Uranium Hexafluoride
Properties
| State | Solid sublimating to gas at 56.5 °C (1 atm); triple point 64.0 °C at 151.7 kPa |
| Color | Colorless |
| Solubility | Reacts violently with water; soluble in liquid fluorocarbons and some organic solvents |
| Melting Point | 64.05 °C (at 151.7 kPa; sublimes at 1 atm) |
| Boiling Point | 56.5 °C (sublimation, 1 atm) |
About Uranium Hexafluoride
Uranium hexafluoride is the only uranium compound volatile enough to handle as a gas at near-ambient temperatures, and that single property is the entire foundation of the commercial nuclear fuel cycle. The molecule is a perfect octahedron — six W-F... pardon, six U-F bonds of 1.99 Angstrom radiating from the central U(VI) — held together by strong covalent W=F sigma bonds with no Jahn-Teller distortion because U(VI) is d^0 f^0. The triple point sits at 64.05 °C and 151.7 kPa, just above atmospheric, so at 1 atm the solid sublimes directly to gas at 56.5 °C without passing through a liquid phase. This solid-to-gas transition with no intermediate liquid is the operational basis of every gas-centrifuge enrichment cascade in operation, from URENCO Almelo to Eurodif's successor Orano in Pierrelatte. Inside a Zippe-type centrifuge spinning at 60,000 to 90,000 RPM, the 0.85% mass difference between 235UF6 (349.03 g/mol) and 238UF6 (352.04 g/mol) is enough to push the lighter isotopologue toward the rotor axis and the heavier toward the wall — feed in 0.72% U-235 natural uranium, cascade thousands of centrifuges in series and parallel, and you can produce 5% LEU for power reactors or above 90% HEU for weapons. The fluorine in UF6 has only one stable isotope (19F), which is essential — any natural variation in F isotope distribution would smear out the U isotope signal.
Where you'll encounter it
If you have ever watched centrifuge cascade footage from URENCO or seen the famous declassified photos of the K-25 gaseous diffusion plant at Oak Ridge (a single U-shaped building covering 44 acres, the largest building in the world when completed in 1945), the working fluid in every pipe and seal was UF6. UF6 is shipped between conversion plants, enrichment plants, and fuel fabrication plants in Type 30B and Type 48Y steel cylinders that look like small submarines — heated to about 100 °C to vaporize the contents for transfer, then cooled and sealed for road or rail transport. The notorious incident at Sequoyah Fuels in Gore, Oklahoma in 1986 saw a single overfilled cylinder rupture during heating, releasing about 14 tonnes of UF6 that hydrolyzed in the air to a yellow cloud of UO2F2 and HF — one worker killed, dozens injured, and the plant never reopened. The subfab piping in any UF6 facility runs heat-traced and double-walled, with continuous HF leak detection at floor level because UF6 is heavier than air and HF will pool wherever the gas leaks.
Common Uses
- Working gas for uranium isotope enrichment in gas-centrifuge cascades worldwide
- Historical working gas for gaseous-diffusion enrichment at Oak Ridge K-25 and Paducah
- Feedstock to fuel fabrication plants where it is converted to UO2 powder for pellet pressing
- Intermediate in the back-end fuel cycle for re-enrichment of reprocessed uranium streams
- Reference material in actinide chemistry for studies of high-oxidation-state fluorides
- Calibration gas for mass spectrometers and centrifuge process diagnostics in enrichment plants
- Working material in laser-isotope separation research (SILEX, AVLIS) for advanced enrichment routes
- Test material for international safeguards verification of enrichment facility throughput and assay
Safety Information
EXTREMELY HAZARDOUS. GHS classification: Acute Toxicity Category 2 (oral, dermal, inhalation), Skin Corrosion Category 1A, Carcinogen Category 1B, Reproductive Toxicity Category 1B, Specific Target Organ Toxicity Repeated Exposure Category 2 (kidney), and Radioactive. The chemical toxicity from kidney-targeting U(VI) plus the HF generated by hydrolysis dominates over the radiological hazard at natural and low enrichments. The Sequoyah Fuels accident in 1986 demonstrated the worst-case scenario: a ruptured cylinder released 14 tonnes of UF6 that hydrolyzed to HF and UO2F2 fumes, killing one worker and injuring dozens. NRC handles regulation under 10 CFR 40 (source material) and 10 CFR 70 (special nuclear material above 1% enrichment), with criticality safety controls at all enriched-material facilities. Transport in Type 30B and 48Y cylinders under DOT class 7 radioactive plus class 8 corrosive. Decommissioning requires N2 purge of all wetted surfaces and neutralization of UF6 condensate to UO2 plus CaF2.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.