Uranium enrichment (стр. 2 из 2)
Figure 6: The electromagnetic separations plant built at Oak Ridge, Tennessee during the Manhattan Project. These devices, also referred to as cauldrons, were used in enriching a part of the uranium for the bomb that was dropped by the United States on Hiroshima.
The electromagnetic separations process is based on the fact that a charged particle moving in a magnetic field will follow a curved path with the radius of that path dependent on the mass of the particle. The heavier particles will follow a wider circle than lighter ones assuming they have the same charge and are traveling at the same speed. In the enrichment process, uranium tetrachloride is ionized into a uranium plasma (i.e. the solid Ucl4 is heated to form a gas and then bombarded with electrons to produce free atoms of uranium that have lost an electron and are thus positively charged). The uranium ions are then accelerated and passed through a strong magnetic field. After traveling along half of a circle (see Figure 6) the beam is split into a region nearer the outside wall which is depleted and a region nearer the inside wall which is enriched in U-235. The large amounts of energy required in maintaining the strong magnetic fields as well as the low recovery rates of the uranium feed material and slower more inconvenient facility operation make this an unlikely choice for large scale enrichment plants, particularly in light of the highly developed gas centrifuge designs that are employed today.
2.4 Jet Nozzle / Aerodynamic Separation
The final type of uranium enrichment process that has been used on a large scale is aerodynamic separation. This technology was developed first in Germany and employed by the apartheid South African government in a facility which was supposedly built to supply low enriched uranium to their commercial nuclear power plants as well as some quantity of highly enriched uranium for a research reactor. In reality, the enrichment plant also supplied an estimated 400 kg of uranium enriched to greater than 80% for military use. In early 1990, President de Klerk ordered the end of all military nuclear activities and the destruction of all existing bombs. This was completed roughly a year and a half later, just after South Africa joined the NPT regime and just before submitting to inspections and safeguards by the International Atomic Energy Agency.
The aerodynamic isotope separation (which includes the jet nozzle and helicon processes) achieves enrichment in a manner similar to that employed with gas centrifuges in the sense that gas is forced along a curved path which moves the heavier molecules containing U-238 towards the outer wall while the lighter molecules remain closer to the inside track. In the jet nozzle plants, uranium hexafluoride gas is pressurized with either helium or hydrogen gas in order to increase the velocity of the gas stream and the mixture is then sent through a large number of small circular pipes which separate the inner enriched stream from the outer depleted stream. This process is one of the least economical enrichment techniques of those that have been pursued, given the technical difficulties in manufacturing the separation nozzles and the large energy requirements to compress the UF6 and carrier gas mixture. As with gaseous diffusion plants, there is a large amount of heat generated during operation of an aerodynamic separations plant which requires large amounts of coolants such as Freon.
2.5 Other Technologies
There are a number of other uranium enrichment technologies such as atomic vapor laser isotope separation (AVLIS), molecular laser isotope separation (MLIS), chemical reaction by isotope selective laser activation (CRISLA), and chemical and ion exchange enrichment that have been developed as well, but they are mostly still in the experimental or demonstration stage and have not yet been used to enrich commercial or military quantities of uranium. The AVLIS, CRISLA, and MLIS processes make use of the slight difference in atomic properties of U-235 and U-238 to allow powerful lasers to preferentially excite or ionize one isotope over the other. AVLIS makes use of uranium metal as a feed material and electric fields to separate the positively charged U-235 ions from the neutral U-238 atoms. MLIS and CRISLA on the other hand use uranium hexafluoride mixed with other process gases as a feed material and use two different lasers to excite and then chemically alter the uranium hexafluoride molecules containing U-235, which can then be separated from those molecules containing U-238 that remained unaffected by the lasers. AVLIS was pursued for commercial use by the U.S. Enrichment Corporation, but was abandoned in the late 1990s as being unprofitable while other countries have also abandoned all known AVLIS and MLIS production programs as well. The chemical and ion exchange enrichment processes were developed by the French and the Japanese. These techniques make use of the very slight differences in the reaction chemistry of the U-235 and U-238 atoms. Through the use of appropriate solvents, the uranium can be separated into an enriched section (contained in one solvent stream) and a depleted stream (contained in a different solvent that does not mix with the first in the same way that oil and water do not mix). This enrichment technique was also pursued by Iraq. Currently all known programs involving this technique have been closed since at least the early 1990s. All of these technologies have been demonstrated on the small scale and some, like AVLIS, have gone further along in the development process that would be necessary to scale up to production level facilities. This would be particularly true if the profitability of the plant was not an issue and it was only meant to enrich the reasonably modest quantities of HEU necessary for one to two bombs per year. Currently, however, the gas centrifuge appears to be the primary technology of choice for both future commercial enrichment as well as for potential nuclear weapons proliferation.
Reference List
1.David Albright, Frans Berkhout and William Walker. “Plutonium and Highly Enriched Uranium 1996”. Stockholm, 1997.
2.Laughter, Mark D. (2007) “Profile of World Uranium Enrichment Programs – 2007”. ORNL/TM – 2007/193
3.David Albright “Irag’s Programs toMake Highly Enriched Uranium and Plutonium for Nuclear Weapons Prior to the Gulf War”, 2002
4.Nuclear Engineering International. 2004 World Nuclear Industry Handbook. Wilmington Pub. Co., 2004