Uranium Dioxide
Properties
| State | Solid (sintered ceramic) |
| Color | Black to dark brown |
| Solubility | Insoluble in water; slowly dissolves in nitric acid (oxidizes to uranyl) |
| Melting Point | 2865 °C |
About Uranium Dioxide
Uranium dioxide is the black ceramic that fuels essentially every commercial light-water and heavy-water reactor in operation. UO2 crystallizes in the cubic fluorite (CaF2) structure — U(IV) on the fcc cation sublattice with O^2- in every tetrahedral hole, giving 8:4 cation-to-anion coordination — and the same structure is shared by ThO2, NpO2, and PuO2, which is why mixed actinide oxides form continuous solid solutions and the PUREX-Mox industrial chain is feasible. The combination of properties that makes UO2 the universal reactor fuel is specific and hard to match: melting point 2865 °C (you can lose 80% of your coolant and the fuel still doesn't melt), thermal conductivity around 8 W/m/K at room temperature dropping to about 3 W/m/K at operating temperature, density 10.97 g/cm^3 (high U loading per pellet), low neutron absorption cross-section of about 7.5 barns at thermal energies (the moderator and U-235 fission dominate), and a fluorite lattice that accepts 7 to 10% atom fraction fission products as solid solutions and traps the gaseous Xe and Kr in closed pores during burnup. Commercial PWR fuel pellets are sintered to about 95% theoretical density, ground to dimensional tolerances of microns, dished and chamfered, and stacked inside Zircaloy-4 or ZIRLO cladding to form 4 m fuel rods that get assembled into 17x17 fuel bundles.
Where you'll encounter it
If you have ever toured Westinghouse Columbia or Framatome Romans-sur-Isere, the long polished black cylinders coming off the centerless grinder are UO2 fuel pellets — each one about 8 mm diameter and 10 mm tall, and each one containing the energy equivalent of about a tonne of coal. Fresh fuel handling is gloves-and-respirator work because the alpha-active U-235 enrichment up to 5% poses an internal hazard if inhaled, but the gamma dose from U-238 daughters is low enough that the work happens in open cleanrooms rather than hot cells. The interesting transformation happens after about 18 months in-core: the pellet has cracked radially under the thermal gradient (1000 °C centerline to 400 °C at the cladding interface), the rim has formed a high-burnup structure with submicron grains, and the pellet now contains Cs, I, Xe, Kr, Ru, Mo, and rare earths as fission products. Reprocessing this spent fuel via PUREX requires dissolution in hot 8 M nitric acid — a process that takes hours per assembly even at La Hague's industrial scale.
Common Uses
- Sintered fuel pellets for pressurized water reactors (PWR) and boiling water reactors (BWR) worldwide
- Natural-uranium fuel pellets for CANDU heavy-water reactors operating at 0.72% U-235 enrichment
- Mixed-oxide (MOX) fuel matrix combining UO2 with reprocessed PuO2 from spent-fuel cycles
- Precursor in the conversion chain from yellowcake (U3O8) through UF4 to UF6 for enrichment
- Fuel kernels in TRISO particle fuel for high-temperature gas-cooled reactor designs
- Reference material in actinide solid-state chemistry and high-temperature thermodynamics research
- Radiation-shielding ceramic in specialty research applications requiring high-Z dense materials
- Calibration standard for gamma spectroscopy and X-ray fluorescence analysis of uranium-bearing samples
Safety Information
HAZARDOUS - RADIOACTIVE AND CHEMICALLY TOXIC. GHS classification: Carcinogen Category 1B, Specific Target Organ Toxicity Repeated Exposure Category 2 (kidney), and Aquatic Chronic Hazard Category 1. The chemical toxicity to the proximal renal tubule from soluble uranium is the dose-limiting hazard for low-enriched material — the radiological hazard from natural-isotopic UO2 is modest (alpha-emitters with low specific activity). NRC regulates handling under 10 CFR 20 with annual occupational dose limits of 5 rem (50 mSv) total effective dose equivalent. Transport classification is UN 2911 (Type A) for unenriched material and UN 3324 for enriched. Fabrication facilities operate under criticality safety controls (geometry-favorable equipment, mass limits per location) to prevent inadvertent assembly of a critical mass — typical safe limits for U-235 in solution are below 700 g, far below the bare critical mass of about 50 kg. Handle only in licensed facilities with trained personnel, calibrated alpha-detection instruments, and respiratory protection rated for alpha-emitting particulate.
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.