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| Names | |
|---|---|
| IUPAC name Plutonium(IV) oxide | |
| Systematic IUPAC name Plutonium(4+) oxide | |
| Other names Plutonium dioxide | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.031.840 |
| EC Number |
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| Properties | |
| O2Pu | |
| Molar mass | 276 g·mol−1 |
| Appearance | Dark yellow crystals |
| Density | 11.5 g cm−3 |
| Melting point | 2,744 °C (4,971 °F; 3,017 K) |
| Boiling point | 2,800 °C (5,070 °F; 3,070 K) |
| Structure | |
| Fluorite (cubic),cF12 | |
| Fm3m, No. 225 | |
a = 539.5 pm[1] | |
| Tetrahedral (O2−); cubic (PuIV) | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | Radioactive |
| NFPA 704 (fire diamond) | |
| Flash point | non-flammable |
| Related compounds | |
Othercations | Uranium(IV) oxide Neptunium(IV) oxide Americium(IV) oxide |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Plutonium(IV) oxide, orplutonia, is achemical compound with theformulaPuO2. This high melting-point solid is a principal compound ofplutonium. It can vary in color from yellow to olive green, depending on the particle size, temperature and method of production.[2]
PuO2 crystallizes in thefluorite motif, with the Pu4+ centers organized in aface-centered cubic array and oxide ions occupying tetrahedral holes.[3] PuO2 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 PuO2 monolith to retain its structural integrity.[citation needed]
At high temperatures PuO2 tends to lose oxygen, becoming sub-stoichiometric PuO2−x, with the introduction of lower valence Pu3+. 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.[4]
Plutonium dioxide is a stable ceramic material with an extremely low solubility in water and with a high melting point (2,744 °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.[5]
As with allplutonium compounds, it is subject to control under theNuclear Non-Proliferation Treaty.
Plutonium spontaneously oxidizes to PuO2 in an atmosphere of oxygen. Plutonium dioxide is mainly produced bycalcination ofplutonium(IV) oxalate, Pu(C2O4)2·6H2O, at 300 °C. Plutonium oxalate is obtained during thereprocessing of nuclear fuel as plutonium is dissolved in a solution ofnitric andhydrofluoric acid.[6] Plutonium dioxide can also be recovered frommolten-salt breeder reactors by adding sodium carbonate to the fuel salt after any remaining uranium is removed from the salt as its hexafluoride.
PuO2, along withUO2, is used inMOX fuels fornuclear reactors.Plutonium-238 dioxide is used as fuel for several deep-space spacecraft such as theCassini,Voyager,Galileo andNew Horizons probes as well as in theCuriosity andPerseverance rovers onMars. The isotope decays by emitting α-particles, which then generate heat (seeradioisotope 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 PuO2 RTGs have reentered the Earth's atmosphere and burned up (Nimbus B-1 in May 1968 and theApollo 13 Lunar Module in April 1970),[7][8] 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 aboardNimbus 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 ofTransit 5-BN-3 (the early-generation plutonium RTG on board disintegrated upon reentry and dispersed radioactive material into the atmosphere north ofMadagascar, prompting a redesign of all U.S. RTGs then in use or under development).[9]
Physicist Peter Zimmerman, following up a suggestion byTed Taylor, calculated that a low-yield (1-kiloton)nuclear weapon could be made relatively easily from plutonium dioxide.[10] Such bomb would require a considerably largercritical 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.[11]
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,[12] 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.[13]
In particulate form, plutonium dioxide at a particle size less than 10 μm[14] is radiotoxic if inhaled due to its strongalpha-emission.[15]
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.
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