Nuclear Fuel Cycle

Principle

In a gas centrifuge, UF₆ gas is placed inside a rapidly spinning rotor (cylinder). The centrifugal force pushes the heavier $^{238}$UF₆ molecules preferentially toward the outer wall, while the lighter $^{235}$UF₆ molecules concentrate closer to the centre of the rotor.

This is analogous to the Earth’s atmosphere, where gravity causes heavier molecules (like oxygen) to be more concentrated near the ground, while lighter molecules (like hydrogen) are found at higher altitudes. In a centrifuge, the “gravitational” acceleration is provided by rotation and can reach approximately 10$^{6}$ g (one million times the Earth’s gravitational acceleration).

To enhance separation, a counter-current flow is established by heating the bottom of the centrifuge:

• Hot gas rises through the centre (enriched in $^{235}$U)
• Cool gas descends near the wall (enriched in $^{238}$U)
• The highest $^{235}$U concentration occurs at the top centre
• The highest $^{238}$U concentration occurs at the bottom, near the wall

A product scoop at the top centre collects the enriched stream, while a waste scoop at the bottom collects the depleted stream.

Separation Factor

The centrifuge separation factor is given by:

$\alpha \approx \exp\left[\frac{r_w^2 \omega^2 (m_h - m_l)}{2RT}\right]$

Typical centrifuge separation factors are approximately 1.06 to 1.5 per stage — much higher than the 1.0043 achieved by gaseous diffusion.

For example, a centrifuge with a 30 cm cylinder spinning at 350 m/s at 300 K achieves a separation factor of approximately 1.1.

Industrial Application

URENCO is a British-Dutch-German consortium that was one of the first companies to offer commercial enrichment using centrifuge technology. The Capenhurst site has a capacity of approximately 4.9 million SWU/yr. The first enrichment at Capenhurst was by gaseous diffusion for military purposes; centrifuges were installed from 1976.

Advantages of Gas Centrifuge over Gaseous Diffusion