Photo Credit: Lawrence Livermore National Laboratory
Introduction to Laser Enrichment of Isotopes
Laser enrichment began in the mid-60’s for the purpose of separating isotopes of elements by the process of selective ionization by lasers. This process was designed to produce higher concentrations of specific desired isotopes of a chemical element by removing them from other isotopes that are not of use. The most popular and developed work has been performed on natural uranium. This process is a large contributor to the nuclear power industry and weapon development. Using the unique frequencies which atoms vibrate in gaseous form, a laser tuned to the vibrational frequency of a U-235 atom can cause the isotope to behave differently from the heavier U-238 atom to allow harvesting. However, technical difficulties have impeded translation from the laboratory to the commercial or weapons settings despite the efforts of more than a dozen countries since the 1970s.
Discovery
Laser enrichment was based around the early work coming from the 1970’s like MLIS (molecular laser isotope separation) and AVLIS (atomic vapor laser isotope separation). These two early discoveries in laser enrichment were both transferred into the United States Enrichment Corporation. The latter of the two earlier developments used tunable dye lasers which were able to make 235-Uranium absorb the photons and undergo excitation. The ions were then electostatically deflected into a collector while the unwanted was passed through. [ref 1].
Different Elements
- 1.Uranium- Laser enrichment of uranium is the most common application of laser enrichment and will discussed in detail below. This process separates isotopes for use nuclear fuel and energy production, and is much more useful and space efficient than older methods of uranium enrichment.
- 2.Carbon- Laser enriched carbon has applications in development of semiconductor material and biomedicine. The enriched Carbon-12 is of use in semiconductors, and the bi-product of this enrichment, Carbon-13 already has known uses in the biomedical field.
- 3.Silicon- Laser enrichment of silicon can be used for creating advanced semiconductor material. Creating isotopically pure silicon may be of use in devices with semiconductors, which include all computers and electronic devices. These devices that use silicon in its current form are reaching performance limits, and may be able to benefit from enrichment of silicon, though there is little demand for this and no economically sound source has been developed.[ref 2]
Success of LANL of laser enrichment of different elements can be seen in the table below. The different types of Dissociation methods will be described below in the Analysis section.[ref 3]
Commercial Energy
General Electric (GE) currently plans to use the Australian laser enrichment technology (SILEX) to enrich natural UF6 gas in the uranium-235 isotope. GE is planning to conduct the project in two phases. The first, a test phase while the latter being a commercial-scale enrichment plant phase. The Test Loop, which is being built at GE's nuclear fuel fabrication facility in Wilmington, North Carolina, USA, will verify performance and reliability data for full scale (commercial-like) facilities [ref 4]. This change in energy source could prove to be a much cheaper way of energy production that would allow for the lowering of cost per unit of power.
Nuclear Weapons
Fuel for nuclear reactors does not come out of the ground ready to used. Rather, fuel suppliers must process natural uranium to extract small amounts of the rare fissile U-235 isotope to produce the fuel pellets that reactors use to generate power. Typically a power reactor will utilize fuel enriched with 3% U-235, a nuclear weapon 80-90%.
Over the decades efforts continued to develop more economical enrichment techniques for both civil and military purposes. Much research centered on laser enrichment.
The Nuclear Regulatory Commission staff contends that its current licensing suffices to deal with security questions, but APS responds that "nonproliferation is not given an adequate level of attention." It gets support from NRC chairman Gregory Jaczko who conceded in a July 12, 2010 speech that "the smaller footprint and lower energy needs of the laser enrichment technology have been the cause of concern."
While SILEX may still fail as a commercial venture, we must prepare ourselves for success and the renewed interest in laser enrichment it will stimulate globally. With construction of more of the SILEX plant looming, it is none to soon for the Agency to consider interfacing with NRC and GE-Hitachi to work out an arrangement to establish a new safeguard precedent. Clearly marginally tethered international laser development is something we must avoid to prevent yet more nuclear weapons states in the future.[ref 5].
