Editing Nuclear reprocessing (section)

=== Fluoride volatility ===
{{Main|Fluoride volatility}}
[[File:Fission yield volatile 2.png|thumb|450px|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 or their oxides are volatile at very high temperatures. Yields at 10<sup>0,1,2,3</sup> years after [[Nuclear fission|fission]], not considering later [[neutron capture]], fraction of 100% not 200%. [[Beta decay]] [[Kr-85]]→[[Rubidium|Rb]], [[Sr-90]]→[[Zirconium|Zr]], [[Ru-106]]→[[Palladium|Pd]], [[Sb-125]]→[[Tellurium|Te]], [[Cs-137]]→[[Barium|Ba]], [[Ce-144]]→[[Neodymium|Nd]], [[Sm-151]]→[[Europium|Eu]], [[Eu-155]]→[[Gadolinium|Gd]] visible.]]
In the fluoride volatility process, [[fluorine]] is reacted with the fuel. Fluorine is so much more reactive than even [[oxygen]] that small particles of ground oxide fuel will burst into flame when dropped into a chamber full of fluorine. This is known as flame fluorination; the heat produced helps the reaction proceed. Most of the [[uranium]], which makes up the bulk of the fuel, is converted to [[uranium hexafluoride]], the form of uranium used in [[uranium enrichment]], which has a very low boiling point. [[Technetium]], the main [[long-lived fission product]], is also efficiently converted to its volatile hexafluoride. A few other elements also form similarly volatile hexafluorides, pentafluorides, or heptafluorides. The volatile fluorides can be separated from excess fluorine by condensation, then separated from each other by [[fractional distillation]] or selective [[redox|reduction]]. [[Uranium hexafluoride]] and [[technetium hexafluoride]] have very similar boiling points and vapor pressures, which makes complete separation more difficult.

Many of the [[fission product]]s volatilized are the same ones volatilized in non-fluorinated, higher-temperature volatilization, such as [[iodine]], [[tellurium]] and [[molybdenum]]; notable differences are that [[technetium]] is volatilized, but [[caesium]] is not.

Some transuranium elements such as [[plutonium]], [[neptunium]] and [[americium]] can form volatile fluorides, but these compounds are not stable when the fluorine partial pressure is decreased.<ref>{{Cite book| url = https://books.google.com/books?id=SJOE00whg44C&pg=PA66| title = The radiochemistry of nuclear power plants with light water reactors| isbn = 978-3-11-013242-7| author = Neeb, Karl-Heinz| publisher = Walter de Gruyter| year = 1997| access-date = 29 November 2021| archive-date = 25 January 2022| archive-url = https://web.archive.org/web/20220125095656/https://books.google.com/books?id=SJOE00whg44C&pg=PA66| url-status = live}}</ref> Most of the plutonium and some of the uranium will initially remain in ash which drops to the bottom of the flame fluorinator. The plutonium-uranium ratio in the ash may even approximate the composition needed for [[fast neutron reactor]] fuel. Further fluorination of the ash can remove all the uranium, [[neptunium]], and plutonium as volatile fluorides; however, some other [[minor actinides]] may not form volatile fluorides and instead remain with the alkaline fission products. Some [[noble metals]] may not form fluorides at all, but remain in metallic form; however [[ruthenium hexafluoride]] is relatively stable and volatile.

Distillation of the residue at higher temperatures can separate lower-boiling [[transition metal]] fluorides and [[alkali metal]] (Cs, Rb) fluorides from higher-boiling [[lanthanide]] and [[alkaline earth metal]] (Sr, Ba) and [[yttrium]] fluorides. The temperatures involved are much higher, but can be lowered somewhat by distilling in a vacuum. If a carrier salt like [[lithium fluoride]] or [[sodium fluoride]] is being used as a solvent, high-temperature distillation is a way to separate the carrier salt for reuse.

[[Molten salt reactor]] designs carry out fluoride volatility reprocessing continuously or at frequent intervals. The goal is to return [[actinide]]s to the molten fuel mixture for eventual fission, while removing [[fission product]]s that are [[neutron poison]]s, or that can be more securely stored outside the reactor core while awaiting eventual transfer to permanent storage.

====Chloride volatility and solubility====
Many of the elements that form volatile high-[[valence (chemistry)|valence]] fluorides will also form volatile high-valence chlorides. Chlorination and distillation is another possible method for separation. The sequence of separation may differ usefully from the sequence for fluorides; for example, [[zirconium tetrachloride]] and [[tin tetrachloride]] have relatively low boiling points of {{convert|331|°C}} and {{convert|114.1|°C}}. Chlorination has even been proposed as a method for removing zirconium fuel cladding,<ref name=advancedheadend>{{cite web |url=http://www.ornl.gov/~webworks/cppr/y2001/pres/123514.pdf |title=Advanced Head-End Processing of Spent Fuel: A Progress Report |author=Guillermo D. Del Cul |work=2005 ANS annual meeting |publisher=[[Oak Ridge National Laboratory]], U.S. DOE |access-date=3 May 2008 |display-authors=etal |url-status=dead |archive-url=https://web.archive.org/web/20060307211536/http://www.ornl.gov/~webworks/cppr/y2001/pres/123514.pdf |archive-date=7 March 2006}}</ref> instead of mechanical decladding.

