Uranyl Fluoride – UO₂F₂

 CAS 13536-84-0

Uranyl fluoride (UO₂F₂), a compound of uranium, is an intermediate in the conversion of uranium hexafluoride UF₆ to an uranium oxide or metal form and is a direct product of the reaction of UF₆ with moisture in the air. It is very soluble in water. Uranyl fluoride also is hygroscopic and changes in color from brilliant orange to yellow after reacting with water. Uranyl fluoride is reported to be stable in air to 300°C, above which slow decomposition to U₃O₈ occurs. When heated to decomposition, UO₂F₂ emits toxic fluoride fumes.

In accidental releases of UF₆, UO₂F₂, as a solid particulate compound, may deposit on the ground. The overall chemical reaction of this event can be represented as: UF₆+ 2H₂O → UO₂F₂+ 4HF. These reactions can take place whether the uranium hexafluoride is a solid or a gas, but will take place almost instantaneously when the UF₆ is in a gaseous state. The resulting hydrofluoric acid and the presence of additional water results in formation of solids (primarily HF adducts of hydrated uranyl fluoride (UO₂F₂-nH₂O).

 Toxicology

Chemical hazards are far more significant than radioactive hazards, though there is a radioactivity concern if prepared with enriched uranium. Material is corrosive, and harmful by inhalation, ingestion or skin absorption. Ingestion or inhalation may be fatal. Effects of exposure may be delayed.

Introduction

A vial containing solid crystals of uranium hexafluoride.
(Source: Argonne National Laboratory)

Uranium hexafluoride (UF₆) is a chemical compound consisting of one atom of uranium

combined with six atoms of fluorine. It is the chemical form of uranium that is used

during the uranium enrichment process. Within a reasonable range of temperature

and pressure, it can be a solid, liquid, or gas. Solid UF₆ is a white, dense,
crystalline material that resembles rock salt.

What’s its structure?

 

 

UF6 is octahedral, with all 6 fluorines pointing towards the corner of an octahedron. Click on the images to see the 3D structure files.

 

 

That’s a lot of fluorines!

If you want a compound of an element in a high oxidation state, fluorine is
the most likely element to help you achieve it; partly favoured by the low F-F
bond energy, partly because fluorine forms strong bonds with almost any other
element. So uranium isn’t the only element to form a hexafluoride. Other
actinides like Np and Pu do it; several of the platinum metals like Pt, Ru, Os,
Rh and Ir form them, so do Tc, Re, Mo and W; and of course there are the famous
noble-gas compound XeF₆ and the unreactive gas SF₆
(important in the electrical industry as an insulating gas and recently the
subject of concern for its potential in global warming) as well as SeF₆
and TeF₆ . But UF₆ is the most important, as we shall see.

What is UF₆ like?

It is a white crystalline solid at room temperature (its triple point is 64°C
(147.3°F) and it sublimes at 56.5°C (133.8°F) at atmospheric pressure). The
liquid phase only exists under pressures greater than about 1.5 atmospheres and
at temperatures above 64°C (see the phase diagram).

UF₆ crystals in a glass vial. (from http://web.ead.anl.gov/uranium/guide/uf6/index.cf)

Phase diagram, showing relationship betweenpressure, temperature, and physical form (from

http://web.ead.anl.gov/uranium/guide/prodhand/sld022.cfm).

Like all the other hexafluorides (except XeF₆) it has an
octahedral structure (U-F = 1.994 Å), with strong covalent bonds within the
molecules but only weak Van der Waals’ forces between neighbours, so that it has
a low boiling point and melting point. Vibrational frequencies are
n₁ (R) 667 cm^-1;
n₂ (R) 535 cm^-1;
n₃ (IR) 624 cm^-1;
n₄ (IR) 186 cm^-1;
n₅ (R) 201 cm^-1;
n₆ (inactive) 140 cm^-1.

Is it reactive?

It does not react with dry air, oxygen, nitrogen or carbon dioxide. However,
when it comes in contact with water, even traces, it undergoes hydrolysis to
uranyl fluoride (UO₂F₂) and the corrosive and toxic gas HF.

UF₆   +   2 H₂O
UO₂F₂
+   4 HF

For this reason, it attacks glass unless absolutely free of either water or
HF. It reacts with many elements (e.g. Group I and II metals; B, Al, Ga,
In, C, Si-Pb etc) forming their fluorides. Most hydrogen-containing
compounds react forming HF, so that hydrocarbons (including arenes) form a
mixture of carbon fluorides, carbonaceous material and HF, but there is no
reaction with fluorocarbons such as Teflon. It attacks many metals, except Ni
(owing to the formation of a surface coating of fluoride) and Al (that bears a
surface oxide layer).

How is it made?

There are many ways of making UF₆. Although reaction of uranium
and fluorine was first noted by Henri Moissan, first isolator of fluorine, c.1900,
UF₆ was originally reported by Ruff (pioneer of syntheses of many
metal fluorides) and Heinzelmann in 1911. They made it by fluorination of
uranium, and also by fluorination of UC₂; fluorine needs to be heated
to react with uranium, unless it is finely divided.

