Thorium(IV) Fluoride
Properties
| State | Solid |
| Color | White |
| Solubility | Essentially insoluble in water; soluble in HF and in fluoride-melt solvents |
| Melting Point | 1110 °C |
| Boiling Point | 1680 °C |
About Thorium(IV) Fluoride
Thorium tetrafluoride is the fuel-form thorium compound — the species that gets dissolved in molten LiF-BeF2 (FLiBe) to make liquid-fueled thorium reactor blankets. Oak Ridge built and ran the Molten Salt Reactor Experiment from 1965 to 1969 on a fluoride-salt fuel that contained ThF4 as the fertile component plus UF4 as the fissile driver, and that 8-MW prototype is still the only operational thorium-fuel reactor experiment in US history. China's TMSR-LF1, which reached criticality in Wuwei in 2021, runs the same chemistry at 2 MW thermal. The choice of fluoride over chloride or oxide for molten-salt reactor fuel comes down to four properties: fluoride salts are stable to radiolysis at the 700°C operating temperature, they have low vapor pressure (so the primary loop runs at near-atmospheric pressure rather than the ~150-bar pressure of a PWR), they dissolve actinides at high concentration (10+ mol% UF4 in LiF-BeF2 is achievable), and they allow continuous online removal of fission products by gas sparging. Crystallographically, ThF4 adopts the monoclinic UF4-type structure with Th(IV) in 8-coordinate square-antiprismatic environments of bridging fluorides — the same coordination geometry that drives its mid-infrared optical properties. From the 1960s through the 1990s, ThF4 was the antireflection coating material of choice for Ge windows, ZnSe lenses, and ZnS optics in CO2-laser delivery systems and FLIR sensors, paired with ZnS in alternating quarter-wave stacks; the radiological liability eventually pushed industry toward YF3 and Y2O3 alternatives that get within 5% of the same optical performance.
Where you'll encounter it
If you've ever ordered an antireflection-coated germanium window for a thermal-imaging camera built before the year 2000, the coating stack on it almost certainly contains a few hundred nanometers of ThF4 — and you can detect this with a sensitive Geiger counter held against the window face. Modern FLIR optics from FLIR Systems, Raytheon, and Edmund Optics have moved to YF3 or BaF2 to avoid the radiological-disposal liability at end of life, but military-surplus night-vision and CO2-laser hardware from the Reagan-era buildup still rely on the ThF4 stacks. Inside a molten-salt reactor research lab — Oak Ridge, Idaho National, or the Shanghai Institute of Applied Physics — ThF4 is the gray-green powder that gets weighed in a glove box, sealed into a nickel-alloy capsule, and added to a FLiBe melt at 700°C to start a fertile-fuel run. Calcium-metal reduction of ThF4 in a tantalum-lined bomb at 1100°C is also the standard production route for thorium metal ingots used in materials-science research.
Common Uses
- Fertile-fuel component (5-15 mol%) in LiF-BeF2-ThF4-UF4 molten-salt reactor fuel mixtures
- Feedstock for thorium-metal production via Ca metallothermic reduction at 1100°C in tantalum bombs
- Mid-infrared (2-12 μm) antireflection coating layer for Ge, ZnSe, and ZnS optics in legacy FLIR systems
- Optical multilayer pairing with ZnS in CO2-laser delivery-window stacks (legacy military hardware)
- Source compound for ThF4 single-crystal growth used in radiation-detector research
- Precursor for thorium-doped fluoride-glass research compositions
- Fluoride-flux additive in actinide-separation research and pyroprocessing studies
- Solid-state ionic-conductor research substrate at moderate temperatures
Safety Information
GHS: H315 skin irritation, H319 eye irritation, H335 respiratory irritation, H350 carcinogenic via inhalation (Category 1A) due to radioactivity. NRC source-material licensed above 6.8 kg elemental Th. ThF4 itself is essentially water-insoluble, so the dust hazard is the dominant route of exposure: inhaled fluoride-actinide particles deposit in the deep lung, dissolve very slowly, and deliver chronic alpha dose from 232Th and its decay daughters (228Ra, 228Th, 224Ra, 220Rn). On contact with hot moisture or strong acids, ThF4 can release HF — particularly when handling spent reactor salts or during decladding operations. OSHA PEL for soluble Th compounds is 0.05 mg/m3 as Th and ACGIH TLV for fluoride dust is 2.5 mg/m3 as F; the more restrictive value applies. Handle inside HEPA-filtered glove boxes or fume hoods, with HF-compatible PPE (neoprene gloves, polycarbonate face shield) and a written radiation-protection program.
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.