Uranium(III) hydride

Uranium hydride is a brownish black pyrophoric powder. 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 and decomposes in nitric acid.

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.{{Cite book |title=The Encyclopedia of the Chemical Elements |chapter=Uranium |year=1968 |author-link=Glenn T. Seaborg |first=Glenn T. |last=Seaborg |publisher=Reinhold Book Corporation |location=Skokie, Illinois |page=782|id=LCCCN 68-29938}} 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.{{cite book|author1=Gerd Meyer|author2=Lester R. Morss|title=Synthesis of lanthanide and actinide compounds|url=https://books.google.com/books?id=bnS5elHL2w8C&pg=PA44|access-date=11 October 2011|year=1991|publisher=Springer|isbn=978-0-7923-1018-1|pages=44–}} Both α- and β-UH₃ are ferromagnetic at temperatures below ~180 K. Above 180 K, they are paramagnetic.{{cite book|author=K. H. J. Buschow|title=Concise encyclopedia of magnetic and superconducting materials|url=https://books.google.com/books?id=N9mvytGEBtwC&pg=PA901|access-date=11 October 2011|year=2005|publisher=Elsevier|isbn=978-0-08-044586-1|pages=901–}}

Exposure of uranium metal to hydrogen at 250 °C gives the trihydride:

:{{chem2|2 U + 3H2 -> 2UH3}}

Bulk uranium metal crumbles into a fine powder during the course of the reaction.{{cite book |doi=10.1002/14356007.a27_281.pub2 |chapter=Uranium, Uranium Alloys, and Uranium Compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2007 |last1=Peehs |first1=Martin |last2=Walter |first2=Thomas |last3=Walter |first3=Sabine |last4=Zemek |first4=Martin |isbn=978-3-527-30385-4 }}{{cite book|author1=Egon Wiberg|author2=Nils Wiberg|author3=Arnold Frederick Holleman|title=Inorganic chemistry|url=https://books.google.com/books?id=Mtth5g59dEIC&pg=PA239|access-date=11 October 2011|year=2001|publisher=Academic Press|isbn=978-0-12-352651-9|pages=239–}}

The process is reminiscent of hydrogen embrittlement but uranium hydride is not an interstitial compound. Instead, according to X-ray crystallography, each uranium atom is surrounded by 12 atoms of hydrogen (defect perovskite structure). Each hydrogen atom occupies a large tetrahedral hole in the lattice.{{cite book|url=https://books.google.com/books?id=u0ZdEdgLlP0C&pg=PA789|title=Text Book Of Inorganic Chemistry|author=Amit Arora|page=789|publisher=Discovery Publishing House|year=2005 |isbn=81-8356-013-X|access-date=2010-02-07}} The density of hydrogen in uranium hydride is approximately the same as in liquid water or in liquid hydrogen.{{cite book|url=https://books.google.com/books?id=VW7Nl0L0GeoC&pg=PA393|title=Alternative Energy Systems in Building Design (GreenSource Books)|author=Peter Gevorkian|page=393|publisher=McGraw Hill Professional|year=2009|isbn=978-0-07-162147-2 |access-date=2010-02-07}} The U-H-U linkage through a hydrogen atom is present in the structure.{{cite book|url=https://books.google.com/books?id=0l3bYxwBWMcC&pg=PA218|title=Environmental Pollution |author=G. Singh|publisher=Discovery Publishing House|year=2007|isbn=978-81-8356-241-6 |access-date=2010-02-07}}

Uranium hydride forms when uranium metal (e.g. in Magnox fuel with corroded cladding) becomes exposed to water or steam, with uranium dioxide as byproduct:

:7 U + 6 H₂O → 3 UO₂ + 4 UH₃

The resulting uranium hydride is pyrophoric; if the metal (e.g. a damaged fuel rod) is exposed to air afterwards, excessive heat may be generated and the bulk uranium metal itself can ignite.{{cite journal|url=https://books.google.com/books?id=bAwAAAAAMBAJ&pg=PA49|page=49|title=Rust never sleeps|journal=Bulletin of the Atomic Scientists |year= 1994|access-date=2010-02-07|volume=50|issue=5}} Hydride-contaminated uranium can be passivated by exposure to a gaseous mixture of 98% helium with 2% oxygen.{{cite web |url=http://teton.if.uidaho.edu/emsp/overviewflowviz.html |title=EMSP |publisher=Teton.if.uidaho.edu |access-date=2010-02-07 |archive-url=https://web.archive.org/web/20090930205434/http://teton.if.uidaho.edu/emsp/overviewflowviz.html |archive-date=2009-09-30 }} Condensed moisture on uranium metal promotes formation of hydrogen and uranium hydride; a pyrophoric surface may be formed in absence of oxygen.{{cite book|url=https://books.google.com/books?id=hcPYAAANpGYC&pg=PT16|title=Advanced nuclear fuel cycles and radioactive waste management|page=176|author=OECD Nuclear Energy Agency|publisher=OECD Publishing|year= 2006|isbn=92-64-02485-9|access-date=2010-02-07}} This poses a problem with underwater storage of very special spent nuclear fuel in spent fuel ponds (nuclear fuel from commercial nuclear plants does not contain any uranium metal). Depending on the size and distribution on the hydride particles, self-ignition can occur after an indeterminate length of exposure to air.{{cite book|url=https://books.google.com/books?id=CrIz5k3ALv8C&pg=PA197|title=Stabilisation/Solidification Treatment and Remediation: Proceedings of the International Conference on Stabilisation/Solidification Treatment and Remediation, 12–13 April 2005, Cambridge, UK|author1=Abir Al-Tabbaa |author2=J. A. Stegemann |page=197| publisher=Taylor & Francis|year=2005|isbn=0-415-37460-X|access-date=2010-02-07}} Such exposure poses risk of self-ignition of fuel debris in radioactive waste storage vaults.{{cite book|url=https://books.google.com/books?id=UWURYnLlQzQC&pg=PA278|page=278|title=International Conference on Nuclear Decom 2001: ensuring safe, secure and successful decommissioning: 16–18 October 2001 Commonwealth Conference and Events Centre, London UK, Issue 8|publisher=John Wiley and Sons|year=2001|access-date=2010-02-07|isbn=1-86058-329-6}}

Uranium hydride exposed to water evolves hydrogen. In contact with strong oxidizers this may cause fire and explosions. Contact with halocarbons may cause a violent reaction.{{cite web |url=http://www.osha.gov/SLTC/healthguidelines/uraniuminsolublecompounds/recognition.html |title=Uranium & Insoluble Compounds |publisher=Osha.gov |access-date=2010-02-07 |archive-url=https://web.archive.org/web/20100322215227/http://www.osha.gov/SLTC/healthguidelines/uraniuminsolublecompounds/recognition.html |archive-date=2010-03-22 }}

UH₃ releases hydrogen upon heating to near 400 °C. In this way bulk uranium can be transformed to a powder with high surface area. The resulting powder is extremely reactive toward H₂ even at -80 °C.{{cite book|author=M. Baudler|chapter=Hydrogen, Deuterium, Water|title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY,NY|volume=2pages=114}}

Hydrogen, deuterium, and tritium can be purified by reacting with uranium, then thermally decomposing the resulting hydride/deuteride/tritide.{{cite book|url=https://books.google.com/books?id=MYCBJIKpC2gC&pg=PA104|title=Thermophysical properties of lithium hydride, deuteride, and tritide and of their solutions with lithium|page=104 |publisher=Springer|year=1987|isbn=0-88318-532-6|author=E. E. Shpil'rain |access-date=2010-02-07}} Extremely pure hydrogen has been prepared from beds of uranium hydride for decades.{{cite book|url=https://books.google.com/books?id=Na8jRpkPffkC&pg=PA263|page=264 |title=Hydrogen energy system: production and utilization of hydrogen and future aspects|publisher=Springer|year= 1995|access-date=2010-02-07|isbn=0-7923-3601-1|author=Yuda Yürüm}} Heating uranium hydride is a convenient way to introduce hydrogen into a vacuum system.{{cite book|url=https://books.google.com/books?id=yBmnnaODnHgC&pg=PA121|title=Handbook of electron tube and vacuum techniques|author=Fred Rosebury|page=121|publisher=Springer|year=1992|isbn=1-56396-121-0 |access-date=2010-02-07}} Uranium tritide (UT) is used for the safe and efficient storage of tritium, since gaseous tritium is harder to contain and work with. UT is formed by combining tritium and uranium at room temperature. The tritium can be later extracted by heating the UT. Tritium and its decay product ³He are extracted at different temperatures.{{Cite web |date=2022 |title=NIS - Tritium Uses |url=https://www.nuclearinfo.org/wp-content/uploads/2022/05/Tritium_uses_nd..pdf |access-date=2024-12-12 |website=NIS Nuclear Info}}