U (s)   +  3 F₂ (g)
UF₆ (g)

UC₂ (s)   +   7 F₂ (g)
UF₆ (g)
+   2 CF₄ (g) (at 350°C)

An unusual reaction which does not use fluorine (but which has not been
employed commercially) is:

2 UF₄ (g)   +   O₂ (g)
UF₆ (g)
+   UO₂F₂ (g)

The main process used industrially employs fluorination of UF₄ at
around 500°C. It is a very exothermic process; temperatures can reach 1100°C in
reactors with capacities of up to 380 kg per hour.

UF₄ (g)   +   F₂ (g)
UF₆ (g)

Presumably UF₄ has to be made first?

Indeed, and its synthesis from the uranium ore is quite a complicated
process. The uranium is usually present in the ore as an oxide, approximating to
U₃O₈. This is dissolved in nitric acid forming UO₂(NO₃)₂.6H₂O.
(In some places, carbonate leaching, affording [UO₂(CO₃)₃]^4-,
is used). This is purified by extraction using a solvent such as
tributylphosphate in kerosene; after evaporation of the solution, the resulting
solid is roasted to form UO₃. The resulting UO₃ is then
converted into UF₆ in a three-stage process.

• UO₃ (s)   +   H₂ (g)
  UO₂(s)
  +   H₂O(g)
• UO₂ (s)   +  4HF(g)
  UF₄(s)
  +   2 H₂O(g)
• UF₄ (g)   +    F₂ (g)
  UF₆ (g)

How is UF₆ used?

It is extremely volatile (it has a vapour pressure of around 120 mmHg at room
temperature) so can be handled as a gas for the isotope separation.

How is the separation achieved?

Several methods have been used:

• Gaseous diffusion
• Electromagnetic isotope separation
• Gas centrifugation
• Laser isotope separation

Electromagnetic isotope separation, using UCl₄, was the original
method used in the Manhattan project in a plant at Oak Ridge Tennessee. Laser
isotope separation has been little used, the best known version being the
Australian Silex process. Of the other two processes, something over 90%

of the world’s enriched uranium is obtained
this way.

Gas diffusion for isotope enrichment.

In gaseous diffusion, UF₆ gas is forced to diffuse under pressure
through porous membranes. The lighter ²³⁵UF₆ molecules
diffuse slightly faster than the ²³⁸UF₆ molecules. As the
gas moves, the two isotopes are separated, increasing (enriching) the U-235
concentration, and decreasing (depleting) the concentration of U-238. However,
over 1000 stages are required to produce a UF₆ product with even 3-4%
enrichment! As the most volatile uranium compound, UF₆ was an obvious
choice for use, but because of its high reactivity and problems associated with
its handling, many alternatives were investigated, such as alkoxides, whilst
unsuccessful attempts were made by Gilman to obtain simple alkyls. Ultimately
the technology was improved to enable full advantage to be taken of its
volatility.

Hazards

As an a-emitter, uranium is not especially
hazardous unless ingested by the body. However, the ready reaction of UF₆
with water makes it a potentially hazardous substance, owing to the toxicity of
the HF produced. In a 1944 accident when a cylinder containing UF₆
was being heated by steam, a weld ruptured, releasing some 400 lb of UF₆;
the reaction of UF₆ with steam released sufficient HF to cause two
fatalities. Another person was killed by inhaled HF in 1986 following rupture of
a heated cylinder; some 31 other people were exposed to the fumes but no one
else appears to have suffered long-term ill effects.

UF₆ and Uranium Processing

Uranium processing. (Source: Argonne National Laboratory)

The gaseous diffusion process used
to enrich uranium requires uranium in the form of uranium hexafluoride (UF₆).
In the first step of UF₆ production, uranium ore is mined and sent to
a mill where uranium oxide (often called “yellowcake”) is produced. The uranium
oxide is then sent to a UF₆ production facility. At the production
facility, the uranium oxide is combined with anhydrous hydrogen fluoride (HF)
and fluorine gas in a series of chemical reactions to form the chemical compound
UF₆. The product UF₆ is placed into steel cylinders and
shipped as a solid to a gaseous diffusion plant for enrichment.

Uranium hexafluoride is used in
uranium processing because its unique properties make it very convenient. It can
conveniently be used as a gas for processing, as a liquid for filling or
emptying containers or equipment, and as a solid for storage—all at temperatures
and pressures commonly used in industrial processes.

Physical Properties

Uranium hexafluoride (UF₆)
at ambient conditions is a volatile, white, crystalline solid. Solid UF₆
is readily transformed into the gaseous or liquid states by the application of
heat. All three phases — solid, liquid, and gas — coexist at 147°F (64°C) (the
triple point). Only the gaseous phase exists above 446°F (230°C), the critical
temperature, at which the critical pressure is 45.5 atm (4.61 mPa). The vapor
pressure above the solid reaches 1 atm (0.1 mPa) at 133°F (56°C), the
sublimation temperature.