Analysis of SILEX Process
The Separation of Isotopes by Laser Excitation, SILEX, process exposes a cold stream of a mixture of uranium hexafluoride (UF6) molecules and a carrier gas to energy from a pulsed laser. The laser used is a CO2 laser operating at a wavelength of 10.8 μm (micrometres) and optically amplified to 16 μm, which is in the infrared spectrum. The amplification is achieved in a Raman conversion cell, a large vessel filled with high-pressure para-hydrogen.
The 16 μm wavelength laser preferentially excites the 235UF6, creating a difference in the isotope ratios in a product stream, which is enriched in 235U, and a tailings stream, which has an increased fraction of the more common 238U. In effect, the laser is tuned to electrically charge the U235 atoms, which can become trapped in an electric field and drawn to a metal plate for collection.
According to John L. Lyman, the Silex Systems Ltd. (SSL) research facility in Australia uses a laser pulsed at a frequency of 50 Hz, a rate that results in great inefficiency. At 50 Hz, only 1% of the UF6 feedstock is processed. This results in a high fraction of feedstock entering the product stream and a low observed enrichment rates. Consequently, a working enrichment plant would have to substantially increase the laser duty cycle. In addition, the preparation time needed is prohibitively long for full-scale production. The SSL research facility requires ten hours of prep time for a one-hour enrichment test run, significantly restricting output.[11]
How Laser Enrichment Works
SILEX uses laser radiation to break bonds and ionize elements to separate isotopes by means of selective ionization. For natural uranium in particular, the laser breaks one of the six Florine bonds in UF6 utilizing photo-dissociation to create UF5+ which contains the U-235. Photo-dissociation is a complex process and will be explained below in its own section, but all in all it uses photon interactions with the chemical bonds to break the bonds themselves. The lasers are specifically tuned to ionize U-235, and not U-238.
With the UF5+ which contains U-235 having a positive charge, the molecules can be separated from the UF6 which contains the U-238. The U-235 ions are attracted to and collected on a negatively charged plate. This process can produce samples of nitrogen that are 5% U-235, versus natural uranium which is only 0.7% U-235.[ref 6] Since the energy of a photon is given by the equation:
this shows that the energy is inversely proportional to the wavelength of the photon. Different isotopes have different electronic energies. The equation above shows that energy is a function of wavelength, meaning isotopes of different energies will respond to different color lasers, that have different wavelengths. [ref 8]
Photo-dissociation
Photo excitation of atoms is nothing new. Stanislaw Mrozowski suggested that mercury isotopes might be separated by selective excitation with the 253.7-nanometer resonance line of a mercury arc lamp and subsequent reaction with oxygen. This separation was achieved experimentally by Kurt Zuber in 1935 [ref 3]. In the early 1940s Harold Urey proposed a photochemical method for separating Uranium isotopes but he lost out to the gaseous diffusion technique. After World War II Carbon and oxygen isotopes were also separated by using a strong spectral line of an iodine lamp to excite carbon monoxide molecules. "These pre-laser experiments involved a one step process in which absorbed photons with frequencies in the visible or ultraviolet spectral region to selectively excite electronic states of one isotopic species"[ref 3]
They were however limited by most molecules having very broad structureless electronic absorptions bands thus making selective excitation by this method impossible and by the intensity of the radiation sources available. Photo-chemical isotope separation requires highly monochromatic, highly intense radiation. High-intensity tunable laser have removed many of the limitations of the early experiments. These laser can be tuned to match any absorption features that show a distinct isotope shift, and because of its high monochromaticity laser light can excite a desired species with reasonable selectivity even when absorption feature of other isotopic species partially overlap those of the desired isotopic species. A high-intensity laser can also saturate the absorbing material as well.
The best part though is that the laser pulses are short compared with the average time for the selectively excited molecules to lose their energies. Short pulses are needed if the excitation process is to be isotopically selective and efficient in its use of laser photons.
