Fluoride volatility

Tendency of highly fluorinated molecules to vaporize
Fluoride volatility is the tendency of highly fluorinated molecules to vaporize at comparatively low temperatures. Heptafluorides, hexafluorides and pentafluorides have much lower boiling points than the lower valence fluorides. Most difluorides and trifluorides have high boiling points, while most tetrafluorides and monofluorides fall in between. The term "fluoride volatility" is jargon used particularly in the context of separation of radionuclides.

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, as well as Se 79. Noble gases (xenon, krypton) are volatile even without fluoridation, and will not condense except at much lower temperatures. Left behind are alkali metals (caesium, rubidium), alkaline earth metals (strontium, barium), lanthanides, the remaining actinides (americium, curium), remaining transition metals (yttrium, zirconium, palladium, silver) and post transition metals (tin, indium, cadmium). This fraction contains the fission products that are radiation hazards on a scale of decades (Cs 137, Sr 90, Sm 151), the four remaining long lived fission products Cs 135, Zr 93, Pd 107, Sn 126 of which only the last emits strong radiation, 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.

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 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.

Fluoride Z Boiling°C Melting°C Key halflife Yield SeF6 34 −46.6 −50.8 79Se:65ky .04% TeF6 52 −39 −38 127mTe:109d IF7 53 4.8 (1 atm) 6.5 (tripoint) 129I:15.7my 0.54% MoF6 42 34 17.4 99Mo:2.75d PuF6 94 62 52 239Pu:24ky TcF6 43 55.3 37.4 99Tc:213ky 6.1% NpF6 93 55.18 54.4 237Np:2.14my UF6 92 56.5 (subl) 64.8 233U:160ky RuF6 44 200 (dec) 54 106Ru:374d RhF6 45 70 103Rh:stable ReF7 75 73.72 48.3 Not FP BrF5 35 40.25 −61.30 81Br:stable IF5 53 97.85 9.43 129I:15.7my 0.54% XeF2 54 114.25 (subl) 129.03 (tripoint) SbF5 51 141 8.3 125Sb:2.76y RuOF4 44 184 115 106Ru:374d RuF5 44 227 86.5 106Ru:374d NbF5 41 234 79 95Nb:35d low PdF4 46 107Pd:6.5my SnF4 50 750 (subl) 705 121m1Sn:44y126Sn:230ky 0.013%? ZrF4 40 905 932 (tripoint) 93Zr:1.5my 6.35% AgF 47 1159 435 109Ag:stable CsF 55 1251 682 137Cs:30.2y135Cs:2.3my 6.19%6.54% BeF2 4 1327 552 RbF 37 1410 795 UF4 92 1417 1036 233U:160ky FLiBe 1430 459 stable FLiNaK 1570 454 stable LiF 3 1676 848 stable KF 19 1502 858 40K:1.25Gy NaF 11 1704 993 stable ThF4 90 1680 1110 CdF2 48 1748 1110 113mCd:14.1y YF3 39 2230 1150 91Y:58.51d InF3 49 >1200 1170 BaF2 56 2260 1368 140Ba:12.75d TbF3 65 2280 1172 GdF3 64 1231 159Gd:18.5h PmF3 61 1338 147Pm:2.62y EuF3 63 2280 1390 155Eu:4.76y NdF3 60 2300 1374 147Nd:11d PrF3 59 1395 143Pr:13.57d CeF3 58 2327 1430 144Ce:285d SmF3 62 2427 1306 151Sm:90y0.419%? SrF2 38 2460 1477 90Sr: 29.1y 5.8% LaF3 57 1493 140La:1.68d