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) 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.

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
SeF634−46.6−50.879Se:65ky.04%
TeF652−39−38127mTe:109d
IF7534.8 (1 atm)6.5 (tripoint)129I:15.7my0.54%
MoF6423417.499Mo:2.75d
PuF6946252239Pu:24ky
TcF64355.337.499Tc:213ky6.1%
NpF69355.1854.4237Np:2.14my
UF69256.5 (subl)64.8233U:160ky
RuF64454106Ru:374d
RhF64570103Rh:stable
ReF77573.7248.3Not FP
BrF53540.25−61.3081Br:stable
IF55397.859.43129I:15.7my0.54%
XeF254114.25 (subl)129.03 (tripoint)
SbF5511418.3125Sb:2.76y
RuOF444184115106Ru:374d
RuF54422786.5106Ru:374d
NbF5412347995Nb:35dlow
PdF446107Pd:6.5my
SnF450750 (subl)705121m1Sn:44y
126Sn:230ky		0.013%
?
ZrF440905932 (tripoint)93Zr:1.5my6.35%
AgF471159435109Ag:stable
CsF551251682137Cs:30.2y
135Cs:2.3my		6.19%
6.54%
BeF241327552
RbF371410795
UF49214171036233U:160ky
FLiBe1430459stable
FLiNaK1570454stable
LiF31676848stable
KF19150285840K:1.25Gy
NaF111704993stable
ThF49016801110
CdF24817481110113mCd:14.1y
YF3392230115091Y:58.51d
InF349>12001170
BaF25622601368140Ba:12.75d
TbF36522801172
GdF3641231159Gd:18.5h
PmF3611338147Pm:2.62y
EuF36322801390155Eu:4.76y
NdF36023001374147Nd:11d
PrF3591395143Pr:13.57d
CeF35823271430144Ce:285d
SmF36224271306151Sm:90y0.419%
?
SrF2382460147790Sr: 29.1y5.8%
LaF3571493140La:1.68d

See also

Notes
  • Missing top fluorides:[4]
    • PrF4 (because it decomposes at 90 °C)
    • TbF4 (because it decomposes at 300 °C)
    • CeF4 (because it decomposes at 600 °C)
  • Without stable fluorides: Kr, Xe, Pd[5]

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 Archived 2010-07-04 at the Wayback Machine. (PDF). Retrieved on 2010-11-14.
  5. Precious metal refining with fluorine gas – Patent 5076839. Freepatentsonline.com. Retrieved on 2010-11-14.
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