Chlorides are likely to be easier than fluorides to later convert back to other compounds, such as oxides.

Chlorides remaining after volatilization may also be separated by solubility in water. Chlorides of alkaline elements like [[americium]], [[curium]], [[lanthanides]], [[strontium]], [[caesium]] are more soluble than those of [[uranium]], [[neptunium]], [[plutonium]], and [[zirconium]].

====Advantages of halogen volatility====
* Chlorine (and to a lesser extent fluorine<ref>{{cite web |title=Fluorine |url=https://www.essentialchemicalindustry.org/chemicals/fluorine.html |website=essentialchemicalindustry.org |access-date=4 October 2022 |archive-url=https://web.archive.org/web/20220425062623/https://www.essentialchemicalindustry.org/chemicals/fluorine.html |archive-date=25 April 2022 |date=10 October 2016 |url-status=live}}</ref>) is a readily available [[industrial chemical]] that is produced in mass quantity<ref>{{cite web |title=Chlorine Manufacturing Industry in the US |url=https://www.ibisworld.com/united-states/market-research-reports/chlorine-manufacturing-industry/ |website=ibisworld.com |access-date=4 October 2022 |archive-url=https://web.archive.org/web/20220223185750/https://www.ibisworld.com/united-states/market-research-reports/chlorine-manufacturing-industry/ |archive-date=2022-02-23 |language=en-US |date=28 Jun 2022 |url-status=live}}</ref>
* Fractional distillation allows many elements to be separated from each other in a single step or iterative repetition of the same step
* Uranium will be produced directly as [[Uranium hexafluoride]], the form used in enrichment
* Many volatile fluorides and chlorides are volatile at relatively moderate temperatures reducing thermal stress. This is especially important as the boiling point of uranium hexafluoride is below that of water, allowing to conserve energy in the separation of high boiling fission products (or their fluorides) from one another as this can take place in the absence of uranium, which makes up the bulk of the mass
* Some fluorides and chlorides melt at relatively low temperatures allowing a "liquid phase separation" if desired. Those low melting salts could be further processed by molten salt electrolysis.
* Fluorides and chlorides differ in water solubility depending on the cation. This can be used to separate them by aqueous solution. However, some fluorides violently react with water, which has to be taken into account.

====Disadvantages of halogen volatility====
* Many compounds of fluorine or chlorine as well as the native elements themselves are toxic, corrosive and react violently with air, water or both
* [[Uranium hexafluoride]] and [[Technetium hexafluoride]] have very similar boiling points ({{convert|329.6|K}} and {{convert|328.4|K}} respectively), making it hard to completely separate them from one another by distillation.
* Fractional distillation as used in [[petroleum refining]] requires large facilities and huge amounts of energy. To process thousands of tons of uranium would require smaller facilities than processing billions of tons of petroleum {{mdash}} however, unlike petroleum refineries, the entire process would have to take place inside radiation shielding and there would have to be provisions made to prevent leaks of volatile, poisonous and radioactive fluorides.
* [[Plutonium hexafluoride]] boils at {{convert|335|K}} this means that any facility capable of separating uranium hexafluoride from Technetium hexafluoride is capable of separating plutonium hexafluoride from either, raising proliferation concerns
* The presence of [[alpha decay|alpha emitters]] induces some (α,n) reactions in fluorine, producing both radioactive {{chem|22|Na|link=sodium-22}} and neutrons.<ref>{{cite journal|url=https://www.sciencedirect.com/science/article/abs/pii/S0969806X23001640|title=Neutron and gamma-ray signatures for the control of alpha-emitting materials in uranium production: A Nedis2m-MCNP6 simulation|date=2023 |doi=10.1016/j.radphyschem.2023.110919 |access-date=2023-08-09 |last1=Vlaskin |first1=Gennady N. |last2=Bedenko |first2=Sergey V. |last3=Polozkov |first3=Sergey D. |last4=Ghal-Eh |first4=Nima |last5=Rahmani |first5=Faezeh |journal=Radiation Physics and Chemistry |volume=208 |page=110919 |bibcode=2023RaPC..20810919V |s2cid=257588532 }}</ref> This effect can be reduced by separating alpha emitters and fluorine as fast as feasible. Interactions between chlorine's two stable isotopes {{chem|35|Cl|link=chlorine-35}} and {{chem|37|Cl|link=chlorine-37}} on the one hand and alpha particles on the other are of lesser concern as they do not have as high a cross section and do not produce neutrons or long lived radionuclides.<ref>[https://www.oecd-nea.org/janisweb/book/alphas/Cl35/MT4/renderer/1082 Dead link] {{Dead link|date=September 2022}}</ref>
* If carbon is present in the spent fuel it'll form [[halogenated hydrocarbons]] which are extremely potent [[greenhouse gas]]es, and hard to chemically decompose. Some of those are toxic as well.