Natural uranium consists of two isotopes of uranium, uranium-235 and uranium-238. Uranium-235 is by far the smaller fraction of the uranium, comprising only 71 parts of a thousand. It is quite valuable because it is the only naturally occurring fissile material (a fissile nucleus can be fissioned by a single slowed-down neutron). But because there is no chemical difference between uranium-235 and uranium-238, separating the two from one another, or “enriching” them, is quite difficult.
In our current approach to generating nuclear power in the United States, we take uranium that has been mined and enrich the amount of uranium-235 from a mere 0.7% up to about 2-3% for use in nuclear fuel. To accomplish this enrichment process, we have to convert the uranium from its natural, chemically stable oxide form to a gas that can be used in the enrichment process. Uranium hexafluoride (UF6), sometimes just called “hex”, is the form of uranium used in enrichment processes.
Then, to make hex, the uranium tetrafluoride is contacted with fluorine gas (F2), to “boost” its number of fluoride ions from four to six.
After enrichment, a large amount of uranium hexafluoride at a natural level of enrichment (NUF6) has been converted into enriched uranium hexafluoride (EUF6) and depleted uranium hexafluoride (DUF6). Most of the material is the depleted UF6, while the enriched UF6 is sent along to make enriched nuclear fuel.
Roughly 700,000 metric tonnes of depleted UF6 has been accumulating at uranium enrichment plants in Portsmouth, Ohio; Oak Ridge, Tennessee; and Paducah, Kentucky for many decades. Since the DUF6 does not have the nuclear fuel our current reactors need, it is of little economic value. The DUF6 is stored primarily in 14-ton cylinders, but the long-term storage of DUF6 is a concern because of the chemical instability of UF6. When UF6 is exposed to moist air, it reacts with the water in the air to produce UO2F2 (uranyl fluoride) and HF (hydrogen fluoride) both of which are toxic. The solution to this problem is to convert the DUF6 into a form that is chemically stable and insoluble, the same form in which is was originally found, uranium dioxide.
But for a future of thorium-fueled liquid-fluoride reactors, there is even another reason to convert DUF6 to DUO2–to recover the valuable fluorine. Each molecule of DUF6 has six fluoride ions, and fluoride reactors will need them.
Converting DUF6 back to depleted uranium dioxide (DUO2) can be done by first reducing UF6 with gaseous hydrogen. The UF6 gives up two of its fluoride ions to the hydrogen, turning into UF4 and forming HF.