Depleted uranium hexafluoride

Depleted uranium hexafluoride is a byproduct of the processing of uranium hexafluoride into enriched uranium. It is one of the chemical forms of depleted uranium, along with depleted triuranium octoxide and depleted uranium metal. DUHF is 1.7 times less radioactive than uranium hexafluoride and natural uranium.

History

The concept of depleted and enriched uranium emerged nearly 150 years after the discovery of uranium by Martin Klaproth in 1789. In 1938, two German physicists Otto Hahn and Fritz Strassmann had made the discovery of the fission of the atomic nucleus of the ²³⁵U isotope, which was theoretically substantiated by Lise Meitner, Otto Robert Frisch and in parallel with them Gottfried von Droste and Siegfried Flügge. This discovery marked the beginning of the peaceful and military use of the nuclear energy of uranium. A year later, Yulii Khariton and Yakov Zeldovich were the first to prove theoretically that with an enrichment of ²³⁵U in natural uranium, a chain reaction could be sustained. This nuclear chain reaction requires on average that at least one neutron, released by the fission of an atom of ²³⁵U, will be captured by another atom of ²³⁵U and will cause it also to fission. The probability of a neutron being captured by a fissile nucleus should be high enough to sustain the reaction. To increase this probability, an increase in the proportion of ²³⁵U is necessary, which in natural uranium constitutes only 0.72%, along with 99.27% ²³⁸U and 0.0055% ²³⁴U.

Competition

By the mid-1960s, the United States had a monopoly on the supply of uranium fuel for Western nuclear power plants. In 1968, the USSR declared its readiness to accept orders for uranium enrichment. As a result, a competitive market formed in the world, and commercial enrichment companies began to appear. In 1971, the first Soviet contract was signed with the French Alternative Energies and Atomic Energy Commission, where nuclear power plants were actively built. In 1973, roughly 10 long-term contracts were signed with power companies from Italy, Germany, Great Britain, Spain, Sweden, Finland, Belgium and Switzerland. By 2017, large commercial enrichment plants have been operating in France, Germany, the Netherlands, Great Britain, the United States, Russia and China. The development of the enrichment market has led to the accumulation of over 2 million tons of DUHF in the world during this period.

Other forms of depleted uranium

Depleted uranium may exist in several chemical forms; in the form of DUHF, the most common form, with a density of 5.09 g/cm³, in the form of depleted triuranium octoxide with a density of 8.38 g/cm³, and in the form of depleted uranium metal with a density of 19.01 g/cm³.

Physical properties

Since the various uranium isotopes share the same chemical properties, the chemical and physical properties of depleted, enriched, and unenriched UF₆ are identical, except for the degree of radioactivity. Like other forms of UF₆, under standard conditions, DUHF forms white crystals, with a density of 5.09 g/cm3. At pressures below 1.5 atm, the solid DUHF sublimes into gas when heated, with no liquid form. At 1 atm, the sublimation point is 56.5 °C. The critical temperature of DUHF is 230.2 °C, and the critical pressure is 4.61 MPa.

Radioactivity

The radioactivity of DUHF is determined by the isotopic composition of uranium because the fluorine in the compound is stable. The radioactive decay rate of natural UF₆ is 1.7×10⁴ Bq/g of which 97.6% is due to ²³⁸U and ²³⁴U.

Uranium isotope	Mass fraction in natural uranium	Half-life, years	Activity of 1 mg of pure isotope	Contribution to the activity of natural uranium
²³⁸U	99.27%	4.51 billion	12.4 Bq	48.8%
²³⁵U	0.72%	704 million	80 Bq	2.4%
²³⁴U	0.0055%	245,000	231000 Bq	48.8%

When uranium is enriched, the content of light isotopes, ²³⁴U and ²³⁵U, increases. Although ²³⁴U, despite its much lower mass fraction, contributes more to the activity, the target isotope for nuclear industry use is ²³⁵U. Therefore, the degree of uranium enrichment or depletion is specified by the content of ²³⁵U. The reduction of ²³⁴U, and to a slight degree ²³⁵U, content reduces the radioactivity below unenriched UF₆.

Type of uranium hexafluoride	Degree of ²³⁵U content	Radioactive decay rate, Bq/g	Activity with respect to natural uranium hexafluoride
Natural	0.72%	1.7×10⁴	100%
Depleted	0.45%	1.2×10⁴	70%
Depleted	-	-	-
Depleted	-	-	-

Production

Low enriched uranium with enrichment of 2 to 5% ²³⁵U is used for nuclear power, in contrast to weapons-grade highly enriched uranium with ²³⁵U content of over 20% and usually over 90%. Various methods of isotope separation are used to produce enriched uranium, mainly gas centrifugation and, in the past, the gaseous diffusion method. Most of them work with gaseous UF₆, which in turn is produced by fluorination of elemental uranium tetrafluoride or uranium oxides, both highly exothermic.
Since UF₆ is the only uranium compound that is gaseous at a relatively low temperature, it plays a key role in the nuclear fuel cycle as a substance suitable for separating ²³⁵U and ²³⁸U. After obtaining enriched UF₆, the remainder is transformed into depleted UF₆, which consists mainly of ²³⁸U, because its ²³⁵U content is reduced by perhaps a factor of three, and its ²³⁴U content by a factor of six. In 2020, nearly two million tons of depleted uranium was accumulated in the world. Most of it is stored in the form of DUHF in special steel tanks.
The methods of handling depleted uranium in different countries depends on their nuclear fuel cycle strategy. The International Atomic Energy Agency recognizes that policy determination is the prerogative of the government. Given the technological capabilities and concepts of the nuclear fuel cycle in each country, with access to separation facilities, DUHF may be considered as a valuable raw material on one hand or low-level radioactive waste on the other. Therefore, there is no unified legal and regulatory status for DUHF in the world. The IAEA expert report, 2001 and the joint report of the OECD, NEA and IAEA Management of Depleted Uranium, 2001 recognize DUHF as a valuable raw material.

Separation plants, country	Accumulated DUHF	Annual increase in DUHF reserves	Form of storage of depleted uranium
USEC / DOE	700	30	UF₆
ROSATOM	640	15	UF₆
EURODIF	200	18	UF₆, U₃O₈
BNFL	44	0	UF₆
URENCO	43	6	UF₆
JNFL, PNC	38	0.7	UF₆
CNNC	30	1.5	UF₆
SA NEC	3	0	UF₆
Others	<1.5	0	-
Total	≈ 1700	≈ 70	UF₆,

Applications

As a result of chemical conversion of DUHF, anhydrous hydrogen fluoride and/or its aqueous solution are obtained, which have a certain demand in non-nuclear energy markets, such as the aluminum industry, in production of refrigerants, herbicides, pharmaceuticals, high-octane gasoline, plastics, etc. It is also applied in the reuse of hydrogen fluoride in the production of UF₆ via the conversion of U₃O₈ into uranium tetrafluoride, before further fluorination into UF₆.

Processing

There are multiple directions in the world practice of DUHF reprocessing. Some of them have been tested in a semi-industrial setting, while others have been and are being operated on an industrial scale with an effort to reduce the reserves of uranium tailings and provide the chemical industry with hydrofluoric acid and industrial organofluorine products.

Method	Reaction	Final product
Pyrohydrolysis	UF₆ + H₂O → UO₂F₂ + 4 HF 3 UO₂F₂ + 3 H₂O → U₃O₈ + 6 HF + ½ O₂	Triuranium octoxide and hydrofluoric acid
Pyrohydrolysis in a fluidized bed		Uranium dioxide density up to 6 g/cm3 and hydrofluoric acid
Hydrogen recovery	UF₆ + H₂ → UF₄ + 2 HF	Uranium tetrafluoride and hydrogen fluoride
Recovery via organic compounds	UF₆ + CHCI = CCI₂ → UF₄ + CHCIF - CCI₂F	Uranium tetrafluoride, refrigerants, including ozone-safe
Recovery via organic compounds	UF₆ + CCI₄ → UF₄ + CF₂CI₂ + CI₂	Uranium tetrafluoride and methane-type refrigerants
Plasma-chemical conversion	UF₆ + 3 H - OH → 1/3 U₃O₈ + 6 HF + 1/6 O₂	Triuranium octoxide and hydrogen fluoride
Radiation-chemical recovery UF₆	UF₆ + 2 e → UF₄ + 2 F	Uranium tetrafluoride and fluorine.

Depending on nuclear fuel cycle strategy, technological capabilities, international conventions and programs, such as the Sustainable Development Goals and the UN Global Compact, each country approaches the issue of the use of accumulated depleted uranium individually. The United States has adopted a number of long-term programs for the safe storage and reprocessing of DUHF stocks prior to their final disposal.