Encyclopedia
Enriched uranium is
uranium whose
uranium-235 content has been increased through the process of
isotope separation. Natural uranium consists mostly of the
238U isotope, with about 0.72 % by weight as
235U, the only isotope existing in nature in any appreciable amount that is
fissionable by
thermal neutrons.
Enriched uranium is a critical component for both civil
nuclear power generation and military
nuclear weapons. The
International Atomic Energy Agency attempts to monitor and control enriched uranium supplies and processes in its efforts to ensure nuclear power generation safety and curb
nuclear weapons proliferation.
During the
Manhattan Project enriched uranium was given the codename
oralloy, a shortened version of
Oak Ridge alloy, after the plant where the uranium was enriched. The term oralloy is still occasionally used to refer to enriched uranium.
The
238U remaining after enrichment is known as
depleted uranium , and is considerably less
radioactive than even natural uranium, though still extremely dense. It is useful for
armor, penetrating weapons, and other applications requiring very dense metals.
Grades
Highly enriched uranium
Highly enriched uranium has a greater than 20% concentration of
235U or
233U.
The fissile uranium in nuclear weapons usually contains 85% or more of
235U known as
weapon-grade, though for a crude, inefficient weapon 20% is sufficient ; some argue that even less is sufficient, but then the
critical mass required rapidly increases. However, judicious use of implosion and neutron reflectors can enable construction of a weapon from a quantity of uranium below the usual critical mass for its level of enrichment, though this would likely only be possible in a country which already had extensive experience in developing nuclear weapons. The presence of too much of the
238U isotope inhibits the runaway
nuclear chain reaction that is responsible for the weapon's power. The critical mass for 85 % of highly enriched uranium is about 50 kilograms.
HEU is also used in
fast neutron reactors as well as in nuclear submarine reactors, where it contains at least 50%
235U, but typically exceeds 90%. The Fermi-1 commercial fast reactor prototype used HEU with 26.5%
235U.
Low-enriched uranium
Low-enriched uranium has a lower than 20% concentration of
235U.
For use in commercial light water reactors , the most prevalent power reactors in the world, uranium is enriched to 3 to 5 %
235U. Fresh LEU used in research reactors is usually enriched 12% to 19.75% U-235, the later concentration being used to replace HEU fuels when converting to LEU.
Slightly enriched uranium
Slightly enriched uranium has a concentration of
235U
between 0.9% and 2%.
This new grade is being used to replace Natural uranium fuel in some heavy water reactors like the
CANDU. Costs are lowered because less uranium and fewer bundles are needed to fuel the reactor. This in turn reduces the quantity of used fuel and its subsequent waste management costs.
Recovered uranium is a variation of SEU. It is based on a
fuel cycle involving used fuel recovered from light water reactors . The spent fuel from a LWR typically contains more U-235 than natural uranium, and therefore could be used to fuel reactors which customarily use natural uranium as fuel.
Methods
Isotope separation is a difficult and energy intensive activity. Enriching uranium is difficult because the two isotopes are very similar in weight:
235U is only 1.26% lighter than
238U. Several production techniques applied to enrichment have been used, and several are under investigation. In general these methods exploit the slight differences in atomic weights of the various isotopes. Some work is being done that would use
nuclear resonance however it is not certain if any of these processes have been scaled up to production.
A feature common to all large-scale enrichment schemes is that they employ a number of identical stages which produce successively higher concentrations of
235U. Each stage concentrates the product of the previous step further before being sent to the next stage. Similarly, the tailings from each stage are returned to the previous stage for further processing. This sequential enriching system is called a cascade.
Thermal diffusion
Thermal diffusion utilizes the transfer of heat across a thin liquid or gas to accomplish isotope separation. The process exploits the fact that the lighter
235U gas molecules will diffuse toward a hot surface, and the heavier
238U molecules will diffuse toward a cold surface. A plant at Oak Ridge was used during
World War II to prepare feed material for the EMIS process. It was abandoned in favor of gaseous diffusion.
Gaseous diffusion
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous
uranium hexafluoride through
semi-permeable membranes. This produces a slight separation between the molecules containing
235U and
238U. Throughout the
Cold War, gaseous diffusion played a major role as a uranium enrichment technique, though it has now been almost completely replaced by newer methods.
