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

Valences for the majority of elements are based on the highest known fluoride. However, elements with high valence tend to reach higher oxidation state in compounds with oxygen rather than fluorine.
Roughly, fluoride volatility can be used to remove elements with a valence of 5 or greater: uranium, neptunium, plutonium, metalloids, nonmetals, halogens, and the middle transition metals. This fraction includes the actinides most easily reusable as nuclear fuel in a thermal reactor, and the two fission product">Nuclear fission">fission products best suited to disposal by transmutation, Tc-99 and I-129, as well as Se-79.
Noble gases are volatile even without fluoridation, and will not condense except at much lower temperatures.
Left behind are alkali metals, alkaline earth metals, lanthanides, the remaining actinides, remaining transition metals and post-transition metals. This fraction contains the fission products that are radiation hazards on a scale of decades, 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 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. Americium finds use in ionization smoke detectors while californium is used as a spontaneous fission based neutron source. Curium has only very limited uses outside nuclear reactors. Fissionable but non-fissile actinoids can be used or disposed of in a subcritical nuclear reactor using an external neutron source such as an Accelerator Driven 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 form nonvolatile fluorides. Few metals in the fission products form volatile fluorides that accompany the uranium and plutonium hexafluorides, together with inert gases. Distillation is then used to separate the uranium hexafluoride from the mixture.
The nonvolatile alkaline fission products and minor actinides fraction is most suitable for further processing with 'dry' electrochemical processing 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 at Řež in the Czech Republic tested screw dosers that fed ground uranium oxide into a fluorinator where the particles were burned in fluorine gas to form uranium hexafluoride.
Hitachi has developed a technology, called FLUOREX, which combines fluoride volatility, to extract uranium, with more traditional solvent extraction, to extract plutonium and other transuranics. The FLUOREX-based fuel cycle is intended for use with the Reduced moderation water reactor.
Some fluorides are water soluble while others aren't and can be separated in aqueous solution. However, all aqueous processes that take place without complete removal of tritium prior to addition of water will contaminate the water with tritiated water which is difficult to remove from water. Some elements which form soluble florides form insoluble chlorides. Addition of a suitable soluble chloride will salt out those cations. One example is silver (I) fluoride which forms silver chloride precipitate upon addition of a soluble chloride.
Some fluorides react aggressively with water and may form highly corrosive hydrogen fluoride. This needs to be taken into account if aqueous processes involving fluorides are to be used.
If desired, a series of further anion-additions similar to the :de:Kationentrennungsgang can be used to separate out different cations for disposal, further processing or use.

Table of relevant properties

Fluoride	Z	Boiling °C	Melting °C	Key halflife	Yield
HF	1	19.5	−83.6	T:12y	Common reagent
BF₃	5	−100.3	−126.8	none over 0.8 s	Neutron poison used for control
SeF₆	34	−46.6	−50.8	⁷⁹Se:327ky	.04%
TeF₆	52	−39	−38	^127mTe:109d
IF₇	53	4.8	6.5	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	64.8	²³³U:160ky
RuF₆	44	200	54	¹⁰⁶Ru:374d
RhF₆	45	73.5	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	129.03		-
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	705	^121m1Sn:44y ¹²⁶Sn:230ky	0.013% ?
ZrF₄	40	905	932	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	¹⁰Be:1.4my
RbF	37	1410	795
UF₄	92	1417	1036	²³³U:160ky
FLiBe		1430	459	stable
FLiNaK		1570	454	stable
LiF	3	1676	848	stable
KF	19	1502	858	⁴⁰K:1.25Gy
NaF	11	1704	993	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:90y	0.419% ?
SrF₂	38	2460	1477	⁹⁰Sr: 29.1y	5.8%
LaF₃	57		1493	¹⁴⁰La:1.68d