Enriched uranium

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 (DU), and is considerably less radioactive than even natural uranium, though still extremely dense. It is useful for armor, armor penetrating weapons and other material density applications.

Grades

Highly enriched uranium (HEU)

Highly enriched uranium (HEU) has a greater than 20% concentration of ²³⁵U.

The fissile uranium in nuclear weapons usually contains 85% or more of ²³⁵U known as weapon(s)-grade, though for a crude inefficient weapon 20% is sufficient (called weapon(s)-usable); some argue that even less is sufficient, but then the critical mass required rapidly increases. The presence of too much of the ²³⁸U isotope inhibits the runaway nuclear chain reaction that is responsible for the weapon's power.

HEU is also used in nuclear submarine reactors, where it contains at least 50% ²³⁵U, but typically exceeds 90%.

Low-enriched uranium (LEU)

Low-enriched uranium (LEU) has a lower than 20% concentration of ²³⁵U. For use in commercial light water reactors (LWR), the most prevalent power reactors in the world, uranium is enriched to 3 to 5 % ²³⁵U. While LEU cannot be used to create a nuclear bomb, there is concern that it could be processed into a component of a dirty bomb using conventional explosive to disperse radioactive contaminants in a populated area. 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 (SEU)

Slightly enriched uranium (SEU) has a concentration of ²³⁵U between 0.9% and 2%.

This new grade is being used to replace Natural uranium (NU) 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 (RU) is a variation of SEU. It is based on a fuel cycle involving used fuel recovered from light water reactors (LWR). 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 (Hex) 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 UF₆ 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 (UCOR) 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 lead 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 (EMIS). 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 (Atomic Vapor Laser Isotope Separation) 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, (Molecular Laser Isotope Separation). 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 UF₆ called SILEX apparently is “fundamentally completely different to what has been tried elsewhere" according to Silex Systems Ltd.[1], the developer. Details of the process are currently not available. In 1996 USEC secured the rights to evaluate and develop SILEX for uranium (it is also useable for silicon and other elements) but relinquished these in 2003.

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 (PSP) 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 (SWU) 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 (from the German Urantrennarbeit )

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 effort expended in separating a mass $F$ of feed of assay $x_{f}$ into a mass P of product assay xp, or product enrichment, and tails of mass T and assay xt is expressed in terms of the number of separative work units needed, given by the expression:

SWU=P*V\left(x_{p}\right)+T*V(x_{t})-F*V(x_{f})

where $V\left(x\right)$ is the Value Function, defined as

V(x)=(1-2x)*\ln({\frac {1-x}{x}})

Feed to product ratio is given by the expression:

F/P = (xp - xt)/(xf - xt).

Tails to product ratio is given by the expression:

T/P = (xp - xf)/(xf - xt).

If, for example, you begin with 100 kilograms (220 pounds) of NU, it takes about 60 SWU to produce 10 kilograms (22 pounds) 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 1,300 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 1,000 kSWU/year is, therefore, able to enrich the uranium needed to fuel about eight large nuclear power stations

Cost Issues

In 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 (as compared to 0.7% in NU) 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.

Uranium Enrichment Calculator designed by: the WISE Uranium Project

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 even more than that of ²³⁵U. High concentrations of ²³⁴U may cause excessive worker radiation exposures during fuel fabrication; 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 (with an ²³⁵U assay of approx. 50%) 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.

Also a more detailed economic analysis of downblending actually performed in Russia suggests that blendstock enrichment even consumes 20% more Separative Work Units (SWU) than can be recovered. This means that no recovery takes place at all, and the whole process is a SWU sink.

A Uranium Downblending Calculator designed by the WISE Uranium Project

External links

Uranium Enrichment and Nuclear Weapon Proliferation, by Allan S. Krass, Peter Boskma, Boelie Elzen and Wim A. Smit, 296 pp., Published for SIPRI by Taylor and Francis Ltd, London, 1983
Uranium enrichment (PDF, 651 KB)
Silex Systems Ltd
Nuclear Issues Briefing Paper 33
[2] Overview and history of U.S. HEU production