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Plutonium Dioxide

PuO2 oxide
Molar Mass
276.000 g/mol
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Properties

StateSolid
ColorYellow-green to dark brown
SolubilityEssentially insoluble in water; dissolves very slowly in hot concentrated HNO3 with HF or Ag(II) catalyst
Melting Point2744 °C

About Plutonium Dioxide

Plutonium dioxide is the standard bulk form for almost every plutonium operation on the planet — separation, storage, transport, fuel fabrication, and waste packaging all run through PuO2 at some point. Crystallographically it sits in the cubic fluorite (CaF2) structure family alongside ThO2, UO2, NpO2, and AmO2: each Pu(IV) center is eight-coordinate to oxide, with oxide ions filling the tetrahedral holes of an FCC plutonium sublattice. The lattice parameter (5.396 Å) and high melting point (2744 °C) reflect the strong ionic-covalent Pu-O bonding that makes the compound chemically inert under ambient conditions — it doesn't react with air, water, or dilute mineral acids at room temperature, which is exactly why it's the form of choice for long-term storage. The PUREX (Plutonium Uranium Reduction Extraction) process at sites like La Hague and Sellafield separates plutonium from spent UO2 fuel as Pu(NO3)4 in nitric acid, then calcines that to PuO2 powder. From there it goes one of two places: blended with depleted UO2 at 5-10% Pu loading to make MOX (mixed-oxide) reactor fuel, or pressed into pellets and stored under IAEA Category I safeguards in vaults like the U.S. K-Area Material Storage at Savannah River Site. The Pu-238 isotope (different from the Pu-239 of MOX fuel) is the heat source in radioisotope thermoelectric generators that have powered every NASA deep-space mission past Jupiter — Voyager 1 and 2, Galileo, Cassini, New Horizons, Curiosity, Perseverance, and Dragonfly all run on PuO2 pellets clad in iridium and silicon carbide.

Where you'll encounter it

If you've ever watched a Mars rover landing or seen images from beyond Saturn, the power source keeping those instruments warm and running is Pu-238 dioxide — a 4.8 kg block of PuO2 in Curiosity's Multi-Mission RTG generates ~2 kW of decay heat that thermoelectric converters turn into ~110 W of electricity, and it'll keep doing that for decades because the half-life is 87.7 years. The DOE makes this material at Oak Ridge by neutron-irradiating Np-237 targets, separating the Pu-238 chemically, and converting it to oxide; production was idled in 1988 and only restarted in 2013, so NASA mission cadence is now constrained by Pu-238 supply. On the safeguards side, an IAEA inspector visiting a reprocessing plant verifies plutonium inventory by counting and weighing PuO2 storage cans (each typically 4.4 kg of Pu) and running gamma-ray spectroscopy to confirm isotopic composition — a fingerprint that distinguishes weapons-grade (>93% Pu-239) from reactor-grade (>20% Pu-240) material. Hanford and Rocky Flats workers from the 1950s-1970s suffered chronic plutonium inhalation injuries that established PuO2 as one of the most radiotoxic respirable substances known, with annual occupational intake limits at the microgram level.

Common Uses

  • MOX (mixed-oxide) reactor fuel blended at 5-10% Pu loading with depleted UO2
  • Pu-238 radioisotope thermoelectric generators powering Voyager, Curiosity, Perseverance, and Dragonfly
  • Standard storage and transport form for separated plutonium under IAEA Category I safeguards
  • Reference waste form for vitrification and geologic-repository performance studies
  • Target material for actinide partitioning and transmutation research at INL and JAEA

Safety Information

Severely radiotoxic. Pu-239 is a long-lived alpha emitter (24,100-year half-life); Pu-238 in RTGs runs at ~390 W/kg specific thermal power for the dioxide, hot enough to glow red on its own. Inhalation of respirable PuO2 dust is the critical exposure pathway because alpha particles delivered to lung tissue cause localized radiation damage that initiates lung cancer at low cumulative doses. ICRP annual limit on intake is roughly 200 Bq for Pu-239 inhalation (Type S aerosol), which corresponds to about 0.08 µg of Pu — a microgram-level limit. NRC Special Nuclear Material under 10 CFR Part 73; international IAEA Category I safeguards. Handled exclusively in nuclear-licensed glove-box facilities with HEPA/ULPA off-gas filtration, multiple containment barriers, and continuous radiation monitoring. Fire scenarios are the dominant accident pathway — a hot fire can disperse PuO2 as respirable particulate, the mode of release that contaminated Rocky Flats in 1957 and 1969.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

Pu — Plutonium O — Oxygen

Frequently Asked Questions

What is the molar mass of plutonium dioxide?
PuO2 has a nominal molar mass of 276.0 g/mol if computed with the longest-lived isotope Pu-244 (244.064) + 2 × 15.999 (O). In practice plutonium is handled as isotope mixtures, so the working molar mass depends on composition: weapons-grade Pu (>93% Pu-239) gives 271.05 g/mol; Pu-238 dioxide for RTGs gives 270.04 g/mol; reactor-grade Pu (significant Pu-240, Pu-241, Pu-242) lands in the 271-272 range. Always use the isotopic composition from the actual material's mass-spectrometry COA when doing fuel-fabrication or safeguards mass-balance calculations.
Why is PuO2 the standard form for plutonium storage?
Three reasons converge. First, chemical inertness — PuO2 doesn't react with air, water, or dilute acids at ambient temperature, so corrosion and self-degradation in storage are minimal. Second, thermal stability — the 2744 °C melting point means even in a major facility fire the material stays as a refractory solid rather than vaporizing and dispersing. Third, uniformity — converting separated plutonium to PuO2 powder gives IAEA inspectors a single, well-defined accounting form that can be weighed, counted, and assayed across every facility worldwide. The U.S. K-Area Material Storage at Savannah River currently holds tens of tonnes of PuO2 in welded steel cans inside vault storage.
How does Pu-238 RTG fuel differ from Pu-239 reactor fuel?
Different isotope, completely different mission. Pu-238 has an 87.7-year half-life and emits alpha particles at 5.5 MeV with no significant gamma — that high specific power (390 W/kg as the oxide) drives RTG thermoelectric converters in deep-space probes where solar panels won't work past Jupiter. Pu-238 cannot sustain a fission chain reaction because its critical mass is impractically large, so it's not weapons-usable. Pu-239 has a 24,100-year half-life and is the workhorse fissile isotope — it sustains chain reactions in MOX-fueled thermal reactors and is the active material in modern nuclear weapons. The two isotopes never get mixed; Pu-238 is made deliberately by Np-237 neutron capture for civilian space-power use, while Pu-239 comes out of every uranium-fueled reactor as a side product of U-238 capture.