Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched grades
U remaining after enrichment is known as depleted uranium
(DU), and is considerably less radioactive
than even natural uranium, though still very dense and extremely hazardous in granulated form – such granules are a natural by-product of the shearing action that makes it useful for armor
. Despite being mildly radioactive, depleted uranium is also an effective radiation shielding
Uranium as it is taken directly from the Earth is not suitable as fuel for most nuclear reactors and requires additional processes to make it usable. Uranium is mined either underground or in an open pit depending on the depth at which it is found. After the uranium ore
is mined, it must go through a milling process to extract the uranium from the ore.
This is accomplished by a combination of chemical processes with the end product being concentrated uranium oxide, which is known as "yellowcake
", contains roughly 60% uranium whereas the ore typically contains less than 1% uranium and as little as 0.1% uranium.
After the milling process is complete, the uranium must next undergo a process of conversion, "to either uranium dioxide
, which can be used as the fuel for those types of reactors that do not require enriched uranium, or into uranium hexafluoride
, which can be enriched to produce fuel for the majority of types of reactors".
Naturally-occurring uranium is made of a mixture of 235
U and 238
U. The 235
U is fissile
, meaning it is easily split with neutrons
while the remainder is 238
U, but in nature, more than 99% of the extracted ore is 238
U. Most nuclear reactors require enriched uranium, which is uranium with higher concentrations of 235
U ranging between 3.5% and 4.5% (although a few reactor designs using a graphite
or heavy water moderator
, such as the RBMK
, are capable of operating with natural uranium as fuel). There are two commercial enrichment processes: gaseous diffusion
and gas centrifugation
. Both enrichment processes involve the use of uranium hexafluoride and produce enriched uranium oxide.
A drum of yellowcake
(a mixture of uranium precipitates)
Reprocessed uranium (RepU) Low enriched uranium (LEU) Low enriched uranium
(LEU) has a lower than 20% concentration of 235
U; for instance, in commercial LWR, the most prevalent power reactors in the world, uranium is enriched to 3 to 5% 235
U. High-assay LEU (HALEU) is enriched from 5–20%.
Fresh LEU used in research reactors
is usually enriched 12 to 19.75% 235
U, the latter concentration is used to replace HEU fuels when converting to LEU.
Highly enriched uranium (HEU)
of highly enriched uranium metal
Highly enriched uranium
(HEU) has a 20% or higher concentration of 235
U. The fissile uranium in nuclear weapon
primaries usually contains 85% or more of 235
U known as weapons-grade
, though theoretically for an implosion design
, a minimum of 20% could be sufficient (called weapon-usable) although it would require hundreds of kilograms of material and "would not be practical to design";
even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the critical mass
for unmoderated fast neutrons
rapidly increases, with for example, an infinite
mass of 5.4% 235
U being required.
experiments, enrichment of uranium to over 97% has been accomplished.
The very first uranium bomb, Little Boy
, dropped by the United States
in 1945, used 64 kilograms of 80% enriched uranium. Wrapping the weapon's fissile core in a neutron reflector
(which is standard on all nuclear explosives) can dramatically reduce the critical mass. Because the core was surrounded by a good neutron reflector, at explosion it comprised almost 2.5 critical masses. Neutron reflectors, compressing the fissile core via implosion, fusion boosting
, and "tamping", which slows the expansion of the fissioning core with inertia, allow nuclear weapon designs
that use less than what would be one bare-sphere critical mass at normal density. The presence of too much of the 238
U isotope inhibits the runaway nuclear chain reaction
that is responsible for the weapon's power. The critical mass for 85% highly enriched uranium is about 50 kilograms (110 lb), which at normal density would be a sphere about 17 centimetres (6.7 in) in diameter.
Later US nuclear weapons usually use plutonium-239
in the primary stage, but the jacket or tamper secondary stage, which is compressed by the primary nuclear explosion often uses HEU with enrichment between 40% and 80%
along with the fusion
fuel lithium deuteride
. For the secondary of a large nuclear weapon, the higher critical mass of less-enriched uranium can be an advantage as it allows the core at explosion time to contain a larger amount of fuel. The 238
U is not said to be fissile but still is fissionable by fast neutrons (>2 MeV) such as the ones produced during D-T fusion.
is difficult because two isotopes of the same element have nearly identical chemical properties, and can only be separated gradually using small mass differences. (235
U is only 1.26% lighter than 238
U.) This problem is compounded because uranium is rarely separated in its atomic form, but instead as a compound (235
is only 0.852% lighter than 238
.) A cascade
of identical stages produces successively higher concentrations of 235
U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.
