View source for Plutonium(IV) oxide

{{short description|Chemical compound}}
{{Chembox
| Verifiedfields = changed
| Watchedfields  = changed
| verifiedrevid  = 403840273
| ImageFile      = Pudioxide.png
| ImageFile_Ref  = {{Chemboximage|correct|??}}
| ImageSize      = 244
| ImageName      = Unit cell, ball and stick model of plutonium(IV) oxide
| ImageFile1     = [[File:Plutoniumdioxid.jpg|frameless|center]]
| IUPACName      = Plutonium(IV) oxide
| SystematicName = Plutonium(4+) oxide
| OtherNames     = Plutonium dioxide
| Section1       = {{Chembox Identifiers
| CASNo = 12059-95-9
| CASNo_Ref = {{cascite|correct|CAS}}
| EINECS = 235-037-3
| ChemSpiderID = 10617028
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| PubChem = 9795444
| SMILES = [O-2].[O-2].[Pu+4]
| StdInChI = 1S/2O.Pu/q2*-2;+4
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = FLDALJIYKQCYHH-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
}}
| Section2       = {{Chembox Properties
| Pu=1 | O=2
| Appearance = Dark yellow crystals
| Density = 11.5 g cm<sup>−3</sup>
| MeltingPtC = 2744
| BoilingPtC = 2800}}
| Section3       = {{Chembox Structure
| CrystalStruct = [[Fluorite structure|Fluorite]] (cubic), [[Pearson symbol|''cF12'']]
| SpaceGroup = Fm<u style="text-decoration:overline">3</u>m, No. 225 
| Coordination = Tetrahedral (O<sup>2−</sup>); cubic (Pu<sup>IV</sup>)
| LattConst_a = 539.5 pm<ref name="cry">{{cite journal |author1=Christine Guéneau |author2=Alain Chartier |author3=Paul Fossati |author4=Laurent Van Brutzel |author5=Philippe Martin |title=Thermodynamic and Thermophysical Properties of the Actinide Oxides |journal=Comprehensive Nuclear Materials 2nd Ed. |date=2020 |volume=7 |pages=111–154 |doi=10.1016/B978-0-12-803581-8.11786-2 |isbn=9780081028667 |language=en}}</ref>}}
| Section4       = {{Chembox Hazards
| MainHazards = Radioactive
| NFPA-H = 4
| NFPA-F = 0
| NFPA-R = 0
| NFPA-S = RA
| FlashPt_notes = non-flammable}}
| Section7       = {{Chembox Related
|OtherAnions = 
|OtherCations = [[Uranium(IV) oxide]]<br>[[Neptunium(IV) oxide]]<br>[[Americium(IV) oxide]]
| OtherCompounds = 
}}
}}
'''Plutonium(IV) oxide''', or '''plutonia''', is a [[chemical compound]] with the [[chemical formula|formula]] [[plutonium|Pu]][[oxygen|O]]<sub>2</sub>. This high melting-point solid is a principal compound of [[plutonium]]. It can vary in color from yellow to olive green, depending on the particle size, temperature and method of production.<ref>{{cite news|url=http://arq.lanl.gov/source/orgs/nmt/nmtdo/AQarchive/3rdQuarter08/page3.shtml|title=Nitric acid processing|publisher=Los Alamos Laboratory}}</ref>

==Structure==
PuO<sub>2</sub> crystallizes in the [[fluorite]] motif, with the Pu<sup>4+</sup> centers organized in a [[face-centered cubic]] array and oxide ions occupying tetrahedral holes.<ref>{{Greenwood&Earnshaw1st|page=1471}}</ref> PuO<sub>2</sub> owes its utility as a nuclear fuel to the fact that vacancies in the octahedral holes allows room for fission products. In nuclear fission, one atom of plutonium splits into two. The vacancy of the octahedral holes provides room for the new product and allows the PuO<sub>2</sub> monolith to retain its structural integrity.{{citation needed|date=December 2012}}

At high temperatures PuO<sub>2</sub> tends to lose oxygen, becoming sub-stoichiometric PuO<sub>2−x</sub>, with the introduction of lower valence Pu<sup>3+</sup>. This continues into the molten liquid state where the local Pu-O coordination number drops to predominantly 6-fold, compared to 8-fold in the stoichiometric fluorite structure.<ref>{{cite journal |last1=Wilke |first1=Stephen |last2=Benmore |first2=Chris |last3=Alderman |first3=Oliver |last4=Sivaraman |first4=Ganesh |last5=Ruehl |first5=Matthew |last6=Hawthorne |first6=Krista |last7=Tamalonis |first7=Anthony |last8=Andersson |first8=David |last9=Williamson |first9=Mark |last10=Weber |first10=Richard |title=Plutonium oxide melt structure and covalency |journal=Nature Materials |date=2024 |volume=23 |issue=7 |pages=884–889 |doi=10.1038/s41563-024-01883-3 |pmid=38671164 |bibcode=2024NatMa..23..884W |url=https://www.nature.com/articles/s41563-024-01883-3|url-access=subscription }}</ref>

