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, caesium) form nonvolatile fluorides. Few metals in the fission products (the transition metals niobium, ruthenium, technetium, molybdenum, and the halogen iodine) form volatile (boiling point <200 °C) fluorides that accompany the uranium and plutonium hexafluorides, together with inert gases. Distillation is then used to separate the uranium hexafluoride from the mixture.
The nonvolatile alkaline fission products and minor actinides is most suitable for further processing with dry electrochemical processing (pyrochemical) non-aqueous methods. The lanthanide fluorides are 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 [https://archive.today/20070609212936/http://www.nri.cz/eng/index.html 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.
Hitachi has developed a technology, called FLUOREX, which combines fluoride volatility, to extract uranium, with more traditional solvent extraction (PUREX), to extract plutonium and other transuranics]. The FLUOREX-based fuel cycle is intended for use with the Reduced moderation water reactor.