-nation Urenco set-up is also similar though with more plants in different countries — UK, Netherlands and Germany — and here the technology is not available to host countries or accessible to other equity holders. On and RWE in Germany has said that if its technology is used in international centres it would not be accessible. Its new plant is in the USA, where the host government has regulatory but not management control. A planned new Areva plant in the USA has no ownership diversity beyond that of Areva itself, so would have been essentially a French plant on US territory.
A number of enrichment processes have been demonstrated historically or in the laboratory but only two, the gaseous diffusion process and the centrifuge process, have operated on a commercial scale.
In both of these, UF 6 gas is used as the feed material. Molecules of UF 6 with U atoms are about one percent lighter than the rest, and this difference in mass is the basis of both processes. Isotope separation is a physical process. New centrifuge plants are being built in France and USA.
Several plants are adding capacity. With surplus capacity, Russian plants operate at low tails assays underfeeding to produce low-enriched uranium for sale.
The feedstock for enrichment consists of uranium hexafluoride UF 6 from the conversion plant. The tails assay concentration of U is an important quantity since it indirectly determines the amount of work that needs to be done on a particular quantity of uranium in order to produce a given product assay.
Feedstock may have a varying concentration of U, depending on the source. Natural uranium will have a U concentration of approximately 0. The capacity of enrichment plants is measured in terms of 'separative work units' or SWU. The SWU is a complex unit which indicates the energy input relative to the amount of uranium processed, the degree to which it is enriched i.
The unit is strictly: kilogram separative work unit, and it measures the quantity of separative work performed to enrich a given amount of uranium a certain amount when feed and product quantities are expressed in kilograms.
The unit 'tonnes SWU' is also used. There is always a trade-off between the cost of enrichment SWU and the cost of uranium. However, especially in relation to new small reactor designs, there is increasing interest in higher enrichment levels. Some small demand already exists for research reactors. The first graph shows enrichment effort SWU per unit of product. The second shows how one tonne of natural uranium feed might end up: as kg of uranium for power reactor fuel, as 26 kg of typical research reactor fuel, or conceivably as 5.
The curve flattens out so much because the mass of material being enriched progressively diminishes to these amounts, from the original one tonne, so requires less effort relative to what has already been applied to progress a lot further in percentage enrichment.
The relatively small increment of effort needed to achieve the increase from normal levels is the reason why enrichment plants are considered a sensitive technology in relation to preventing weapons proliferation, and are very tightly supervised under international agreements.
Where this safeguards supervision is compromised or obstructed, as in Iran, concerns arise. About , SWU is required to enrich the annual fuel loading for a typical MWe light water reactor at today's higher enrichment levels. Enrichment costs are substantially related to electrical energy used. In the past it has also accounted for the main greenhouse gas impact from the nuclear fuel cycle where the electricity used for enrichment is generated from coal.
However, it still only amounts to 0. The utilities which buy uranium from the mines need a fixed quantity of enriched uranium in order to fabricate the fuel to be loaded into their reactors.
This is the contracted or transactional tails assay, and determines how much natural uranium must be supplied to create a quantity of Enriched Uranium Product EUP — a lower tails assay means that more enrichment services notably energy are to be applied.
The enricher, however, has some flexibility in respect to the operational tails assay at the plant. This is known as underfeeding. In respect to underfeeding or overfeeding , the enricher will base its decision on the plant economics together with uranium and energy prices. 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.
With forecast overcapacity, it is likely that some older cascades will be retired. Obsolete diffusion plants have been retired, the last being some belated activity at Paducah in Natural uranium is usually shipped to enrichment plants in type 48Y cylinders, each holding about 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. The three enrichment processes described below have different characteristics.
Diffusion is flexible in response to demand variations and power costs but is very energy-intensive. With centrifuge technology it is easy to add capacity with modular expansion, but it is inflexible and best run at full capacity with low operating cost. Laser technology can strip down to very low level tails assay, and is also capable of modular plant expansion.
The gas centrifuge process was first demonstrated in the s but was shelved in favour of the simpler diffusion process. It was then developed and brought on stream in the s as the second-generation enrichment technology. It is economic on a smaller scale, e.
It is much more energy efficient than diffusion, requiring only about kWh per SWU. China has two small centrifuge plants imported from Russia. China has several centrifuge plants, the first at Hanzhun with 6th generation centrifuges imported from Russia. The Lanzhou plant is operating at 3. Others are under construction. Brazil has a small plant which is being developed to 0. Pakistan has developed centrifuge enrichment technology, and this appears to have been sold to North Korea.
In both France and the USA plants with late-generation Urenco centrifuge technology have been built to replace ageing diffusion plants, not least because they are more economical to operate. Full initial capacity of 3. In it applied for doubling in capacity to 6. It is now cancelled, and in Orano requested the NRC to terminate the licence. It was designed to have an initial annual capacity of 3. To enrich uranium, yellowcake is first turned into a gas called uranium hexafluoride.
This is pumped into centrifuges that spin so fast the ever-so-slightly heavier gas containing uranium is forced to the outside, while the lighter gas containing uranium stays in the middle. In enrichment plants, thousands of centrifuges are connected in cascades. Each unit enriches the gas a little and then passes it on to the next centrifuge to enrich some more. Under the nuclear deal, Iran is permitted to enrich uranium to 3.
There is no technical barrier facing the Iranians. It is the early stages of enrichment that consume the most energy and the process becomes easier down the line. The process gets easier because less material has to be moved around at higher levels of enrichment.
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