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| Fig. 1: A laser separation experiment conducted at Lawrence Livermore National Labs (Source: Wikimedia Commons) |
All modern nuclear power, whether its fusion or fission, for energy or for weapons, depends on the separation of isotopes. One of the primary limitation in the production of fissile U-235 as well as the isolation of lithium-6 capable of generating tritium for fusion reactions has been the difficulty of isotope separation. [1,2] Traditionally, isotope separation has been conducted mainly through centrifugal separation, a process that is difficult and costly. With laser separation technology like the phone shown in Fig. 1, the cost of isotope separation has been drastically reduced. [3]
Nuclear Isotopes describe atoms which contain the same amount of protons but different amounts of neutrons. While many of these isotopes have very similar chemical and physical properties, they often have special nuclear properties that can be exploited for nuclear technologies. Depending on the isotopic stability of a particular nuclear isotope in nature, they are present in differing amounts in naturally occurring deposits of bulk material on earth. For example, U-235, which is the primary fissile material used in some nuclear reactors as well as nuclear weapons, composes 0.74% of natural uranium deposits on earth. However, in order to exploit the nuclear properties of U-235 as a fissile material - a material capable of sustaining an exothermic nuclear fission chain reaction - they must be isolated from their U-238 isotopes. [4]
The earliest desire for isotope separation at a large scale came from the development of nuclear sciences prior to WWII. The first technology used to separation nuclear isotopes was the centrifugal isotope separator developed in 1934. The technique spins gaseous material in rapidly spinning centrifuges in order to concentrate slightly heavier isotopes at the outer edges of the centrifuge. The resulting enriched uranium stream is then pumped to the next stage of centrifuge in order to be further enriched. [5]
Another method used to separate isotopes is the gaseous diffusion separation method. This method relies on the differing diffusion speeds of gasses with different masses. By forcing gasses of the isotope material through membranes, this technique can progressively separate the lighter isotopes from the heavier isotopes. This method was ultimately used in order to produce the fissile material used in the atomic weapons of WWII. [6]
The premise of Laser Isotope Separation comes from the differing hyperfine structures of isotopes. The different isotopes contain differing number of neutrons which influences the nuclear magnetic dipole moment and, in turn, the hyperfine structure. These differences in the absorption spectrum of the isotopes means that a precisely tuned laser can be used in order to only excite one specific isotope and not the other isotope. [3]
In atomic vapor laser isotope separation, the target material is first vaporized into a gaseous vapor phase, then a laser is applied to selectively excite the target vapor material. The excited vapor material then undergoes photoionization, which results in a positively charged stream which can be collected through a electromagnetic field. This technique is shown in Fig. 2. [7]
Another method of isotope separation uses the differing molecular absorbance of molecular compounds containing the isotope material. Instead of vaporizing the bulk elemental material, this method relies on gas phased molecular compounds containing the target isotope. For example, in Uranium molecular isotope separation, 235UF6 gas is selectively ionized by the laser into UF5+ molecules which is separated from depleted stream of 238UF6. [8]
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| Fig. 2: A diagram showing atomic laser isotope separation. [10] (Courtesy of the NRC) |
While laser isotope separation has been around for several decades, the economic cost of this technology was too high compared to centrifugation as well as the membrane method used for the production of fissile material. However, recent advances in this technology such as the SILEX process has drastically decreased the energy as well as material costs for last isotope separation. This reduction in cost has several key benefits. [9]
The lowered cost of separating nuclear isotopes impacts energy, medicine, as well as nuclear proliferation. In terms of producing fissile materials capable of being used in nuclear power plants, the cheaper production in price will lower the cost of atomic energy. The ease of separating nuclear isotopes can also reduce the cost of acquiring radioisotopes for imaging purposes in medicine. However, some groups have warned that laser isotope separation technology might increase the difficulties of policing nuclear non-proliferation. [9]
While traditional isotope separation processes required processing facilities with huge scales due to the nature of their separation mechanism, the newer, more efficient laser isotope separation technology could drastically reduce the energy and physical footprint for a enrichment facility. In a traditional centrifugal isotope separation process, up to a thousand stages of centrifugation might be required in oder to attain a weapons grade enrichment. Similarly, the diffusion process also can take up to a thousand stages to get the enrichment factor up to weapons requirement. The cheaper and more efficient laser isotope separation technology can allow nation states to more easily hide their clandestine enrichment operations from the global community.
Laser isotope separation is a different method of isotopes separation from the traditional centrifugal or diffusion method that can generate enriched fissile material. Recent technological advancements have made them economically competitive with traditional separation methods. The development and further efficiencies brought on by laser isotope separation can create cheaper sources of radioisotopes for nuclear energy and medicine. However, this technology, if fallen into the wrong hands, might also increase the risk of nuclear proliferation.
© Peter Wang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] G. Janes et al. "Two-Photon Laser Isotope Separation of Atomic Uranium: Spectroscopic Studies, Excited-State Lifetimes, and Photoionization Cross Sections," IEEE J. Quantum Elect. 12, 111 (1976).
[2] E. A. Symons, "Lithium Isotope Separation: A Review of Possible Techniques," Separ. Sci. Technol. 20, 633 (1985).
[3] P. T. Greenland, "Laser Isotope Separation," Contemp. Phys. 31, 405 (1990).
[4] N. Bohr and J. A. Wheeler, "The Mechanism of Nuclear Fission," Phys. Rev. 56, 426 (1939).
[5] R. S. Kemp, "Gas Centrifuge Theory and Development: A Review of U.S. Programs," Sci. Global Sec. 17, 1 (2009).
[6] R. C. Jones and W. H. Furry, "The Separation of Isotopes by Thermal Diffusion," Rev. Mod. Phys. 18, 151 (1946).
[7] J. A. Paisner, "Atomic Vapor Laser Isotope Separation," in Laser Technology in Chemistry, ed. by H. Medin and S. Svanberg (Springer, 1988) pp. 253-260.
[8] P. Parvin, "Molecular Laser Isotope Separation Versus Atomic Vapor Laser Isotope Separation," Prog. Nucl. Energy 44, 331 (2004).
[9] W. Fuss, "Laser Isotope Separation and Proliferation Risks," Max-Planck-Institut für Quantenoptik, MPQ 346, February 2015.
[10] "Laser Enrichment Methods (AVLIS and MLIS)," U.S. Nuclear Regulatory Commission, September 2009.