On heating with diborane, uranium hydride produces uranium boride.{{cite book|url=https://books.google.com/books?id=qi0vujzNSz4C&pg=PA235|title=Advances in inorganic chemistry and radiochemistry|volume=16|author= Harry Julius Emeléus|page=235|publisher=Academic Press|year=1974|isbn=0-12-023616-8 |access-date=2010-02-07|author-link=Harry Julius Emeléus}} With bromine at 300 °C, uranium(IV) bromide is produced. With chlorine at 250 °C, uranium(IV) chloride is produced. Hydrogen fluoride at 20 °C produces uranium(IV) fluoride. Hydrogen chloride at 300 °C produces uranium(III) chloride. Hydrogen bromide at 300 °C produces uranium(III) bromide. Hydrogen iodide at 300 °C produces uranium(III) iodide. Ammonia at 250 °C produces uranium(III) nitride. Hydrogen sulfide at 400 °C produces uranium(IV) sulfide. Oxygen at 20 °C produces triuranium octoxide. Water at 350 °C produces uranium dioxide.{{cite book|url=https://books.google.com/books?id=SvAbtU6XvzgC&pg=PT12|title=Lanthanide and actinide chemistry|author=Simon Cotton|page=170|publisher=John Wiley and Sons|year=2006|isbn=0-470-01006-1 |access-date=2010-02-07}}

Polystyrene-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.{{cite book|url=https://books.google.com/books?id=KoTve97yYB8C&pg=PA211|title=Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945|isbn=0-521-54117-4|publisher=Cambridge University Press|year=2004|author=Lillian Hoddeson|page=211 |access-date=2010-02-07|display-authors=etal}}

Uranium hydride enriched to about 5% uranium-235 has been proposed as a combined nuclear fuel/neutron moderator for the Hydrogen Moderated Self-regulating Nuclear Power Module. According to the aforementioned patent application, the reactor design in question begins producing power when hydrogen 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.{{cite web |url=https://patents.google.com/patent/US20080069289|title=Patent Application 11/804450: Self-regulating nuclear power module |last=Peterson|first=Otis G. |date=2008-03-20 |work=United States Patent Application Publication |publisher=United States Patent and Trademark Office, Federal Government of the United States, Washington, DC, USA |access-date=2009-09-05}} Uranium hydride is both a nuclear fuel and a neutron moderator; 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 {{convert|800|°C|-2}}, 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 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 hydride ion may interfere with some mass spectrometry measurements, appearing as a peak at mass 239, creating false increase of signal for plutonium-239.{{cite book|url=https://books.google.com/books?id=W3FnEOg8tS4C&pg=PA243|title=Nuclear forensic analysis|author1=Kenton James Moody |author2=Ian D. Hutcheon |author3=Patrick M. Grant |page=243|publisher=CRC Press|year=2005|isbn=0-8493-1513-1 |access-date=2010-02-07}}

Uranium hydride slugs were used in the "tickling the dragon's tail" series of experiments to determine the critical mass of uranium.{{cite web |url=http://www.mphpa.org/classic/LA/Photo-Pages-2/LAP-217.htm |title=Photo – Tickling the Dragon's Tail |publisher=Mphpa.org |date=2005-08-03 |access-date=2010-02-07 |archive-date=2010-02-18 |archive-url=https://web.archive.org/web/20100218112800/http://www.mphpa.org/classic/LA/Photo-Pages-2/LAP-217.htm }}

Uranium hydride and uranium deuteride were suggested as a fissile material for a uranium hydride bomb. The tests with uranium hydride and uranium deuteride during Operation Upshot–Knothole were disappointing, however. During the early phases of the Manhattan Project, 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.{{cite journal|url=https://books.google.com/books?id=VAwAAAAAMBAJ&pg=PA2|page=2|title=Lying well|journal=Bulletin of the Atomic Scientists |date= July 1994|access-date=2010-02-07|volume=50|issue=4|doi=10.1080/00963402.1994.11456528|bibcode=1994BuAtS..50d...2M|last1=Moore|first1=Mike|url-access=subscription}}

• UH₄, Uranium(IV) hydride

• {{cite book|url=https://books.google.com/books?id=eSAkBkAZ-J4C&pg=PA232|title=Plasma and high frequency processes for obtaining and processing materials in the nuclear fuel cycle|publisher=Nova Publishers|year=2003|isbn=1-59033-009-9 |page=232|author=I. N. Toumanov|access-date=2010-02-07}}
• {{Cite book |title=The Encyclopedia of the Chemical Elements |chapter=Uranium |year=1968 |author-link=Glenn T. Seaborg |first=Glenn T. |last=Seaborg |publisher=Reinhold Book Corporation |location=Skokie, Illinois |page=782|id=LCCCN 68-29938}}

{{reflist|30em}}

{{uranium compounds}}

{{hydrides by group}}

Category:Metal hydrides

Category:Nuclear materials

Category:Nuclear fuels

Category:Neutron moderators

Category:Reducing agents

Category:Uranium(III) compounds

Category:Ferromagnetic materials

Category:Pyrophoric materials