Enrichment of uranium is the process by which the percentage of fissile uranium in a sample naturally obtained is increased using several artificial processes. Uranium obtained from mining contains several isotopes of uranium in different compositions, such as U-238 (~99%), U-235 (~0.7%) and U-234 (~0.02%). [1] Out of these, Uranium 235 is the only one that is fissile, i.e. can be used in a nuclear reactor to produce heat (and consequently electricity) in a controlled manner. As such, the concentration of U-235 as a fuel used in a reactor needs to be increased, which is done using several processes, the main ones being described in this paper. This leads to the production of several grades of uranium fuel as well, which is then used for different purposes.
Uranium when mined is in the form of a stable oxide (U3O8) or a peroxide. After initial purification to remove impurities, which is done by heating strongly and subsequent agglomeration and crushing, the purified uranium oxide is then reduced in a kiln by hydrogen to form uranium dioxide by the following reaction. [2]
The reduced oxide of uranium is then reacted with hydrogen fluoride to form uranium tetrafluoride which is further reacted with gaseous fluorine to produce uranium hexafluoride which is then used as a starting input material in an enrichment plant. The reactions by which the above mentioned conversion takes place are given below.
Gaseous diffusion: Uranium hexafluoride is first heated and converted from a solid to a gas. It is then made to pass through a series of semi-permeable membranes which are a part of compressors and converters. Since the U-235 isotope molecules are lighter than the U-238 isotope molecules, they tend to diffuse faster through the pores of the membranes and hence can be separated. The two streams of gas produced have different U-235 concentrations and hence are separated. Several repetitions of this process are required to obtain concentrations of U-235 isotope in the gas are high enough to the extent that it can be used in a nuclear reactor. [3] The enriched UF6 gas is then allowed to liquefy and subsequently solidify before being transported to a nuclear power plant. This technology contributes to about 27% of the world's enriched uranium. [3]
Gas centrifuge: This method of uranium enrichment also utilizes the difference in mass of the uranium isotopes are an aid in their separation. A gas centrifuge system consists of several cylinders which rotate at high speeds that create a strong centrifugal force within. The U-238 molecules being heavier tend to line the outer wall of the cylinder while the lighter U-235 molecules stay more towards the center. The two streams are separated and the depleted stream is pumped back for further U-235 extraction. This method of enrichment proves to be more energy efficient as compared to the diffusion method by a factor of 50:1 and hence is currently used to produce 50% of the world enriched uranium. [3]
Laser separation: The use of laser separation for uranium enrichment is based on the principle of differential photoexicitation of isotopes of uranium by the use of monochromatic radiation. One such process is the Atomic Vapor Laser Isotope Separation (AVLIS) in which the laser light used photo ionizes a particular isotope while not affecting the others and changes its physical or chemical properties which are then utilized to separate the desired isotope, which in uranium enrichment is U-235. In doing so, the U-235 ions are positively charged and hence are attracted to a negatively charged plate and collected.
Different grades of uranium can be produced through enrichment which then has different applications. These are broadly classified under the following categories.
Slightly Enriched Uranium (SEU): This has a concentration of 0.9% to 2% of U-235 isotope and is most commonly used as a substitute to natural uranium in heavy water reactors. One benefit of using this instead of natural uranium is that lesser quantity of fuel is required to produce the same amount of energy and as a result, the amount of nuclear waste to manage at the end of the production is significantly reduced.
Low Enriched Uranium (LEU): This has a concentration lower than 20% of U-235 isotope. It is commonly used in light water reactors and research reactors.
Highly Enriched Uranium (HEU): This has a concentration higher than 20% of U-235 isotope. When the concentration is over 80%, it is termed as weapons grade uranium which is necessary for a nuclear chain reaction to occur. Highly Enriched Uranium with concentrations greater than 40% is used in naval reactors in powering submarines and in fast neutron reactors. [4]
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[1] A. Lordello, A. Abrao and R. Gomes, "Spectrochemical Procedures for Analytical Control of Eighteen General Impurities and Fourteen Rare Earth Elements in UO2 Pellets and Other Uranium-Base Materials," J. Nucl. Mat. 81, 241 (1979).
[2] M.-C. Lee and C.-J. Wu, "Conversion of UF6 to UO2: A Quasi-Optimization of the Ammonium Uranyl Carbonate Process." J. Nucl. Mat. 185, 190 (1991).
[3] J. H. Hill and J. W. Parks, "Uranium Enrichment in the United States," U.S. Energy Research and Development Administration, CONF-750324-7, 5 Mar 75.
[4] S. Nazare, "Low Enrichment Dispersion Fuels for Research and Test Reactors," J. Nucl. Mat. 124, 14 (1984).