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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.

Additional recommended knowledge

Contents

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

Fluoride Z Boiling point Melting point Key halflife Yield
Hexafluorides, heptafluorides
SeF6 34 -46.6ºC -50.8ºC 79Se:65ky .04%
TeF6 52 -39°C -38°C 127mTe:109d
IF7 53 4.8°C (1 atm) 6.5°C (tripoint) 129I:15.7my 0.54%
MoF6 42 34°C 17.4°C 99Mo:2.75d
PuF6 94 52°C (subl) 62°C 239Pu:24ky
TcF6 43 55.3°C 37.4°C 99Tc:213ky 6.1%
UF6 92 56.5°C (subl) 64.8°C 233U:160ky
RuF6 44 54°C 106Ru:374d
RhF6 45 70°C 103Rh:stable
Pentafluorides, tetrafluoride oxides
BrF5 35 40.25°C −61.30°C 81Br:stable
IF5 53 97.85°C 9.43°C 129I:15.7my 0.54%
SbF5 51 141°C 8.3°C 125Sb:2.76y
RuOF4 44 184°C 115°C 106Ru:374d
RuF5 44 227°C 86.5°C 106Ru:374d
NbF5 41 234°C 79°C 95Nb:35d low
Tetrafluorides, monofluorides
PdF4 46 107Pd:6.5my
SnF4 50 705°C 121m1Sn:44y
126Sn230ky
0.013%
?
ZrF4 40 905°C 932°C (tripoint) 93Zr:1.5my 6.35%
AgF 47 1159°C 435°C 109Ag:stable
CsF 55 1251°C 682°C 137Cs:30.2y
135Cs:2.3my
6.19%
6.54%
RbF 37 795°C 87Rb:49by
UF4 92 1417°C 1036°C 233U:160ky
FLiNaK 1570°C 454°C stable
LiF 3 1676°C 848°C stable
ThF4 90 1680°C 1110°C
Trifluorides, difluorides
CdF2 48 1748°C 1110°C 113mCd:14.1y
YF3 39 2230°C 1150°C 91Y:58.51d
InF3 49 1170°C 115In:441ty
BaF2 56 2260°C 1368°C 140Ba:12.75d
NdF3 60 2300°C 1374°C 147Nd:11d
CeF3 58 2327°C 1430°C 144Ce:285d
SmF3 62 2427°C 1306°C 151Sm:90y
146Sm:108y
0.419%
?
SrF2 38 2460°C 1477°C 90Sr: 29.1y 5.8%

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
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Fluoride_volatility". A list of authors is available in Wikipedia.
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