The triple point of UF₆
occurs at 22 pounds per square inch, absolute (psia) and 147°F (64°C). These are
the only conditions at which all three states — liquid, solid, and gas — can
exist in equilibrium. If the temperature or pressure is greater than at the
triple point, there will only be gas or liquid.

UF₆ phase diagram, showing relationship between
pressure, temperature, and physical form. (Source: Argonne National Laboratory)

Density of solid UF₆ at
68°F (20°C) is 317.8 lb/ft³ (5.1 g/cm³). A large decrease
in UF₆ density occurs when UF₆ changes from the solid to
the liquid state, which results in a large increase in volume. The thermal
expansion of the liquid with increasing temperature is also high. Therefore, it
is important to maintain control of the total mass and physical state of UF₆
throughout an operational cycle. To avoid hydraulic rupture, when items with
restricted volumes, such as traps and containers, are filled with UF₆,
full allowance must be made for the volume changes that will arise over the
working temperature range to which the vessels will be subjected.

For UF₆ to be handled as
a liquid, the pressure must be in excess of 0.15 mPa (1.5 atm) and the
temperature above 147°F (64°C) because the sublimation temperature lies below
the triple point. Thus, any process using liquid UF₆ is above
atmospheric pressure and is subject to a potential leakage of UF₆ to
the environment, with vapor loss and cooling occurring simultaneously.
Solidification occurs exothermically when the pressure falls below 1.5 atm (0.15
mPa). Thus, if a cylinder heated above the triple point is breached, a rapid
outflow of the UF₆ occurs until the pressure drops sufficiently to
start the solidification process. The rate of outflow then decreases but
continues until the contents cool to about 133°F (56°C), which is the
atmospheric sublimation temperature. Some release of material may continue,
depending on the type and location of the breach.

UF₆ is hygroscopic (i.e.,
moisture-retaining) and, in contact with water (H₂O), will decompose
immediately to uranyl fluoride (UO₂F₂). When heated to
decomposition, UF₆ emits toxic fluoride fumes.

Chemical Properties

Uranium hexafluoride (UF₆)
does not react with oxygen, nitrogen, carbon dioxide, or dry air. However, UF₆
combines with water to form the soluble reaction products uranyl fluoride (UO₂F₂)
and hydrogen fluoride (HF). For this reason, UF₆ is always handled in
leak-tight containers and processing equipment. When UF₆ comes into
contact with water, such as water vapor in the air, the UF₆ and water
react, forming corrosive HF and a uranium-fluoride compound called uranyl
fluoride (UO₂F₂). UF₆ is essentially inert to
clean aluminum, steel, Monel, nickel, aluminum, bronze, copper, and Teflon.
Teflon is commonly used in the packing and cap gasket for cylinders storing
depleted UF₆.

When released to the atmosphere,
gaseous UF₆ combines with humidity to form a cloud of particulate UO₂F₂
and HF fumes. The reaction is very fast and is dependent on the availability of
water vapor. Following a large-scale release of UF₆ in an open area,
the dispersion is governed by meteorological conditions, and the plume could
still contain unhydrolyzed material even after traveling a distance of several
hundred meters. After hydrolysis, UO₂F₂ can be deposited
as a finely divided solid, while HF remains as part of the gas plume.

In enclosed situations, the reaction
products form a dense fog, reducing visibility for occupants of the area and
hindering evacuation and emergency response. Fog can occur in unconfined areas
if the humidity is high.

In a fire, the reaction of UF₆
with water is accelerated because of the increased UF₆ vapor pressure
and the large quantities of water formed in combustion of organic materials or
hydrocarbons. Reaction of liquid UF₆ with hydrocarbon vapors is
extremely vigorous in flames, with formation of UF₄ and low-molecular
weight fluorinated compounds. More heat is generally released in these
hydrocarbon interactions with UF₆ than in the corresponding reactions
of hydrocarbons with oxygen.

Health Effects

Uranium hexafluoride (UF₆)
and related compounds have radiological and chemical characteristics that pose
potential health risks. Depleted uranium hexafluoride also has potential health
and environmental effects.

Radiation Effects

The characteristics of UF₆
pose potential health risks, and the material is handled accordingly. Uranium is
radioactive and decays into a series of other radioactive elements. Therefore,
UF₆ in storage emits low levels of radiation. The radiation levels
measured on the outside surface of filled depleted UF₆ storage
cylinders are typically about 2 to 3 millirem per hour (mrem/h), decreasing to
about 1 mrem/h at a distance of 1 ft (0.3 m).

Chemical Effects

In addition, if UF₆ is
released to the atmosphere, the uranium compounds and hydrogen fluoride (HF)
that are formed by reaction with moisture in the air can be chemically toxic.
Uranium is a heavy metal that, in addition to being radioactive, can have toxic
chemical effects (primarily on the kidneys) if it enters the bloodstream by
means of ingestion or inhalation. HF is an extremely corrosive gas that can
damage the lungs and cause death if inhaled at high enough concentrations.