==Properties==
Plutonium dioxide is a stable ceramic material with an extremely low solubility in water and with a high melting point (2,744&nbsp;°C). The melting point was revised upwards in 2011 by several hundred degrees, based on evidence from rapid laser melting studies which avoid contamination by any container material.<ref name="PuO2_melt">{{cite journal|last1=De Bruycker|first1=F.|last2=Boboridis|first2=K.|last3=Pöml|first3=P.|last4=Eloirdi|first4=R.|last5=Konings|first5=R. J. M.|last6=Manara|first6=D.|title=The melting behaviour of plutonium dioxide: A laser-heating study|journal=Journal of Nuclear Materials|volume=416|issue=1–2|pages=166–172|doi=10.1016/j.jnucmat.2010.11.030|bibcode=2011JNuM..416..166D|year=2011}}</ref>

As with all [[plutonium]] compounds, it is subject to control under the [[Nuclear Non-Proliferation Treaty]].

==Synthesis==
Plutonium spontaneously oxidizes to PuO<sub>2</sub> in an atmosphere of oxygen. Plutonium dioxide is mainly produced by [[calcination]] of plutonium(IV) oxalate, Pu(C<sub>2</sub>O<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O, at 300&nbsp;°C. Plutonium oxalate is obtained during the [[Nuclear reprocessing|reprocessing]] of nuclear fuel as plutonium is dissolved in a solution of [[Nitric acid|nitric]] and [[hydrofluoric acid]].<ref name="NETS2013">{{cite journal|last1=Jeffrey A. Katalenich Michael R. Hartman Robert C. O’Brien Steven D. Howe|title=Fabrication of Cerium Oxide and Uranium Oxide Microspheres for Space Nuclear Power Applications|journal=Proceedings of Nuclear and Emerging Technologies for Space 2013|date=Feb 2013|page=2|url=https://inldigitallibrary.inl.gov/sti/5680982.pdf|access-date=2016-07-27|archive-date=2016-10-07|archive-url=https://web.archive.org/web/20161007222613/https://inldigitallibrary.inl.gov/sti/5680982.pdf|url-status=dead}}</ref> Plutonium dioxide can also be recovered from [[molten-salt reactor|molten-salt breeder reactors]] by adding sodium carbonate to the fuel salt after any remaining uranium is removed from the salt as its hexafluoride.

==Applications==
[[File:Plutonium pellet.jpg|thumb|left|A pellet of dioxide of plutonium-238 displays [[incandescence]] after prolonged time of thermal isolation under [[asbestos]].]]
PuO<sub>2</sub>, along with [[uranium dioxide|UO<sub>2</sub>]], is used in [[MOX fuel]]s for [[nuclear reactor]]s. [[Plutonium-238]] dioxide is used as fuel for several deep-space spacecraft such as the [[Cassini–Huygens|Cassini]], [[Voyager program|Voyager]], [[Galileo (spacecraft)|Galileo]] and [[New Horizons]] probes as well as in the [[Curiosity (rover)|Curiosity]] and [[Perseverance (rover)|Perseverance]] rovers on [[Mars]]. The isotope decays by emitting α-particles, which then generate heat (see [[radioisotope thermoelectric generator]]). There have been concerns that an accidental re-entry into Earth's atmosphere from orbit might lead to the break-up and/or burn-up of a spacecraft, resulting in the dispersal of the plutonium, either over a large tract of the planetary surface or within the upper atmosphere.  However, although at least two spacecraft carrying PuO<sub>2</sub> RTGs have reentered the Earth's atmosphere and burned up ([[Nimbus B|Nimbus B-1]] in May 1968 and the [[Apollo 13]] Lunar Module in April 1970),<ref>{{cite book|title=Space Nuclear Power|author=A. Angelo Jr. and D. Buden|year=1985|publisher=Krieger Publishing Company|isbn=0-89464-000-3}}</ref><ref>{{cite web|url=http://fti.neep.wisc.edu/neep602/SPRING00/lecture39.pdf|title=General Safety Considerations|format=PDF lecture notes|publisher=Fusion Technology Institute, [[University of Wisconsin–Madison]]|date=Spring 2000|access-date=2017-10-20|archive-date=2018-09-15|archive-url=https://web.archive.org/web/20180915105431/http://fti.neep.wisc.edu/neep602/SPRING00/lecture39.pdf|url-status=dead}}</ref> the RTGs from both spacecraft survived reentry and impact intact, and no environmental contamination was noted in either instance; in fact, the Nimbus RTG was recovered intact from the Pacific Ocean seafloor and launched aboard [[Nimbus 3]] one year later. In any case, RTGs since the mid-1960s have been designed to remain intact in the event of reentry and impact, following the 1964 launch failure of [[Transit (satellite)|Transit 5-BN-3]] (the early-generation plutonium RTG on board disintegrated upon reentry and dispersed radioactive material into the atmosphere north of [[Madagascar]], prompting a redesign of all U.S. RTGs then in use or under development).<ref>{{cite web|url=http://www.astronautix.com/craft/transit.htm |archive-url=https://web.archive.org/web/20020624022922/http://www.astronautix.com/craft/transit.htm |url-status=dead |archive-date=June 24, 2002 |title=Transit |publisher=Encyclopedia Astronautica |access-date=2013-05-07}}</ref>

