Surprisingly, for such a heavy metal, uranium can form a compound that is a gas at moderate temperatures. Uranium combines with six atoms of fluorine to form uranium hexafluoride UF6. Coincidentally, fluorine has only one stable isotope, F This is important because, if the fluorine atoms had different weights, there would be no way to distinguish whether the difference in weight of the UF 6 molecule was due to the uranium or the fluorine atoms.
Since fluorine atoms are identical, any change in mass has to be due to the different uranium isotopes. The heart of a gas centrifuge is a tube, called the rotor, that spins at high speed around its long axis.
The performance of the centrifuge depends critically on the speed of the rotor. Technical advances in materials, high-speed bearings, and precision machining are what have made increased rotor speeds possible and made centrifuges practical. Today, rotors may spin in excess of 60, rpm with the outside surface of the rotor moving well in excess of the speed of sound.
Hence, the pressure and resulting density decrease as we go up through the atmosphere. A similar effect occurs in a centrifuge but, because the forces created by the fast spinning centrifuge might be a million times stronger than gravity, everything happens on a much smaller scale. In the spinning rotor, the slightly heavier UF 6 that contains U will be slightly more compressed along the rim relative to the lighter UF 6 that contains U, which would have a relatively greater concentration near the axis.
The separation between the two isotopes created by the centrifugal forces is quite small. However, there is a simple way to substantially increase the degree of separation by creating a circulation flow along the length of the centrifuge. Eventually enriched and depleted uranium are drawn from the cascade at the desired assays.
To obtain efficient separation of the two isotopes, centrifuges rotate at very high speeds, with the outer wall of the spinning cylinder moving at between and metres per second to give a million times the acceleration of gravity.
Although the volume capacity of a single centrifuge is much smaller than that of a single diffusion stage, its capability to separate isotopes is much greater. Centrifuge stages normally consist of a large number of centrifuges in parallel. Such stages are then arranged in cascade similarly to those for diffusion. In the centrifuge process, however, the number of stages may only be 10 to 20, instead of a thousand or more for diffusion.
Centrifuges are designed to run for about 25 years continuously, and cannot simply be slowed or shut down and restarted according to demand. Western cascades are designed for 0. Laser enrichment processes have been the focus of interest for some time. They are a possible third-generation technology promising lower energy inputs, lower capital costs and lower tails assays, hence significant economic advantages.
One of these processes is almost ready for commercial use. Laser processes are in two categories: atomic and molecular. In the US Government backed it as the new technology to replace its gaseous diffusion plants as they reached the end of their economic lives early in the 21st century. French work on SILVA ceased following a 4-year program to to prove the scientific and technical feasibility of the process.
Some kg of 2. Atomic vapour processes work on the principle of photo-ionisation, whereby a powerful laser is used to ionise particular atoms present in a vapour of uranium metal. An electron can be ejected from an atom by light of a certain frequency.
The laser techniques for uranium use frequencies which are tuned to ionise a U atom but not a U atom. The positively-charged U ions are then attracted to a negatively-charged plate and collected. Atomic laser techniques may also separate plutonium isotopes.
This then enables the ionized UF 5 to be separated from the unaffected UF 6 molecules containing U atoms, hence achieving a separation of isotopes. It provided for GE now GE Hitachi to construct in the USA an engineering-scale test loop, then a pilot plant or lead cascade, which could be operating in , and expanded to a full commercial plant.
The agreement is contingent upon US government approvals. Silex said: "The Paducah commercial opportunity represents an ideal path to market for our disruptive SILEX laser enrichment technology.
GLE is completing the test loop programme, the initial phase of which has already been successful in meeting performance criteria, and engineering design for a commercial facility has commenced. GLE will now decide in the light of commercial considerations on whether to proceed with a full-scale enrichment facility at Wilmington.
There is about , tonnes of these at Paducah and Portsmouth among a total of , t tails. In November the DOE announced that it would proceed with contract negotiations to this end. GLE expects licensing to take years. Negotiations with the DOE continued into , and in November an agreement was signed with the DOE for it to supply about , tonnes of high-assay tails, justifying construction by GLE of the plant in the early s.
PLEF would become a commercial uranium enrichment production facility under a US NRC licence, producing about , tonnes of natural-grade uranium over 40 years or more. The DOE would dispose of the reduced-assay balance. The estimated plant size is 0. Applications to silicon and zirconium stable isotopes are also being developed by Silex Systems near Sydney. CRISLA is another molecular laser isotope separation process which is the early stages of development. In this a gas is irradiated with a laser at a particular wavelength that would excite only the U isotope.
The entire gas is subjected to low temperatures sufficient to cause condensation on a cold surface or coagulation in the un-ionised gas. The excited molecules in the gas are not as likely to condense as the unexcited molecules. Hence in cold-wall condensation, gas drawn out of the system is enriched in the U isotope that was laser-excited. NeuTrek, the development company, is aiming to build a pilot plant in USA. The energy-intensive gaseous diffusion process of uranium enrichment is no longer used in the nuclear industry.
It involves forcing uranium hexafluoride gas under pressure through a series of porous membranes or diaphragms. As U molecules are lighter than the U molecules they move faster and have a slightly better chance of passing through the pores in the membrane.
The UF 6 which diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is depleted in U This process is repeated many times in a series of diffusion stages called a cascade. Each stage consists of a compressor, a diffuser and a heat exchanger to remove the heat of compression. The enriched UF 6 product is withdrawn from one end of the cascade and the depleted UF 6 is removed at the other end.
Diffusion plants typically have a small amount of separation through one stage hence the large number of stages but are capable of handling large volumes of gas. Russia phased out the process in and the last diffusion plant was USEC's Paducah facility, which shut down in mid It was used to enrich some high-assay tails before being finally shut down after 60 years' operation. At Tricastin, in southern France, a more modern diffusion plant with a capacity of This Georges Besse I plant could produce enough 3.
It was shut down in mid, after 33 years' continuous operation. Its replacement GB II, a centrifuge plant — see above has commenced operation. However, though they have proved durable and reliable, gaseous diffusion plants reached the end of their design life and the much more energy-efficient centrifuge enrichment technology has replaced them. But how is uranium enriched?
This is the starting material and it needs some processing. Before the uranium can be used in nuclear reactors or atomic bombs, it has to be enriched. This is because natural uranium contains too little uranium, the form of uranium that is easily split to release energy in the process known as fission.
Natural uranium contains only 0. The isotopes differ only in the number of neutrons found in their atoms. Former U. President Donald Trump pulled the U. Iran responded by intensifying its enrichment of uranium and building centrifuges in plain violation of the accord, while insisting that its nuclear development is for civilian not military purposes.
Israel maintains Iran still maintains the ambition of developing nuclear weapons, pointing to Tehran's ballistic missile program and research into other technologies.
Iran denies it is pursuing nuclear weapons, and says its nuclear program is for peaceful purposes. The amount of the material was 17 kilograms in January.
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