Uranium enrichment by gaseous diffusion: before diffusion through porous barriers, natural uranium is converted to which gaseous compound?

Introduction / Context:
Isotopic enrichment increases the fraction of U-235 in uranium. Historical diffusion plants and many modern processes first convert uranium to a volatile compound so that mass transfer and separation can occur in the gas phase.

Given Data / Assumptions:

• Gaseous diffusion or gas-centrifuge front-end chemistry.
• Need for a thermally stable, sufficiently volatile uranium compound at moderate temperatures.
• Compatibility with corrosion-resistant process materials (e.g., nickel, fluorinated alloys).

Concept / Approach:
UF6 (uranium hexafluoride) is uniquely suitable because it sublimes readily and remains molecular in the gas phase. The slight mass difference between 235UF6 and 238UF6 enables separation by diffusion or centrifugation. Other uranium compounds (oxides, carbides, sulfates) are nonvolatile under operating conditions and cannot be used in gaseous-diffusion trains.

Step-by-Step Solution:
Convert U3O8 (yellowcake) → UF6 via fluorination steps.Heat to obtain gaseous UF6 for separation.Pass through porous barriers (diffusion) or centrifuges (rotation) to exploit mass differences.Collect product and tails streams for further processing.

Verification / Alternative check:
Materials balance and phase-diagram data confirm UF6 is the only common gaseous uranium compound used industrially for enrichment, aligning with plant designs worldwide.

Why Other Options Are Wrong:
UO2, UC2, UO2SO4, UCl4: not suitably volatile under practical conditions for diffusion plants.

Common Pitfalls:
Confusing UF4 (green salt) used earlier in the chain with UF6 used for enrichment.Assuming oxide powders can be directly diffused; they cannot.

Final Answer:
Uranium hexafluoride (UF6)