The gas centrifuge
The gas centrifuge process uses a large number of rotating cylinders in series and parallel formations. This rotation creates a strong centrifugal force so that the heavier gas molecules containing
238U move toward the outside of the cylinder and the lighter gas molecules rich in
235U collect closer to the center. It requires far less energy to achieve the same separation than the older gaseous diffusion process, which it has largely replaced.
The Zippe centrifuge
The Zippe centrifuge is an improvement on the standard gas centrifuge, the primary difference being the use of heat. The bottom of the rotating cylinders are heated, producing currents that move the
235U up the cylinder, where it can be collected by scoops. This improved centrifuge design is used by the commercial company Urenco to produce nuclear fuel. However this process was also used by
Pakistan in their nuclear weapons program and the Pakistani government sold the Zippe-Type technology on to
North Korea and
Iran allowing them to develop their nuclear industry.
Aerodynamic processes
Aerodynamic enrichment processes include the Becker Jet Nozzle Techniques developed by EW Becker and associates and the
vortex tube separation process. These aerodynamic separation processes depend upon diffusion driven by pressure gradients, as does the gas centrifuge. In effect, aerodynamic processes can be considered as non-rotating centrifuges. Enhancement of the centrifugal forces is achieved by dilution of UF
6 with
hydrogen or
helium as a carrier gas achieving a much higher flow velocity for the gas than could be obtained using pure uranium hexafluoride. The Uranium Enrichment Corporation of South Africa developed and deployed the Helikon vortex separation process based on the vortex tube and a demonstration plant was built in
Brazil by NUCLEI, a consortium led by Industrias Nucleares do Brasil that used the separation nozzle process. However both methods have high energy consumption and substantial requirements for removal of waste heat; neither is currently in use.
Electromagnetic Isotope Separation
Electromagnetic Isotope Separation process . An electromagnetic separation process, the metallic uranium is first vaporized, and then ionized to positively charged ions. They are then accelerated and subsequently deflected by magnetic fields on to their respective collection targets. A production-scale
mass spectrometer named the
Calutron was developed during World War II that provided much of the
235U used for the
Little Boy nuclear device, which was deployed over
Hiroshima in 1945. Properly the term 'Calutron' applies to a multistage device arranged in a large oval around a powerful electromagnet. Magnetic separation has been largely abandoned in favour of more effective methods; however international inspectors found that
Iraq had secretly constructed dozens of calutrons, purportedly for development of a nuclear bomb.
Laser processes
Laser processes are a possible third-generation technology promising lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages.
AVLIS is a method by which specially tuned lasers are used to separate isotopes of uranium using selective ionization of hyperfine transitions. The technique uses
lasers which are tuned to frequencies that ionize a
235U atom and no others. The positively-charged
235U ions are then attracted to a negatively-charged plate and collected.
A second method of laser separation is known as MLIS, . In this method, an infrared laser is directed at uranium hexafluoride gas, exciting molecules that contain a
235U atom. A second laser frees a
fluorine atom, leaving
uranium pentafluoride which then precipitates out of the gas.
An Australian development which is molecular and utilises UF
6 called SILEX apparently is “fundamentally completely different to what has been tried elsewhere" according to Silex Systems Ltd., the developer. Details of the process are currently not available. After a protracted development process involving US enrichment company USEC acquiring and then relinquishing commercialization rights to the technology,
General Electric has signed a commercialization agreement with Silex Systems in 2006 .
None of these processes is yet ready for commercial use, though SILEX is well advanced.
Chemical methods
One
chemical process has been demonstrated to pilot plant stage but not used. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change valency in
oxidation/reduction, utilising immiscible aqueous and organic phases.
An
ion-exchange process was developed by the Asahi Chemical Company in
Japan which applies similar chemistry but effects separation on a proprietary resin ion-exchange column.
Plasma separation
Plasma separation process describes a technique potentially more efficient at uranium-enrichment that makes use of
superconducting magnets and
plasma physics. In this process, the principle of
ion cyclotron resonance is used to selectively energize the
235U isotope in a
plasma containing a mix of ions. The French developed their own version of PSP, which they called RCI. Funding for RCI was drastically reduced in 1986, and the program was suspended around 1990, although RCI is still used for stable isotope separation.