They were however limited by most molecules having very broad structureless electronic absorptions bands thus making selective excitation by this method impossible and by the intensity of the radiation sources available. Photo-chemical isotope separation requires highly monochromatic, highly intense radiation. High-intensity tunable laser have removed many of the limitations of the early experiments. These laser can be tuned to match any absorption features that show a distinct isotope shift, and because of its high monochromaticity laser light can excite a desired species with reasonable selectivity even when absorption feature of other isotopic species partially overlap those of the desired isotopic species. A high-intensity laser can also saturate the absorbing material as well.
The best part though is that the laser pulses are short compared with the average time for the selectively excited molecules to lose their energies. Short pulses are needed if the excitation process is to be isotopically selective and efficient in its use of laser photons.
There are three methods of Photo-dissociation that have been used successfully. A single photon process where a visible or an ultraviolet photon excites a molecule to a "predissociative state". A two step process, in which an infrared photon excites a vibrational state of a molecule and an ultraviolet photon dissociates the excited molecule, and a multistep infrared process in which infrared photons excite successively higher and higher vibrational states until the molecule dissociation limit is reached.
Every one of these processes take advantage that in a vibrational state the nuclei of a molecule undergo oscillatory motion about the ground state configuration at some frequency. This frequency depends on the masses of the nuclei thus the vibrational excitation of a molecule containing a lighter isotope requires absorption of a photon at a higher frequency. This mass dependent shift in the absorption spectrum is exploited to dissociate molecules of one isotopic species selectively to achieve isotope separation.
Every one of these processes take advantage that in a vibrational state the nuclei of a molecule undergo oscillatory motion about the ground state configuration at some frequency. This frequency depends on the masses of the nuclei thus the vibrational excitation of a molecule containing a lighter isotope requires absorption of a photon at a higher frequency. This mass dependent shift in the absorption spectrum is exploited to dissociate molecules of one isotopic species selectively to achieve isotope separation.
A single photon excitation relies on predissociation in which a photon induced transition from a bound ground electronic state to an electronic state for which the internuclear forces are always repulsive. The lifetime of such a repulsive state is so short that dissociation follows the transition to the excited state is almost unity probability.
Los Almos National laboratory has been experimenting using selective photo-dissociation of molecules. Molecules can be excited to dissociate in many different ways, and this is the two step process described above. The two main steps that Los Almos was as follows. The first one was the use of an infrared laser that selectively excites the vibrations of gaseous UF6 that contains the molecule of U-235. As you can see below in part (a) of the figure there is a difference in the energy to excite the vibrational modes of one isotope to another (the solid line is one isotope and the dash line is another isotope.
The arrows in part (a) show the absorption of infrared photons that raise a molecule from the ground state to the first vibrational state. The difference in the lengths of the arrows show the different photon energies, or frequencies needed to excite the two isotopic species but the difference is quite small (less than 1.25X10^-4 eV).
Monochromatic lasers can achieve selective excitation process though. Part (b) of the diagram shows one of the vibrational transitions in (a) that is split into many rotational states labeled by J (number of rotational angular momentum quanta of the states) At room temperatures molecules populate rotational states with high J values which causes a problem for the monochromatic laser as the laser is tune to excite the ground state to the first vibrational state. If the UF6 are at rotational states above the ground state they will not be excited to the vibrational state and thus you will be unable to dissociate these unexcited molecules.
Part (b) also shows that during a transition between vibrational states the change in J is restricted to -1, 0, +1 and are denoted as P-, Q-, and R- Branch transitions respectively. You can see in part (c) of the figure above the infrared absorption bands of 235-UF6 and 238-UF6 from 620 to 630 cm^-1 including transition from the ground state to the first excited state of the V3 vibrational mode. The absorptions occur over a broad band of frequencies because molecules in the ground state occupy many rotational states (J) and the molecules in each rotational state can undergo P-, Q-, or R- branch transitions to the first excited vibrational state. As you can see in part (c) the absorption band of 235-UF6 is shifted to slightly higher frequencies relative to that of 238-UF6
An ultraviolet laser is then shot onto the vibrational excited UF6 to dissociate the molecule into UF5 plus a fluorine atom. which cause it to reach the repulsive potential dissocation limted as show in the figure below
As you can see above the dissociation probability as a function of ultraviolet photon frequency of both the excited and unexcited states. Without infrared excitation the dissociation cross-section for different isotopic species are nearly the same. Infrared excitation increase the photo-dissociation cross section at a given frequency and shifts the threshold for dissociation to lower frequencies. This allows you to choose an ultraviolet laser frequency at which the dissociation cross section is large for excited molecules and small for unexcited molecules.
In multiple-photon dissociation they try to make the most of the fact that the threshold frequency for ultraviolet dissociation shifts more and more to the color red as the infrared laser fluence (flux integrated over time) increases. The figure blow shows the ultraviolet dissociation cross section spectrum for CF3I.
These shifts clearly show that the laser is exciting the molecules to very high vibrational states and if molecular vibrations were governed by forces that increased linearly with displacement as if they were a harmonic oscillator the energy difference between vibrational states would be constant and photons with this constant energy could resonantly induce transitions to higher and higher vibrational states.
Improvements Over Past Techniques
To be of use in nuclear power uranium must be enriched to Uranium-235. The first enrichment process used was gaseous diffusion of uranium, which involves forcing gaseous uranium through porous membranes which essentially filters through U-235 because it is lighter and diffuses faster than U-238. In each successive chamber the concentration of U-235 to U-238 is slightly higher, but more than a thousand chambers are needed to increase the concentration of U-235 to 3.2% which is required for light water reactors. The next process to be developed, which was more cost effective and is now the main commercial process currently in use, involves the centrifuging of uranium. Gaseous uranium is placed in a centrifuge and the heavier isotope U-238 moves to the outside of the centrifuge and U-235 remains in the center. This process is repeated up to 20 times, which is much less than the 1000 stages used in the diffusion process.[ref 9] Using laser excitation, uranium can be enriched to 5% U-235 after only a few stages of the process, but the centrifugal process would require thousands of stages to achieve these results.
Public View and Concerns
The main worry in developing this technology is that laser enrichment is so space and energy effective, so that those using this technology could go undetected by nuclear inspectors. Experts from the Council on Foreign Relations worry that these facilities could be “hidden in a warehouse,” [Ref 10] as the SILEX process is 75% smaller than current techniques. New facilities would not be able to be easily detected by current observation satellites. [ref 11]
It is important to note that the risk of proliferation is always present with the emergence of new nuclear techniques, although precautions are of course taken by the US government and the UN to prevent the spread of new technologies.
Summary
Laser enrichments purpose of separating isotopes of elements by the process to produce higher concentrations of specific desired isotopes of a chemical element. This process has the potential to be large contributor to the nuclear power industry and weapon development. Using the unique frequencies which atoms vibrate, a laser tuned to the vibrational frequency of an atom allows for harvesting. Because of technical difficulties translation from the laboratory to the commercial or weapons settings despite the efforts of more than a dozen countries since the 1970s, have impeded further advancements.
References
- ↑ http://www.imp.kiae.ru/_eng/tehn/tehn_txt.htmf
- ↑ http://www.silex.com.au/s03_about_silex/s30_1_content.html
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 http://library.lanl.gov/cgi-bin/getfile?04-01.pdf
- ↑ http://www.nrc.gov/materials/fuel-cycle-fac/laser.html
- ↑ http://www.huffingtonpost.com/bennett-ramberg-phd/new-peaceful-nuclear-tech_b_830937.html
- ↑ https://www.llnl.gov/str/Hargrove.html
- ↑ http://www.peacetasmania.org/weapons_disarm/U_enrichment/NPT_April05_SILEX_Newman4.jpg
- ↑ https://www.llnl.gov/str/Hargrove.html
- ↑ http://www.world-nuclear.org/info/inf28.html
- ↑ http://world-nuclear.org/info/inf28.html
- ↑ http://www.abc.net.au/news/stories/2010/04/13/2870904.htm
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