Fluoride volatility

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.

Volatility and valence

Blue elements have volatile fluorides or are already volatile; green elements do not but have volatile chlorides; red elements have neither, but the elements themselves are volatile at very high temperatures. Yields at 100,1,2,3 years after fission, not considering later neutron capture, fraction of 100% not 200%. Beta decay Kr-85→Rb, Sr-90→Zr, Ru-106→Pd, Sb-125→Te, Cs-137→Ba, Ce-144→Nd, Sm-151→Eu, Eu-155→Gd visible.

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, cadmium) and other metals (tin, indium). 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.

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, 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.[1][2]

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.[3]

Table of relevant properties

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 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:44y
126Sn:230ky		 0.013%
?
ZrF4 40 905 932 (tripoint) 93Zr:1.5my 6.35%
AgF 47 1159 435 109Ag:stable
CsF 55 1251 682 137Cs:30.2y
135Cs: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
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

See also

Notes

References

  1. ↑ Uhlir, Jan. "An Experience on Dry Nuclear Fuel Reprocessing in the Czech Republic" (PDF). OECD Nuclear Energy Agency. Retrieved 2008-05-21.
  2. ↑ Uhlir, Jan. "R&D of Pyrochemical Partitioning in the Czech Republic" (PDF). OECD Nuclear Energy Agency. Retrieved 2008-05-21.
  3. ↑ Markvart, Milos. "Development of Uranium Oxide Powder Dosing for Fluoride Volatility Separation Process" (PDF). Archived from the original (PDF) on November 17, 2004. Retrieved 2008-05-21.
  4. ↑ CRC Handbook of Chemistry and Physics, 88th Edition. (PDF). Retrieved on 2010-11-14.
  5. ↑ Precious metal refining with fluorine gas – Patent 5076839. Freepatentsonline.com. Retrieved on 2010-11-14.

External links

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