The traditional method of recovering uranium and plutonium from process streams is to dissolve actinide-containing residues and wastes in strong acidic media and put them through a series of separation processes such as precipitation, filtration, and ion exchange. Over the half century of nuclear materials processing activities, this method has produced voluminous nuclear wastes stored at various nuclear sites. The search for safe disposal methods continues to this day. From a processing efficiency standpoint, a better way of separating actinides from their major constituents is to act upon only the minor components without dissolving the entire matrices. It would be ideal if a gaseous reactant could be flowed through the residue matrices, and only the volatile actinides recovered as gaseous products, leaving behind the actinide-depleted residues. This article describes identification and analysis of an exotic chemical that holds a great promise in actinide recovery, decontamination of process equipment, and waste reduction in actinide processing.
In the early 1980s the Los Alamos laser isotope separation program for plutonium had completed the scientific concept demonstration and moved toward a small-scale process demonstration. This program was called molecular laser isotope separation (MLIS) and code-named Lippizan. Before this we had an uranium isotope separation program code-named Jumper. Our sister laboratory at Livermore had two similar programs; their process was called atomic vapor laser isotope separation.
An MLIS process uses stable molecules that bear the isotope of interest in their molecular structures. Both uranium and plutonium form stable gaseous molecules called hexafluorides. Six fluorine atoms occupy the six apexes in an octahedral geometry with a uranium or plutonium atom in the center. This particular soccer-ball-like geometry accounts for the chemical stability as a gaseous molecule. Most of the world's supply of separated uranium isotope 235U comes largely from a gaseous diffusion process employing uranium hexafluoride as the working medium. Plutonium hexafluoride is the only known stable gaseous molecule at room temperature, so it is not surprising that several isotope separation schemes were devised in the early years using this molecule.
Once produced, plutonium hexafluoride is fairly stable under process conditions such as circulation, compression, and expansion, inside the process pipes. However, it is not so easy to make plutonium hexafluoride under mild chemical conditions. This problem is further complicated when the MLIS end product is solid particles, and plutonium hexafluoride also decomposes slowly to a solid product that coats the inside wall of the process pipes as the process continues. So we needed a way or ways to recover the solid product from the process line. The only way to accomplish this recovery was then to turn the solid product back to gaseous product in situ. We were searching for chemicals to accomplish the product recovery and cleanup of the process line.
By the early to mid 1980s we had identified a few chemicals that could be used for the purpose. Krypton difluoride (KrF2), dioxygen difluoride (FOOF), and the fluorine atom were the most promising among all we investigated. Krypton and oxygen in these molecules are merely carriers of F atoms. In the case of FOOF we discovered subsequently that the active species involved in the actual fluorination reaction was a radical species-O2F-derivable from FOOF. This story is about the discovery of O2F and characterization of some of its properties. A radical has unpaired electrons unlike a stable molecular electronic configuration so that it is usually very reactive with neighboring species, and it does not hang around long (less than several seconds at most) for us to detect readily in its isolated state.
The structure of FOOF had been studied previously by microwave spectroscopy by other researchers. This compound is also very unstable. It can be stored indefinitely only at very low temperature, such as that of liquid nitrogen. The infrared spectrum of solid (frozen) FOOF at 77 K had been recorded, and all fundamental vibrations assigned also. But the spectral quality was very poor because these measurements had been made on a minute quantity of impure samples. A serious obstacle to the study of FOOF in the gas phase had been the difficulty in obtaining sufficient amounts of the pure compound and preserving it for a sufficient length of time for spectroscopic investigation. In late 1984 we recorded for the first time a complete infrared spectrum of gaseous FOOF obtained under well-defined conditions using a special spectrometer that we had developed at Los Alamos. For the first time we were able to make accurate vibrational frequency assignments and determine their intensities.
Figure 1. The first complete infrared spectrum of gaseous FOOF. The laser produced O2F spectrum is shown in the inset.
Immediately after recording the first complete spectrum of FOOF, we noticed an unexplainable spectral feature centered at 1490 cm-1 (See Figure 1.) This spectral feature did not vary in its relative intensity with the rest of the peaks. Furthermore, it is the only feature in the entire fundamental infrared region apart from those of FOOF. The molecular vibrational theory tells us that whatever species is responsible for this feature must be a compound simpler than FOOF or a heteronuclear, diatomic molecule. We did not consider the idea of a radical because of the attributes of radicals discussed above. The half-life of a radical species is typically so short that one cannot record its spectrum over many minutes as required for a spectral scan. And yet this unexplained feature persisted in all samples of FOOF. We also eliminated early on the possibility that it might be the diatomic radical OF because molecules such as FOOF do not break apart at their stronger O-O bonds spontaneously under these experimental conditions.
Within the first week of successfully recording the FOOF spectrum, we had to speculate that we might have an unusual radical species-O2F-coexisting with FOOF in our sample. How then could we prove our hypothesis that a reactive radical species like O2F coexists with another unstable molecule-FOOF? To answer this question unambiguously we devised an experiment to produce O2F directly and record its spectrum in the absence of FOOF. Now, this experimental design is based on the assumption that the half-life of O2F is sufficiently long (for example, seconds) so that we could probe continually while we produced this short-lived species in an experimental design, described below.
The experimental design thus conceived is laser flash photolysis combined with coaxial probing. A strong laser beam produces F atoms along its beam path, the F atoms react with an oxygen molecule to form the transient radical O2F, and its presence is detected simultaneously by probing the sample volume within the laser beam path. Although easily described, this arrangement requires that the laser beam and the probe beam should be made co-linear, and the probe beam should be inside the laser beam volume for maximum sensitivity. We would make a mixture of two basic ingredients, fluorine and oxygen, which we considered as the starting materials. An intense ultraviolet laser beam at 248 nm was injected into the sample chamber that contained the mixture. The laser beam was pulsed at less than 10 Hz. The probe infrared beam from the spectrometer was combined using a dichroic mirror and made co-linear along the entire beam path. Our hope was that as the laser beam produced the expected radical species at some low-frequency interval, a steady-state concentration of O2F would be established between the production and disappearance, and the probe infrared beam would be monitoring this steady-state concentration continually. (A dichroic optical material reflects and transmits different wavelength lights so that two beams of different wavelengths can be combined coaxially and propagated together along the same beam path.)
Success was immediate. The spectrum of O2F was recorded without the interference of FOOF and analyzed. With this discovery we were able to study the properties of O2F and its chemical relationship with FOOF, another Los Alamos first in laser photochemistry. It was one of the most wonderful detective stories of an exotic chemical species and it helped to meet some of the important isotope separation program goals. The discovery of O2F in FOOF found other applications in later years for recovery of plutonium from plutonium-containing residues and waste. In the future it may be a preferable alternative to acid dissolution of radioactive waste and can be used to recover plutonium from glove box systems and process lines in reactor vessels.
This article was contributed by Kyu C. Kim, NMT Chief Scientist.
This article was based on the author's work and papers published in the mid 1980's.
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