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.

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 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. [ cite web
url= http://www.nea.fr/html/pt/docs/iem/mol98/session2/SIIpaper9.pdf
title= An Experience on Dry Nuclear Fuel Reprocessing in the Czech Republic
last= Uhlir | first= Jan
date= |year= |month= |format= |work= |publisher= OECD Nuclear Energy Agency
accessdate= 2008-05-21 ] [ cite web
url= http://www.nea.fr/html/pt/docs/iem/madrid00/Proceedings/Paper13.pdf
title= R&D of Pyrochemical Partitioning in the Czech Republic
last= Uhlir | first= Jan
date= |year= |month= |format= |work= |publisher= OECD Nuclear Energy Agency
accessdate= 2008-05-21 ]

The nonvolatile residue of alkaline fission products and minor actinides is most suitable for further processing with 'dry' electrochemical processing (pyrochemical) 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 [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. [ cite web
url= http://www.fjfi.cvut.cz/Stara_verze/k417/web_ads/papers/P-g6.pdf
title= Development of Uranium Oxide Powder Dosing for Fluoride Volatility Separation Process
last= Markvart | first= Milos
date= |year= |month= |format= |work= |publisher=
pages= |language= |doi= |archiveurl= |archivedate= |quote=
accessdate= 2008-05-21 ]

Volatility and valence

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 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), 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.

Fluorides by boiling and melting points

Missing: [http://www.freepatentsonline.com/5076839.html 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

Notes

ee also

*FLiNaK
*Molten salt reactor

External links

* [http://www.nea.fr/html/pt/docs/iem/lasvegas04/07_Session_II/S2_03.pdf STUDY OF ELECTROCHEMICAL PROCESSES FOR SEPARATION OF THE ACTINIDES AND LANTHANIDES IN MOLTEN FLUORIDE MEDIA] (PDF)
* [http://www.fjfi.cvut.cz/con_adtt99/papers/P-g5.pdf Separation and purification of UF6 from volatile fluorides by rectification] (PDF)
* [http://www.energyfromthorium.com/pdf/ORNL-4577.pdf Low-pressure distillation of a portion of the fuel carrier salt from the Molten Salt Reactor Experiment] (PDF)
* [http://www.studentpipeline.org/afci/theses/milliron.pdf USE OF THE FLUORIDE VOLATILITY PROCESS TO EXTRACT TECHNETIUM FROM TRANSMUTED SPENT NUCLEAR FUEL] (PDF)
* [https://e-reports-ext.llnl.gov/pdf/238572.pdf A Peer Review of the Strategy for Characterizing Transuranics and Technetium Contamination in Depleted Uranium Hexafluoride Tails Cylinders] (PDF)