There are currently two generic commercial methods employed internationally for enrichment: gaseous diffusion
(referred to as first
generation) and gas centrifuge
generation), which consumes only 2% to 2.5%
as much energy as gaseous diffusion (at least a "factor of 20" more efficient).
Some work is being done that would use nuclear resonance
; however there is no reliable evidence that any nuclear resonance processes have been scaled up to production.
Gaseous diffusion uses semi-permeable membranes to separate enriched uranium
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 235
U and 238
U. Throughout the Cold War
, gaseous diffusion played a major role as a uranium enrichment technique, and as of 2008 accounted for about 33% of enriched uranium production,
but in 2011 was deemed an obsolete technology that is steadily being replaced by the later generations of technology as the diffusion plants reach their ends-of-life.
In 2013, the Paducah
facility in the US ceased operating, it was the last commercial 235
U gaseous diffusion plant in the world.
Thermal diffusion uses the transfer of heat across a thin liquid or gas to accomplish isotope separation. The process exploits the fact that the lighter 235
U gas molecules will diffuse toward a hot surface, and the heavier 238
U gas molecules will diffuse toward a cold surface. The S-50
plant at Oak Ridge, Tennessee
was used during World War II
to prepare feed material for the EMIS
process. It was abandoned in favor of gaseous diffusion.
A cascade of gas centrifuges at a U.S. enrichment plant
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 238
U move tangentially toward the outside of the cylinder and the lighter gas molecules rich in 235
U collect closer to the center. It requires much less energy to achieve the same separation than the older gaseous diffusion process, which it has largely replaced and so is the current method of choice and is termed second generation
. It has a separation factor per stage of 1.3 relative to gaseous diffusion of 1.005,
which translates to about one-fiftieth of the energy requirements. Gas centrifuge techniques produce close to 100% of the world's enriched uranium.
Diagram of the principles of a Zippe-type gas centrifuge with U-238 represented in dark blue and U-235 represented in light blue
The Zippe centrifuge
is an improvement on the standard gas centrifuge, the primary difference being the use of heat. The bottom of the rotating cylinder is heated, producing convection currents that move the 235
U up the cylinder, where it can be collected by scoops. This improved centrifuge design is used commercially by Urenco
to produce nuclear fuel and was used by Pakistan
in their nuclear weapons program.
Laser processes promise lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages. Several laser processes have been investigated or are under development. Separation of isotopes by laser excitation
) is well developed and is licensed for commercial operation as of 2012.
Atomic vapor laser isotope separation (AVLIS) Molecular laser isotope separation (MLIS) Separation of isotopes by laser excitation (SILEX) Separation of isotopes by laser excitation
is an Australian development that also uses UF6
. After a protracted development process involving U.S. enrichment company USEC
acquiring and then relinquishing commercialization rights to the technology, GE Hitachi Nuclear Energy
(GEH) signed a commercialization agreement with Silex Systems
GEH has since built a demonstration test loop and announced plans to build an initial commercial facility.
Details of the process are classified and restricted by intergovernmental agreements between United States, Australia, and the commercial entities. SILEX has been projected to be an order of magnitude more efficient than existing production techniques but again, the exact figure is classified.
In August, 2011 Global Laser Enrichment, a subsidiary of GEH, applied to the U.S. Nuclear Regulatory Commission
(NRC) for a permit to build a commercial plant.
In September 2012, the NRC issued a license for GEH to build and operate a commercial SILEX enrichment plant, although the company had not yet decided whether the project would be profitable enough to begin construction, and despite concerns that the technology could contribute to nuclear proliferation
Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.
The X-ray based LIGA
manufacturing process was originally developed at the Forschungszentrum Karlsruhe, Germany, to produce nozzles for isotope enrichment.
Aerodynamic enrichment processes include the Becker jet nozzle techniques developed by E. W. Becker and associates using the LIGA
process and the vortex tube
separation process. These aerodynamic
separation processes depend upon diffusion driven by pressure gradients, as does the gas centrifuge. They in general have the disadvantage of requiring complex systems of cascading of individual separating elements to minimize energy consumption. In effect, aerodynamic processes can be considered as non-rotating centrifuges. Enhancement of the centrifugal forces is achieved by dilution of UF6
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 continuous Helikon vortex separation cascade for high production rate low enrichment and the substantially different semi-batch Pelsakon low production rate high enrichment cascade both using a particular vortex tube separator design, and both embodied in industrial plant.
