Uranium hexafluoride

Uranium hexafluoride is an inorganic compound used in the nuclear fuel cycle. Scientists often call it "hex" for short. It is a volatile, white solid that plays a critical role in nuclear technology. Hex is used to enrich uranium for use in nuclear reactors. It is also used in the production of nuclear weapons. Because it can turn into a gas easily, it is a very useful tool for processing uranium.

Creating uranium hexafluoride involves a specific chemical process. First, uranium dioxide is converted into uranium tetrafluoride. This happens when the dioxide is treated with hydrofluoric acid (HF). Next, the uranium tetrafluoride is oxidized using fluorine gas. This final step produces the hexafluoride. Both of these fluorination steps are highly exothermic, meaning they release a lot of heat. If the sample contains uranium trioxide, a different compound called uranyl fluoride may form. This oxyfluoride can also be fluorinated to create the final uranium hexafluoride product.

Hex has unique physical and chemical properties. At standard atmospheric pressure, it undergoes sublimation at 56.5 °C. Sublimation is when a solid turns directly into a gas. The triple point of the substance is 64 °C and 152 kPa. Because this triple point is close to normal conditions, changing its phase requires very little thermodynamic work.

Chemically, hex is a mild oxidant and a Lewis acid. This means it can bind to other substances, such as forming heptafluorouranate(VI). It also reacts strongly with water. When hex meets water, it releases hydrofluoric acid, which is both toxic and corrosive. However, it can react with aluminum to form a protective surface layer that stops further reactions.

The most important use for hex is uranium enrichment. This process separates different isotopes of uranium. Fluorine has only one naturally occurring stable isotope. This means any difference in the weight of a hex molecule comes only from the uranium isotope. Scientists use two main methods for this: gaseous diffusion and the gas centrifuge method. The centrifuge method is much more efficient. Gaseous diffusion requires about 60 times more energy than the centrifuge process.

While diffusion produces fuel that yields 25 times more energy than it uses, centrifuge-produced fuel yields 1,500 times more energy than the process consumes.

Beyond enrichment, hex is used in advanced reprocessing called fluoride volatility. This method was developed in the Czech Republic. In this process, spent nuclear fuel is treated with fluorine gas. This transforms oxides or metals into a mixture of various fluorides. The mixture is then distilled to separate different materials. Some fission products form nonvolatile fluorides, which stay as solids. These solids can be stored as waste or processed further using electrochemical or solvation-based methods.

Enrichment creates a large amount of depleted uranium hexafluoride, known as D or D-. This is a waste product that must be managed carefully. Because it is chemically unstable, storing D presents risks to health and the environment. If D is exposed to moist air, it reacts with water to produce uranyl fluoride and hydrofluoric acid. In 2005, the United States held about 686,000 tonnes of D. This material was kept in 57,122 storage cylinders. These cylinders were located in Portsmouth, Ohio, as well as Oak Ridge, Tennessee, and Paducah, Kentucky.

Safety and disposal are major concerns for the nuclear industry. The steel cylinders used to store hex have an estimated lifetime of only a few decades. They must be inspected regularly for leaks or corrosion. Accidents have occurred, such as a release at the Sequoyah Fuels Corporation in 1986. During that event, an estimated 29,500 pounds of gaseous material escaped. To manage the waste, the U.S. government converts D into solid uranium oxides. Converting the entire stockpile of D could cost between $15 million and $450 million.