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Appendix B Alternative Fluorinating Agents Fluorinating agents other than molecular fluorine and hydrogen fluoride (HF) are identified and discussed briefly below. The challenge of volatilizing the uranium by fluorination in the Molten Salt Reactor Experiment (MSRE) system, particularly in places where heating is ineffective, may provide an opportunity to use one of these fluorinating agents. As a general rule, the short-lived fluorinating agents listed below would be more effective in a flow system rather than a closed system. The list below is not exhaustive; for example, xenon fluorides, because they are reasonably well known in fluoride chemistry work, are not included. . Chlorine trifluoride (CIF3) is a liquid that boils at ~ T.7°C and is a good oxidizer that converts uranium tetrafluoride (UF4) to uranium hexafluoride (UFO) even at room temperature. Its main drawback is that if oxygen-containing compounds are present, it may produce chlorine dioxide (ClO2) or chlorine monoxide (CI2O) as a reaction by-product. Gaseous ClO2 is condensed easily to liquid at about 9°C, which can be achieved by fluids boiling under vacuum. Liquid ClO2 is a potent explosive, whereas gaseous ClO2 appears to be reasonably safe. Therefore, CIF3 should not be used in systems containing oxygen in any form, and cold trapping (which would condense moisture) should not be used for collection of volatile compounds (UF61. . Bromine pentafluoride (BrF5) is a liquid at room temperature (meIting point, -61.3°C; boiling point, 40.5°C) and is a good oxidizing agent capable of converting all uranium fluorides, including urany} difluoride (UO2F2,) to uranium hexafluoride at room temperature; its oxidizing capability improves with temperature (Jerry and SteindIer, 1967, 1968; Holmes et al., 1969~. If excess BrFs is used, the reduction B.1
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B.2 ANEVALUATION OF DOE ALTERNATIVES FOR MSRE by-product is bromine trifluoride (BrF3), which is also a strong oxidizing agent; if oxidizable materials are present in excess, the product is bromine, which can be seriously corrosive to stainless steels, less so to high-nickel alloys. The BrF3 can be refluorinated with fluorine and recycled for reuse if desired. It does not have any known explosive reaction products. . Atomic fluorine can be produced from diatomic fluorine (F2) at low pressure, up to 1 or 2 mm (of mercury) in 20 mm of argon, by the use of microwave heating in a sapphire tube, ultraviolet (UV) light, laser activation, or a furnace at 300-600°C. Because of its low dissociation energy, diatomic fluorine (F2) can be dissociated extensively into atomic fluorine, with a reasonably long lifetime (several minutes) that is dependent on its partial pressure in an inert gas stream. Atomic fluorine is a strong oxidizing agent, capable of oxidizing both UF4 and PuF4 (plutonium tetrafluoride) to the hexafluoride at room temperature, and the equipment to produce it is very simple to use and easy to place close to the point of use. Atomic fluorine is practically noncorrosive to most alloys and can be used with copper or stainless piping. Unused fluorine can be redissociated easily by any of the generation mechanisms noted above; for example, if UFO is trapped from the gas stream in carbon dioxide (CO2) or sodium fluoride (NaF) traps, the unused fluorine can be reactivated. . Dioxygen difluoride (O2F2; FOOF) is a fast-acting oxidizing agent capable of converting either UF4 or PuF4 to its hexafluoride (UFO or PuF6) at room temperature (Eller et al., 1988~. This compound is stable at liquid nitrogen temperatures but has a life of only a few seconds at room temperature. It can be generate`d at a few hundred grams per hour in a simple coaxial reactor consisting of a liquid nitrogen-cooled, 1~/2-inch outer-diameter stainless steel jacket with an inner car-rod-heated, 3/~-inch nickel tube. The O2F2 condenses on the inner surface of the outer jacket at liquid nitrogen temperatures and can be eluted for later use, or the reactor can be operated in a continuous production mode. It can be transported easily in stainless equipment held at liquid nitrogen temperature. · Dioxygen monofluoride (O2F; FOO) can be made in the same type of reactor that is also equipped with a sapphire window for
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APPENDIXB ALTERNATIVE FLUORINATING AGENTS B.3 injecting W light from a mercury discharge lamp. It is considerably longer lived than, but has similar oxidizing capabilities to, O2F2. · Krypton difluoride (KrF2) is a powerful oxidizing agent capable of producing UFO or PuF6 at room temperature from any lower fluoride or oxide. It has a room-temperature lifetime of many hours or even days and can be piped considerable distances in standard copper or stainless tubing. It can be made in the same W-pumped system described for O2F production. The production efficiency is much lower than for O2F, but better stability makes KrF2 usable at a considerable distance from the generation area. It was used at Los Alamos National Laboratory (LANL) to decontaminate the prototype M-LIS (Molecular Laser isotope Separation) plutonium isotopic enrichment equipment and remove internal plutonium contamination from the processing equipment at room temperature. KrF2 may be a practical (but costly) candidate for stripping uranium residues from MSRE off-gas pipes, traps, and valves after a more readily available agent is used to oxidize the bulk deposits (e.g., BrFs). The production rates of KrF2 should be able to be scaled up from the current LANL coaxial laboratory-type reactor systems to enable sufficient feed for stripping lower uranium fluoride residues from the freeboard regions in MSRE drain tanks and off-gas traps. Krypton difluoride cannot be used to fluorinate the molten salt at 460°C because it is not stable at elevated temperature.
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Representative terms from entire chapter: