Metallic uranium in pure form is a solid shiny silvery metal which when exposure to air covers with a thin oxide film. It exists in three crystalline modifications — α, β and γ: α-uranium when heated to 662оC transforms into β-uranium which in its turn at 772оC transforms into γ-uranium. Metallic uranium is a poor electrical conductor, its electrical conductivity is almost twice as low as that of iron. Heat capacity of metallic uranium is 3.3 times lower than that of copper, and thermal conductivity is approximately three times less than that of stainless steel, and 13 times less than that of copper. By magnetic susceptibility it is a paramagnetic element and may form non-ferromagnetic alloys.
Atomic mass of natural uranium is 238.0289 atomic units. It is a mixture of three isotopes:
All uranium isotopes are radioactive. Two of them — U238 and U235 — form decay chains that end in stable non-radioactive isotopes of lead Pb206 and Pb207 and helium. U234 isotope is one of intermediate nuclides of U238 decay chain. Intermediate products that have practical value are radium Ra226 and radon Rn222. When nuclear decay of U238 and U235 occurs they emit secondary neutrons which under certain conditions may cause decay of other nucleus. Fission chain reaction is possible if the number of secondary neutrons is at least equal to the number of neutrons causing fission reaction, i.e. neutron multiplication factor must be ≥ 1. If it is maintained at the level of one, it causes nuclear combustion which forms the basis for wide use of uranium in nuclear power engineering.