The photochemistry of uranium hexafluoride as applied to laser isotope separation

Abstract

The subject of this dissertation is the very relevant field of laser instigated photochemistry. Its application to the industrially important process of uranium separation is carefully studied and reported. The isotope separation is accomplished by selectively vibrationally exciting the minor isotope, 235U, with an infrared laser. Subsequently, adequate laser energy of another infrared laser/s is coupled to the UF 6 molecule which causes bond rupture. The photochemistry of this laser based process will determine its economical viability. At the onset of this study very scant knowledge about the photochemistry of the uranium hexafluoride molecule was available in the scientific literature and important phenomena were unexplained. Many of these were clarified and explained during the course of this investigation. Furthermore, many previously assumed and accepted aspects concerning the photochemistry of uranium hexafluoride were proven to be incorrect or unsubstantiated. Firstly, the laser instigated unimolecular dissociation rate of UF6 was calibrated with experimental data. The RRKM statistical theory was employed to correctly describe the unimolecular dissociation process. Excellent agreement between theory predictions and experimental thermal dissociation data was obtained. This verifies the chosen geometrical configuration of the activated complex as proper and reasonable. A simple bond rupture reaction, where the activated complex is represented by a weak bonding of the UF 5 moiety to a fluorine atom, seems appropriate. In establishing the features of the activated complex, the vibrational frequencies of the UF5 molecule were allocated to the transition configuration together with two hindered rotations. The U-F bond that is ruptured, is stretched to three times the equilibrium bond length in the activated complex. The well known Whitten Rabinovitch approximation to determine the density of the vibrational states was employed but a less cumbersome formulation for UF 6 was subsequently derived. This methodology was for the first time applied to UF6 with very good results. Previous two efforts, reported in the scientific literature, were unsuccessful in the sense that agreement between theory and experimental observations was poor. This treatise shows that the unimolecular dissociation rate for UF6 is at least two orders faster than previously advocated. This impacts very positively on the entire separation process. The influence of molecular collisions and the quenching effect this has on the unimolecular dissociation rate must normally be taken cognisance off. In chapter III the transfer of vibrational energy between UF 6 molecules is studied with the aid of a pump and probe experiment. When a hydrogen containing molecule like CH4 is present in the gas mixture, strong HF fluorescence is detected when the F radical reacts with the former. This can be utilized to measure the transfer rate of vibrational energy. It was established that very fast transfer occurs when CH4 is present and also the transfer mechanism between UF 6 molecules was identified to be of a long range nature. It is proposed that a dipole-dipole mechanism for vibrational energy exchange is operative. This seems to be confirmed by the inverse temperature dependence with the rate measured. This is in contrast to the hard sphere gas kinetic theory which predicts a direct dependence with temperature. In the final instance, however, the measured transfer rates are still much slower than the unimolecular dissociation rates of UF 6 determined in chapter III and its contribution can consequently be neglected at the envisaged gas density. Much effort was expended during this study to experimentally measure the unimolecular dissociation rate of UF 6, after laser excitation, inside a molecular beam apparatus. The measurement technique that is suitable for such an application is the mass spectrometry. However, the requirements to measure in the corrosive environment of UF6 as well as the very fast time resolution needed, posed terrific challenges. Various ionization techniques to ionize UF6 and its photo product were developed and tested. A commercial quadrupole mass spectrometer as well as a locally built time-of-flight mass spectrometer was utilized for the detection of ion fragments. It was established that the formation of negative molecular UF 6 ions in a pulsed mode represents the best solution to the requirements. Nevertheless, the actual rates could not be measured due to the demise of the MLIS project. The success of accurately modelling the thermal unimolecular dissociation of UF 6 , to a large degree compensated for this disappointment. In the last chapter, the secondary reactions following on the unimolecular dissociation of UF 6, are reported and scrutinized. This section represents a major overhaul of the established picture that prevailed up to this study. At least nineteen different conclusions, stemming from experimental observations, are put forward. Each of these differs profoundly from previously held views on the secondary reactions following laser photolysis of UF 6 • Amongst the more prominent findings are: 1. The presence of the uranium compound, UF 4, in the laser photolysis products of UF 6 has unambiguously been established. This is true for both infrared and ultraviolet laser photolysis. 2. The internal energy content of nascent photolysis products is extremely important to determine the secondary reactions. Equally important is the rate of internal energy loss in various environments to promote or inhibit reactions. 3. There is a great danger in equating the results of static gas cell irradiations, i.e., repeated laser shots, with the photolysis results in dynamic systems. This has previously led to incorrect interpretations.