Separative work unit
Separative Work Unit is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched, and as such is the extent of increase in the concentration of the
235U isotope relative to the remainder.
Separative work is expressed in SWUs, kg SW, or kg UTA
- 1 SWU = 1 kg SW = 1 kg UTA
- 1 kSWU = 1 tSW = 1 t UTA
- 1 MSWU = 1 ktSW = 1 kt UTA
The unit is strictly:
Kilogram Separative Work Unit, and it measures the quantity of separative work, indicative of energy used in enrichment, when feed, tails and product quantities are expressed in kilograms. The work necessary to separating a mass of feed of assay into a mass of product assay , and tails of mass and assay is expressed in terms of the number of separative work units needed, given by the expression
where is the Value Function, defined as
The feed to product ratio is given by the expression
whereas the tails to product ratio is given by the expression
If, for example, you begin with 100 kilograms of NU, it takes about 60 SWU to produce 10 kilograms of LEU in
235U content to 4.5%, at a tails assay of 0.3%.
The number of Separative Work Units provided by an enrichment facility is directly related to the amount of energy that the facility consumes. Modern gaseous diffusion plants typically require 2,400 to 2,500 kilowatt-hours of electricity per SWU while gas centrifuge plants require just 50 to 60 kilowatt-hours of electricity per SWU.
Example:A large nuclear power station with a net electrical capacity of 1300 MW requires about 25 000 kg of LEU annually with a
235U concentration of 3.75%. This quantity is produced from about 210 000 kg of NU using about 120 000 SWU. An enrichment plant with a capacity of 1000 kSWU/year is, therefore, able to enrich the uranium needed to fuel about eight large nuclear power stations.
Cost IssuesIn addition to the Separative Work Units provided by an enrichment facility, the other important parameter that must be considered is the mass of NU that is needed in to order to yield a desired mass of enriched uranium. As with the number of SWUs, the amount of feed material required will also depend on the level of enrichment desired and upon the amount of
235U that ends up in the depleted uranium. However, unlike the number of SWUs required during enrichment which increases with decreasing levels of
235U in the depleted stream, the amount of NU needed will decrease with decreasing levels of
235U that end up in the DU.
For example, in the enrichment of LEU for use in a light water reactor it is typical for the enriched stream to contain 3.6%
235U while the depleted stream contains 0.2% to 0.3%
235U. In order to produce one kilogram of this LEU it would require approximately 8 kilograms of NU and 4.5 SWU if the DU stream was allowed to have 0.3%
235U. On the other hand, if the depleted stream had only 0.2%
235U, then it would require just 6.7 kilograms of NU, but nearly 5.7 SWU of enrichment. Because the amount of NU required and the number of SWUs required during enrichment change in opposite directions, if NU is cheap and enrichment services are relatively more expensive, then the operators will typically choose to allow more
235U to be left in the DU stream whereas if NU is relatively more expensive and enrichment is less so, then they would choose the opposite.
Downblending
The opposite of enriching is
downblending; Surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel.
The HEU feedstock, can contain unwanted uranium isotopes:
234U is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases but remains well below 1%. High concentrations of
236U is a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. HEU reprocessed from nuclear weapons material production reactors may contain
236U concentrations as high as 25%, resulting in concentrations of approx. 1.5% in the blended LEU product.
236U is a neutron poison; therefore the actual
235U concentration in the LEU product must be raised accordingly to compensate for the presence of
236U.
The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt-%
235U may used as a blendstock to dilute the unwanted byproducts that may contained in the HEU feed. Concentrations of these isotopes in the LEU product in some cases could exceed ASTM specifications for nuclear fuel, if NU, or DU were used. So, the HEU downblending generally cannot contribute to the waste management problem posed by the existing large stockpiles of depleted uranium.
A major downblending undertaking called the Megatons to Megawatts Program converts ex-Soviet weapons-grade HEU to fuel for U.S. commercial power reactors. From 1995 through mid-2005, 250 metric tons of high-enriched uranium were recycled into low-enriched-uranium. The goal is to recycle 500 metric tons by 2013.
See also
External links
- Overview and history of U.S. HEU production
