Uranium hydride, also called uranium trihydride (UH₃), is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...

and a

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

Properties

Uranium hydride is a highly toxic, brownish grey to brownish black

pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...

powder or brittle solid. Its density at 20 °C is 10.95 g cm⁻³, much lower than that of uranium (19.1 g cm⁻³). It has a metallic conductivity, is slightly soluble in

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...

and decomposes in

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

. Two crystal modifications of uranium hydride exist, both cubic: an α form that is obtained at low temperatures and a β form that is grown when the formation temperature is above 250 °C. After growth, both forms are metastable at room temperature and below, but the α form slowly converts to the β form upon heating to 100 °C. Both α- and β-UH₃ are

ferromagnetic Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

at temperatures below ~180 K. Above 180 K, they are paramagnetic.

Formation in uranium metal

Hydrogen gas reaction

Exposure of uranium metal to hydrogen leads to

hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...

. Hydrogen diffuses through metal and forms a network of brittle hydride over the

grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and therma ...

. Hydrogen can be removed and ductility renewed by annealing in

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...

. Uranium metal heated to 250 to 300

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...

(482 to 572 °F) reacts with

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

to form uranium hydride. Further heating to about 500 °C will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium

carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of t ...

nitride In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...

, and

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a flu ...

compounds. The reversible reaction proceeds as follows: :2 U + 3 H₂ 2 UH₃ Uranium hydride is not an

interstitial compound In materials science, an interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure. When the atom is of the same type as those alread ...

, causing the metal to expand upon hydride formation. In its

lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...

, each uranium atom is surrounded by 6 other uranium

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...

s and 12 atoms of

; each hydrogen atom occupies a large tetrahedral hole in the lattice. The density of hydrogen in uranium hydride is approximately the same as in liquid water or in

liquid hydrogen Liquid hydrogen (LH2 or LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully l ...

. The U-H-U linkage through a hydrogen atom is present in the structure.

Water reaction

Uranium hydride forms when uranium metal (e.g. in

Magnox Magnox is a type of nuclear power/production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The n ...

fuel with corroded

cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...

) becomes exposed to water; the reaction proceeds as follows: :7 U + 6 H₂O → 3 UO₂ + 4 UH₃ The resulting uranium hydride is pyrophoric; if the metal (e.g. a damaged

fuel rod Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...

) is exposed to air afterwards, excessive heat may be generated and the bulk uranium metal itself can ignite. Hydride-contaminated uranium can be passivated by exposure to a gaseous mixture of 98%

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

with 2%

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

. Condensed moisture on uranium metal promotes formation of hydrogen and uranium hydride; a pyrophoric surface may be formed in absence of oxygen. This poses a problem with underwater storage of

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...

in spent fuel ponds. Depending on the size and distribution on the hydride particles, self-ignition can occur after an indeterminate length of exposure to air. Such exposure poses risk of self-ignition of fuel debris in radioactive waste storage vaults. Uranium metal exposed to steam produces a mixture of uranium hydride and

uranium dioxide Uranium dioxide or uranium(IV) oxide (), also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear re ...

. Uranium hydride exposed to water evolves hydrogen. In contact with strong oxidizers this may cause fire and explosions. Contact with

halocarbon Halocarbon compounds are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms (fluorine, chlorine, bromine or iodine – ) resulting in the formation of organofluorine compounds, organochlor ...

s may cause a violent reaction.

Other chemical reactions

Polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the Aromatic hydrocarbon, aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin pe ...

-impregnated uranium hydride powder is non-pyrophoric and can be pressed, however its hydrogen-carbon ratio is unfavorable. Hydrogenated polystyrene was introduced in 1944 instead. Uranium deuteride is said to be usable for design of some types of

neutron initiator A modulated neutron initiator is a neutron source capable of producing a burst of neutrons on activation. It is a crucial part of some nuclear weapons, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration i ...

s. Uranium hydride enriched to about 5%

uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exi ...

is proposed as a combined

nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergo ...

neutron moderator In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely m ...

