Thermal diffusion
Uranium enrichment

Thermal diffusion uses heat transfer through a thin film of liquid or gas: UF ₆ is subjected to significant temperature differences to separate the isotopes ²³⁸U and ²³⁵U. The principle of the thermal diffusion column is to confine the mixture of isotopes to be separated in an annular space between two concentric cylinders heated to different temperatures.

The combination of diffusion movements and convection currents leads to the concentration of ²³⁵UF₆ and ²³⁸UF₆ molecules at the upper and lower ends of the column respectively. The separation factor does not exceed the value of 1.0003 and enrichments are very low.

Thermal diffusion in liquid UF₆ was used to power calutrons for the US program during the Second World War.