Fluoride volatility

Fluoride volatility is a method for the extraction of elements which form volatile fluorides. It is being studied for reprocessing of nuclear fuel, either of the conventional fuel rods used in today's LWRs, or as an integral part of a molten salt reactor system.

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Reprocessing methods

Uranium oxides react with fluorine to form gaseous uranium hexafluoride, most of the plutonium reacts to form gaseous plutonium hexafluoride, a majority of fission products (especially electropositive elements: lanthanides, strontium, barium, yttrium, cesium) form solid fluorides dropping to the fluorinator bottom, and only a few of the fission product elements (the transition metals niobium, ruthenium, technetium, molybdenum, and the halogen iodine) form gaseous fluorides that accompany the uranium and plutonium hexafluorides, together with inert gases. Distillation is then used to remove the other volatile metal fluorides and iodine fluorides from the uranium hexafluoride[1] [2].

The nonvolatile residue of alkaline fission products and minor actinides is most suitable for further processing with 'dry' electrochemical processing (pyrochemical) Nuclear reprocessing#Non aqueous methods. The lanthanide fluorides would be difficult to dissolve in the nitric acid used for aqueous reprocessing methods, such as PUREX, DIAMEX and SANEX, which use solvent extraction. Fluoride volatility is only one of several pyrochemical processes designed to reprocess used nuclear fuel.

The Řež nuclear research institute at Řež in the Czech Republic tested screw dosers that fed ground uranium oxide (simulating used fuel pellets) into a fluorinator where the particles were burned in fluorine gas to form uranium hexafluoride. [3]

Volatility, valence, and chemical series

Valences for the majority of elements are based on the highest known fluoride.

Roughly, fluoride volatility can be used to remove elements with a valence of 5 or greater: Uranium, Neptunium, Plutonium, Metalloids (Tellurium, Antimony), Nonmetals (Selenium), Halogens (Iodine, Bromine), and the middle Transition metals (Niobium, Molybdenum, Technetium, Ruthenium, and possibly Rhodium). This fraction includes the actinides most easily reusable as nuclear fuel in a thermal reactor, and the two long-lived fission products best suited to disposal by transmutation, Tc-99 and I-129.

Noble gases (Xenon, Krypton) are volatile even without fluoridation, and will not condense except at much lower temperatures.

Left behind are Alkali metals (Cesium, Rubidium), Alkaline earth metals (Strontium, Barium), Lanthanides, the remaining Actinides, remaining Transition metals (Yttrium, Zirconium, Palladium, Silver, Cadmium) and Poor metals (Tin, Indium). This fraction contains the fission products that are radiation hazards on a scale of decades (Cs-137, Sr-90, Sm-151), four long-lived but less dangerous fission products (Cs-135, Zr-93, Pd-107, Sn-126), most of the neutron poisons, and the higher actinides (Americium, Curium, Californium) that are radiation hazards on a scale of hundreds or thousands of years and are difficult to work with because of gamma radiation but are fissionable in a fast reactor.

Fluorides by boiling and melting points

Missing: Pd 46, La 57, Pr 59, Pm 61, Eu 63 and up

Missing top fluorides: TcF₇ AgF₄ XeF₆ LaF₃ CeF₄ PrF₄ PmF₃ EuF₃ GdF₃ TbF₄

Inert: Kr 36, Xe 54

See also

• FLiNaK
• Molten salt reactor
• AN EXPERIENCE ON DRY NUCLEAR FUEL REPROCESSING IN THE CZECH REPUBLIC
• STUDY OF ELECTROCHEMICAL PROCESSES FOR SEPARATION OF THE ACTINIDES AND LANTHANIDES IN MOLTEN FLUORIDE MEDIA
• Separation and purification of UF6 from volatile fluorides by rectification
• Low-pressure distillation of a portion of the fuel carrier salt from the Molten Salt Reactor Experiment
• USE OF THE FLUORIDE VOLATILITY PROCESS TO EXTRACT TECHNETIUM FROM TRANSMUTED SPENT NUCLEAR FUEL
• A Peer Review of the Strategy for Characterizing Transuranics and Technetium Contamination in Depleted Uranium Hexafluoride Tails Cylinders