for the Hydrogen Moderated Self-regulating Nuclear Power Module. According to the aforementioned patent application, the reactor design in question begins producing power when

gas at a sufficient temperature and pressure is admitted to the core (made up of granulated uranium metal) and reacts with the uranium metal to form uranium hydride. Uranium hydride is both a

and a

; apparently it, like other neutron moderators, will slow neutrons sufficiently to allow for fission reactions to take place; the uranium-235 atoms within the hydride also serve as the nuclear fuel. Once the nuclear reaction has started, it will continue until it reaches a certain temperature, approximately , where, due to the chemical properties of uranium hydride, it chemically decomposes and turns into hydrogen gas and uranium metal. The loss of neutron moderation due to the chemical

decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and ...

of the uranium hydride will consequently slow — and eventually halt — the reaction. When temperature returns to an acceptable level, the hydrogen will again combine with the uranium metal, forming uranium hydride, restoring moderation and the nuclear reaction will start again. Uranium zirconium hydride (UZrH), a combination of uranium hydride and zirconium(II) hydride, is used as a fuel/moderator in the

TRIGA TRIGA (Training, Research, Isotopes, General Atomics) is a class of nuclear research reactor designed and manufactured by General Atomics. The design team for TRIGA, which included Edward Teller, was led by the physicist Freeman Dyson. Design ...

-class reactors. On heating with

diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...

, uranium hydride produces

uranium boride Uranium boride (UB2), a compound of uranium and boron, is a very stable glassy boride material that is insoluble in water. It is being explored as an ingredient in high entropy alloys, and as a method of immobilizing uranium-based radioactive was ...

. With

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

at 300 °C, uranium(IV) bromide is produced. With

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

at 250 °C, uranium(IV) chloride is produced.

Hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock ...

at 20 °C produces uranium(IV) fluoride.

Hydrogen chloride The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chlorid ...

at 300 °C produces

uranium(III) chloride Uranium(III) chloride, UCl3, is a water soluble salt of uranium. UCl3 is used mostly to reprocess spent nuclear fuel. Uranium(III) chloride is synthesized in various ways from uranium(IV) chloride; however, UCl3 is less stable than UCl4. Prepar ...

Hydrogen bromide Hydrogen bromide is the inorganic compound with the formula . It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room tem ...

at 300 °C produces uranium(III) bromide.

Hydrogen iodide Hydrogen iodide () is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under ...

at 300 °C produces uranium(III) iodide.

Ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...

at 250 °C produces uranium(III) nitride.

Hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...

at 400 °C produces uranium(IV) sulfide.

Oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

at 20 °C produces triuranium octoxide. Water at 350 °C produces

. Uranium hydride ion may interfere with some

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is u ...

measurements, appearing as a peak at mass 239, creating false increase of signal for plutonium-239.

History

Uranium hydride slugs were used in the " tickling the dragon's tail" series of experiments to determine the

critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fi ...

of uranium. Uranium hydride and uranium

deuteride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

were suggested as a

fissile material In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be t ...

for a

uranium hydride bomb The uranium hydride bomb was a variant design of the atomic bomb first suggested by Robert Oppenheimer in 1939 and advocated and tested by Edward Teller. It used deuterium, an isotope of hydrogen, as a neutron moderator in a uranium-deuterium ce ...

. The tests with uranium hydride and uranium deuteride during

Operation Upshot–Knothole Operation Upshot–Knothole was a series of eleven nuclear test shots conducted in 1953 at the Nevada Test Site. It followed ''Operation Ivy'' and preceded ''Operation Castle''. Over 21,000 soldiers took part in the ground exercise Desert Ro ...

were disappointing, however. During the early phases of the

Manhattan Project The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...

, in 1943, uranium hydride was investigated as a promising bomb material; it was abandoned by early 1944 as it turned out that such a design would be inefficient.

Applications

Hydrogen,

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...

, and

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...

can be purified by reacting with uranium, then thermally decomposing the resulting hydride/deuteride/tritide. Extremely pure hydrogen has been prepared from beds of uranium hydride for decades. Heating uranium hydride is a convenient way to introduce hydrogen into a vacuum system. The swelling and pulverization at uranium hydride synthesis can be used for preparation of very fine uranium metal, if the powdered hydride is thermally decomposed. Uranium hydride can be used for

isotope separation Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" ...

, preparing uranium metal powder, and as a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth met ...

References

{{uranium compounds Metal hydrides Nuclear materials Nuclear fuels Neutron moderators Reducing agents Uranium