1. Field of the Invention
The present invention relates generally to an isotopic separation and extraction process for uranium in particular but not exclusively.
2. Description of the Prior Art
An essential step in the processing cycle of uranium fuel is the enrichment of the U.sup.235 isotope content of the natural UF.sub.6 from a concentration of 0.7% U.sup.235 to a concentration of 2 to 4% U.sup.235 for the "conventional" light-water power reactor fuels, or to a concentration of about 90% U.sup.235 for the high-temperature gas cooled type power reactor. The enrichment step is not only important but it accounts for about 1/3 of the total fuel-cycle cost. Experiments in the Soviet Union and US have demonstrated that efficient isotope separation is possible using a laser method based on the selective 2-step photo-dissociation of molecules by the simultaneous application of infra-red and ultra-violet radiation (JETP letters 17, 63 (1973)) or multi-photon absorption from a laser source (Appl. Phys. Letters 27, 87 (1975)). Similar methods could be used for the selective photo-dissociation of UF.sub.6 isotopes, UF.sub.6 .fwdarw.UF.sub.5.sup.+ + F.sup.-. A laser isotope separation technique has also been disclosed in U.S. Pat. No. 3,443,087 isused to J. Robieux et al.
The selective two-step photo-dissociation takes place by first exciting one of the isotopic molecules from the vibrational ground state to an excited state, both molecules initially being in the electronic ground state. This vibrational-rotational transition is in the infrared region and is produced by a pulse from a laser tuned to this frequency. Immediately following this infrared pulse, an ultra-violet pulse excites the selected molecules to an unbound level in the electronic excited state and the combined two pulses produce the selective photo-dissociation. Another method of preferentially dissociating a molecular isotope species is by multiple photon absorption, which transports the molecule up the vibrational manifold to dissociation. Multiple photon absorption occurs through either of the following processes: (1) Sequential absorption of n photons by the molecule where the molecule passes through real intermediate quantum states (until dissociation occurs); (2) simultaneous absorption of n photons by the molecule where the molecule passes through virtual intermediate quantum states (until dissociation occurs). The number n&gt;1 is determined by the dissociation energy of the selected molecular isotope and the laser frequency. In these multiple photon processes, the isotopic selectivity is established by the much higher transition probability in the first few intermediate steps for laser frequencies tuned to match one isotopic species. The latter method eliminates the need for a high power ultraviolet radiation source, and is more efficient since the multiphoton absorption cross section is in general much larger than the single photon absorption cross section in the ultraviolet region.
A problem as important as isotope separation is that of effective extraction of the selected isotope ions from the molecular isotope mixture. The Robieux et al. patent teaches that the produced molecular ions may be separated somehow from the neutral molecules by use of the electric or magnetic fields. However, there is no specific disclosure of how to maximize the efficiency of the extraction process so as to permit the achievement of continuous and efficient collection of isotopes.