Uranium is a dense, silvery-grey metal that is ubiquitous in the Earth’s crust at low concentrations. The following reference table summarises its key physical and nuclear properties.
Table: Key Properties of Uranium
| Property | Value |
|---|---|
| Atomic number | 92 |
| Atomic weight | 238.03 g/mol |
| Density | 19.1 g/cm³ (70% denser than lead) |
| Melting point | 1,132 °C |
| Boiling point | 4,131 °C |
| Crystal structure | Orthorhombic (alpha phase, below 660 °C) |
| Appearance | Silvery-grey metal; oxidises in air forming a black coating |
| Natural isotopes | U-234 (0.0055%), U-235 (0.711%), U-238 (99.284%) |
| Crustal abundance | 2—4 ppm (average); up to 20 wt% in richest deposits |
| Seawater concentration | ~3 ppb (~4 billion tonnes total in the world’s oceans) |
Uranium is chemically reactive, forming compounds with most non-metallic elements. The most important uranium compounds in the fuel cycle are: UO₂ (the ceramic fuel form), U₃O₈ (yellowcake, the naturally occurring oxide), UF₄ (green salt, an intermediate in conversion), and UF₆ (uranium hexafluoride, used for enrichment). UF₆ sublimes at 56.5 °C, making it convenient for handling as a gas in enrichment cascades. Fluorine has only one natural isotope (F-19), so the mass difference between ²³⁵UF₆ (349 amu) and ²³⁸UF₆ (352 amu) depends solely on the uranium isotopes.