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The enriched UF 6 product is withdrawn from one end of the cascade and the depleted UF 6 is removed at the other end.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.
The large Georges Besse I enrichment plant at Tricastin in France beyond cooling towers was shut down in Most of the output from the nuclear power plant 4xMWe net was used to power the enrichment facility. A very early endeavour was the electromagnetic isotope separation EMIS process using calutrons.
This was developed in the early s in the Manhattan Project to make the highly enriched uranium used in the Hiroshima bomb, but was abandoned soon afterwards. However, it reappeared as the main thrust of Iraq's clandestine uranium enrichment program for weapons discovered in EMIS uses the same principles as a mass spectrometer albeit on a much larger scale.
Ions of uranium and uranium are separated because they describe arcs of different radii when they move through a magnetic field. The process is very energy-intensive — about ten times that of diffusion.
Two aerodynamic processes were brought to demonstration stage around the s. One is the jet nozzle process, with demonstration plant built in Brazil, and the other the Helikon vortex tube process developed in South Africa. They depend on a high-speed gas stream bearing the UF6 being made to turn through a very small radius, causing a pressure gradient similar to that in a centrifuge.
The light fraction can be extracted towards the centre and the heavy fraction on the outside. Thousands of stages are required to produce enriched product for a reactor. It is based on Helikon but pending regulatory authorisation it has not yet been tested on UF6 - only light isotopes such as silicon.
However, extrapolating from results there it is expected to have an enrichment factor in each unit of 1. One chemical process has been demonstrated to pilot plant stage but not used.
In some countries used fuel is reprocessed to recover its uranium and plutonium, and to reduce the final volume of high-level wastes. The plutonium is normally recycled promptly into mixed-oxide MOX fuel, by mixing it with depleted uranium. Where uranium recovered from reprocessing used nuclear fuel RepU is to be re-used, it needs to be converted and re-enriched. This is complicated by the presence of impurities and two new isotopes in particular: U and U, which are formed by or following neutron capture in the reactor, and increase with higher burn-up levels.
U is largely a decay product of Pu, and increases with storage time in used fuel, peaking at about ten years. Both decay much more rapidly than U and U, and one of the daughter products of U emits very strong gamma radiation, which means that shielding is necessary in any plant handling material with more than very small traces of it. U is a neutron absorber which impedes the chain reaction, and means that a higher level of U enrichment is required in the product to compensate.
For the Dutch Borssele reactor which normally uses 4. Being lighter, both isotopes tend to concentrate in the enriched rather than depleted output, so reprocessed uranium which is re-enriched for fuel must be segregated from enriched fresh uranium. The presence of U in particular means that most reprocessed uranium can be recycled only once - the main exception being in the UK with AGR fuel made from recycled Magnox uranium being reprocessed.
U is also present in RepU, but as an alpha emitter it does not pose extra problems. Traces of some fission products such as Tc may also carry over. All these considerations mean that only RepU from low-enriched, low-burnup used fuel is normally recycled directly through an enrichment plant.
Much smaller quantities have been used elsewhere, in France and Japan. Some re-enrichment, e. It assayed about 0. Recycling of MDU was discontinued in due to economic factors. A laser process would theoretically be ideal for enriching RepU as it would ignore all but the desired U, but this remains to be demonstrated with reprocessed feed. Tails from enriching reprocessed uranium remain the property of the enricher. Some recycled uranium has been enriched by Tenex at Seversk for Areva, under a ten-year contract covering about tonnes UF 6.
French media reports in alleging that wastes from French nuclear power plants were stored at Seversk evidently refer to tails from this. Early enrichment activities often left depleted uranium tails with about 0. With the wind-down of military enrichment, particularly in Russia, there was a lot of spare capacity unused. Consequently, since the mid s some of the highest-assay tails have been sent to Russia by Areva and Urenco for re-enrichment by Tenex.
These arrangements however cease in , though Tenex may continue to re-enrich Russian tails. Tenex now owns all the tails from that secondary re-enrichment, and they are said to comprise only about 0. The enriched UF 6 is converted to UO 2 and made into fuel pellets — ultimately a sintered ceramic, which are encased in metal tubes to form fuel rods, typically up to four metres long. A number of fuel rods make up a fuel assembly, which is ready to be loaded into the nuclear reactor.
See Fuel Fabrication paper. With the minor exception of reprocessed uranium, enrichment involves only natural, long-lived radioactive materials; there is no formation of fission products or irradiation of materials, as in a reactor. Feed, product, and depleted material are all in the form of UF 6 , though the depleted uranium may be stored long-term as the more stable U 3 O 8.
Uranium is only weakly radioactive, and its chemical toxicity — especially as UF 6 — is more significant than its radiological toxicity. The protective measures required for an enrichment plant are therefore similar to those taken by other chemical industries concerned with the production of fluorinated chemicals.
The uranium found in nature contains only 0. Centrifuges spin at enormous speeds and the heavier isotope, U, moves to the outside and is then removed, leaving a higher concentration of U behind, which can be further enriched. This fuel does not represent a proliferation threat primarily because of the critical mass issue — the amount of material necessary to maintain a self-sustaining neutron chain reaction. If the level of enrichment is low, then it holds that the amount of the material must go up in order for a chain reaction to be sustained.
The size can quickly become impractical for weapons delivery, so low enriched uranium LEU is not a threat. Uranium versus plutonium: the see-sawing proliferation threat in research reactors. An inherent problem in research reactors, those built for scientific or medical purposes, is that either the uranium fuel or the plutonium in the spent fuel could potentially have strategic value as fuel in a nuclear weapon.
This chemical is in its solid form under normal conditions, but transforms into a gas if the temperature is raised slightly or the pressure is lowered. Thus if uranium hexafluoride is passed through a very long pipe, the gas that emerges at the far end of the pipe will have a slightly higher percentage of U. However, the pipe must be extremely long as the lighter UF 6 diffuses only 0. Today, enrichment is achieved using a special centrifuge called a gas centrifuge.
The separation process here relies on the mass difference of the molecules see gaseous diffusion above. Here, uranium hexafluoride is fed into an evacuated cylinder containing a rotor.
When these rotors are spun at a high speed , the heavier UF 6 collects near the walls of the cylinder while the slightly lighter UF 6 collects near the central axis. 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.
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