A demonstration plant was built in Brazil
by NUCLEI, a consortium led by Industrias Nucleares do Brasil
that used the separation nozzle process. However all methods have high energy consumption and substantial requirements for removal of waste heat; none are currently still in use.
Electromagnetic isotope separation
Schematic diagram of uranium isotope separation in a calutron
shows how a strong magnetic field is used to redirect a stream of uranium ions to a target, resulting in a higher concentration of uranium-235 (represented here in dark blue) in the inner fringes of the stream.
In the electromagnetic isotope separation
process (EMIS), metallic uranium is first vaporized, and then ionized to positively charged ions. The cations are then accelerated and subsequently deflected by magnetic fields onto their respective collection targets. A production-scale mass spectrometer
named the Calutron
was developed during World War II that provided some of the 235
U used for the Little Boy
nuclear bomb, which was dropped over Hiroshima
in 1945. Properly the term 'Calutron' applies to a multistage device arranged in a large oval around a powerful electromagnet. Electromagnetic isotope separation has been largely abandoned in favour of more effective methods.
One chemical process has been demonstrated to pilot plant stage but not used for production. The French CHEMEX process exploited a very slight difference in the two isotopes' propensity to change valency
, using immiscible aqueous and organic phases. An ion-exchange process was developed by the Asahi Chemical Company
that applies similar chemistry but effects separation on a proprietary resin ion-exchange
Plasma separation process (PSP) describes a technique that makes use of superconducting magnets
and plasma physics
. In this process, the principle of ion cyclotron resonance
is used to selectively energize the 235
U 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" – the amount of separation done by an enrichment process – is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units that are so calculated as to be proportional to the total input (energy / machine operation time) and to the mass processed. Separative work is not energy. The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology. Separative work is measured in Separative work units SWU, kg SW, or kg UTA (from the German Urantrennarbeit – literally uranium separation work)
- 1 SWU = 1 kg SW = 1 kg UTA
- 1 kSWU = 1 tSW = 1 t UTA
- 1 MSWU = 1 ktSW = 1 kt UTA
In addition to the separative work units provided by an enrichment facility, the other important parameter to be considered is the mass of natural uranium (NU) that is needed 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 more expensive, then the operators will typically choose to allow more 235U to be left in the DU stream whereas if NU is more expensive and enrichment is less so, then they would choose the opposite.
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
are 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 235
U assay of approx. 50%) may contain 236
U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. 236U
is a neutron poison
; therefore the actual 235
U concentration in the LEU product must be raised accordingly to compensate for the presence of 236
The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt% 235
U may used as a blendstock to dilute the unwanted byproducts that may be 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. At present, 95 percent of the world's stocks of depleted uranium remain in secure storage.
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 tonnes of high-enriched uranium (enough for 10,000 warheads) was recycled into low-enriched-uranium. The goal is to recycle 500 tonnes by 2013. The decommissioning programme of Russian nuclear warheads accounted for about 13% of total world requirement for enriched uranium leading up to 2008.
The United States Enrichment Corporation
has been involved in the disposition of a portion of the 174.3 tonnes of highly enriched uranium (HEU) that the U.S. government declared as surplus military material in 1996. Through the U.S. HEU Downblending Program, this HEU material, taken primarily from dismantled U.S. nuclear warheads, was recycled into low-enriched uranium (LEU) fuel, used by nuclear power plants
to generate electricity.
Global enrichment facilities
The following countries are known to operate enrichment facilities: Argentina, Brazil, China, France, Germany, India, Iran, Japan, the Netherlands, North Korea, Pakistan, Russia, the United Kingdom, and the United States.
Belgium, Iran, Italy, and Spain hold an investment interest in the French Eurodif
enrichment plant, with Iran's holding
entitling it to 10% of the enriched uranium output. Countries that had enrichment programs in the past include Libya and South Africa, although Libya's facility was never operational.
Australia has developed a laser enrichment
process known as SILEX, which it intends to pursue through financial investment in a U.S. commercial venture by General Electric.
It has also been claimed that Israel has a uranium enrichment program housed at the Negev Nuclear Research Center
site near Dimona
During the Manhattan Project
, weapons-grade highly enriched uranium was given the codename oralloy
, a shortened version of Oak Ridge
alloy, after the location of the plants where the uranium was enriched.
The term oralloy is still occasionally used to refer to enriched uranium.
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Last edited on 24 March 2021, at 04:55
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