Uranium (U)

Uranium (U) is an actinide in period 7 with atomic number 92. Naturally occurring uranium is a mixture of isotopes, primarily:

• \(^{238}\mathrm{U}\) (~99.27%)
• \(^{235}\mathrm{U}\) (~0.72%)
• \(^{234}\mathrm{U}\) (trace)

\(^{235}\mathrm{U}\) is fissile, meaning a thermal (slow) neutron can induce fission, releasing energy and more neutrons. A representative reaction is:

The emitted neutrons can cause additional fissions, enabling a chain reaction when moderated and controlled in a reactor.

• Fissile: Can undergo fission with thermal (slow) neutrons, e.g., \(^{235}\mathrm{U}\).
• Fertile: Not fissile itself but can convert to a fissile nuclide via neutron capture and decay; e.g., \(^{238}\mathrm{U}\) \(\xrightarrow{(n,\gamma)}\) \(^{239}\mathrm{U} \to ^{239}\mathrm{Np} \to ^{239}\mathrm{Pu}\).
• Fissionable: Can undergo fission with fast neutrons (broader set than fissile).

Enrichment is the process of increasing the percentage of \(^{235}\mathrm{U}\) relative to \(^{238}\mathrm{U}\) in natural uranium. For many light-water reactors, the fuel is low-enriched uranium (LEU) (typically \(\sim 3\text{–}5\%\) \(^{235}\mathrm{U}\)). Natural uranium (~0.72% \(^{235}\mathrm{U}\)) is used directly in some heavy-water or graphite-moderated reactor designs.

Uranium commonly exhibits oxidation states +4 and +6 (and sometimes +3, +5). Important compounds include:

• UO₂ (uranium(IV) oxide): typical ceramic reactor fuel.
• UF₆ (uranium hexafluoride): used in enrichment because it is gaseous at moderate temperatures.
• U₃O₈ (triuranium octoxide): stable oxide found in yellowcake.

Yellowcake is a concentrated uranium oxide powder (commonly U₃O₈) produced at mills from mined uranium ores. It is a midstream product that is later converted (e.g., to UF₆) for enrichment or to UO₂ for fuel fabrication.

Natural uranium is only weakly radioactive (long half-lives). The main hazards are chemical toxicity (as a heavy metal) and internal exposure if dusts are inhaled/ingested. Safety measures include dust control, proper PPE, engineering controls (ventilation), and radiation monitoring in regulated facilities.

\(^{238}\mathrm{U}\) is fertile. In a neutron field it can form \(^{239}\mathrm{Pu}\) via:

The \(^{239}\mathrm{Pu}\) produced can be fissile with thermal neutrons, contributing to reactor power.

Representative values (approx.):

• \(^{238}\mathrm{U}\): \(4.47\times10^{9}\) years
• \(^{235}\mathrm{U}\): \(7.04\times10^{8}\) years
• \(^{234}\mathrm{U}\): \(2.46\times10^{5}\) years

Long half-lives imply low specific activity but persistent radiological presence.

A commonly cited ground-state configuration is [Rn] 5f3 6d1 7s2. Participation of 5f/6d orbitals leads to variable oxidation states, rich coordination chemistry, and formation of uranyl species (U(VI)) such as \(\mathrm{UO_2^{2+}}\) in aqueous systems.