Physicist Peter Zimmerman, following up a suggestion by [[Ted Taylor (physicist)|Ted Taylor]], calculated that a low-yield (1-[[kiloton]]) [[nuclear weapon]] could be made relatively easily from plutonium dioxide.<ref>{{cite news |author1=Michael Singer |author2=David Weir |author3=Barbara Newman Canfield |name-list-style=amp |date=Nov 26, 1979 |title=Nuclear Nightmare: America's Worst Fear Come True |url=https://books.google.com/books?id=2OECAAAAMBAJ&pg=PA43 |publisher=New York Magazine}}</ref>  Such bomb would require a considerably larger [[critical mass]] than one made from elemental plutonium (almost three times larger, even with the dioxide at maximum crystal density; if the dioxide were in powder form, as is often encountered, the critical mass would be much higher still), due both to the lower density of plutonium in dioxide compared with elemental plutonium and to the added inert mass of the oxygen contained.<ref>{{cite web|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.7.1|title=4.1 Elements of Fission Weapon Design|last=Sublette|first=Carey|website=The Nuclear Weapon Archive|at=4.1.7.1.2.1 Plutonium Oxide|access-date=20 October 2017|quote=The critical mass of reactor grade plutonium is about 13.9 kg (unreflected), or 6.1 kg (10 cm nat. U) at a density of 19.4. A powder compact with a density of 8 would thus have a critical mass that is (19.4/8)^2 time higher: 82 kg (unreflected) and 36 kg (reflected), not counting the weight of the oxygen (which adds another 14%). If compressed to crystal density these values drop to 40 kg and 17.5 kg.}}</ref>

==Toxicology==
{{see also|Plutonium#Toxicity}}

The behavior of plutonium dioxide in the body varies with the way in which it is taken. When ingested, most of it will be eliminated from the body quite rapidly in body wastes,<ref>United States Nuclear Regulatory Commission, [https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/plutonium.html Fact sheet on plutonium] (accessed Nov 29 2013)</ref> but a small part will dissolve into ions in acidic gastric juice and cross the blood barrier, depositing itself in other chemical forms in other organs such as in phagocytic cells of lung, bone marrow and liver.<ref>{{Citation |last=Gwaltney-Brant |first=Sharon M. |title=Chapter 41 - Heavy Metals |date=2013-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780124157590000418 |work=Haschek and Rousseaux's Handbook of Toxicologic Pathology (Third Edition) |pages=1315–1347 |editor-last=Haschek |editor-first=Wanda M. |place=Boston |publisher=Academic Press |language=en |isbn=978-0-12-415759-0 |access-date=2022-04-10 |editor2-last=Rousseaux |editor2-first=Colin G. |editor3-last=Wallig |editor3-first=Matthew A.}}</ref>

In particulate form, plutonium dioxide at a particle size less than 10 μm<ref name="WNS">World Nuclear Society, [http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Fuel-Recycling/Plutonium/ Plutonium] {{Webarchive|url=https://web.archive.org/web/20150818074517/http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Fuel-Recycling/Plutonium/ |date=2015-08-18 }} (accessed Nov 29 2013)</ref> is radiotoxic if inhaled due to its strong [[Alpha Radiation|alpha-emission]].<ref name="Toxic">{{cite web|url=http://www.atsdr.cdc.gov/toxprofiles/tp143.pdf|title=Toxicological Profile For Plutonium|publisher=U.S. Department of Health and Human Services|date=2007-09-27|access-date=2009-04-23}}</ref>

==See also==
*[[International Atomic Energy Agency]]
*[[Uranium dioxide]]

==References==
{{reflist}}

==External links==
*[https://web.archive.org/web/20090225155213/http://www.ne.doe.gov/pdfFiles/SRPS_safety.pdf Space Radioisotope Power Systems Safety]

{{Plutonium compounds}}
{{Oxides}}

[[Category:Plutonium(IV) compounds]]
[[Category:Oxides]]
[[Category:Nuclear materials]]
[[Category:Fluorite crystal structure]]