Vacuum equipment plays a key role uranium enrichment

Paul Boughton

Oerlikon Leybold Vacuum has received a double-digit million Euro order for vacuum technology from a global manufacturer of uranium centrifuges for nuclear power facilities.

Oerlikon Leybold’s vacuum solutions will play a key role in a nuclear plant presently under construction in the United States. Oerlikon Leybold Vacuum will deliver vacuum equipment for 12 different systems and will offer customised service to fit the customer’s needs, including assembly, testing and documentation in accordance with requested standards.

The RUVAC roots pumps have been modified to withstand prevailing conditions and comply with the high requirements of their application.

Uranium centrifuge technology is used not only in modern energy solutions and civil power generation, but for medical and industrial purposes as well. U-235 is the uranium isotope used in nuclear fission.

Nuclear reactors require a 4-5 per cent U-235 content for safe and efficient operation, but natural uranium ore only has a 0.7 per cent U-235 content ratio.

The uranium for use in reactors must therefore be mined, milled and converted to the appropriate gaseous chemical form, which allows those molecules with higher amounts of U-235 to be selected out. This process is known as uranium enrichment, and it requires the use of special centrifuges. The process involves forcing pressurised uranium hexafluoride gas through a series of porous membranes or diaphragms. As U-235 molecules are lighter than U-238 molecules (a similar isotope that exists alongside U-235), they move faster and have a slightly better chance of passing through the pores in the membrane.

The UF6 that diffuses through the membrane has higher levels of U-235, while the gas that did not pass through has depleted levels of U-235. This process is repeated over and over in a series of diffusion stages called a cascade. Each stage involves a compressor, a diffuser and a heat exchanger to remove the heat of compression.

Finally, the enriched UF6 product is withdrawn from one end of the cascade and the depleted UF6 is removed at the other end. The gas must be processed through some 1,400 stages to obtain a product with a concentration of 3 per cent to 4 per cent U-235. The plants typically achieve only modest levels of isotope separation in each individual stage of enrichment; they compensate for this by having the capacity to handle large volumes of gas. The process requires a robust, stable vacuum.

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