Nuclear Fuel Cycle

Mining - After the uranium is mined it is milled at the mine and leached with sulphuric acid to separate out the uranium. It is then recovered from solution and precipitated as U3O8 − yellowcake

Conversion - Uranium needs to be in the form of a gas before it can be enriched. The U3O8 is converted into UF6 − uranium hexafluoride. UF6 is a gas at relatively low temperatures and as there is only one naturally occuring isotope of florine, any weight difference will be due to the different isotopes of uranium.

Enrichment - For most nuclear reactors, notable exceptions are the Magnox and Candu designs, the uranium fuel has to be enriched by increasing the 235U content from 0.7% to 3 - 4%. During enrichment two streams are produced - enriched and depleted uranium. The two most common methods of enrichment are by using centrifuges or by a diffusion process.

Fuel Fabrication - For enriched fuel the resulting enriched UF6 is converted to UO2 powder and pressed into pellets.

Reprocessing - Refuelling of the reactor is necessary when the 235U content of the fuel is reduced to less than 1%. Overall only about 4% of the uranium is burnt up plus some plutonium has been created from the fission reactions. Reprocessing separates the uranium and plutonium from waste products by chopping up the fuel rods and dissolving them in acid. The uranium can then be returned to the conversion stage for re-enrichment while the plutonium can be blended with uranium to produce Mixed Oxide (MOX) fuel.

Vitrification - The Highly Active Liquid (HAL) waste concentrate obtained from the reprocessing of the fuel is essentially a solution of metal nitrates. This can be converted into mixed oxides and reacted at 1050 °C with glass forming materials to form a vitreous product. The remaining raffinate is placed into a calciner, with a maximum temperature of 500 °C, until a granular solid is formed. It is then discharged into an Inconel melter with some borosilicate glass frit. After melting for several hours at 1050 °C it is poured into stainless steel canisters and allowed to cooled. The UK vitrified waste is stored at the Sellafield site.

The UK is now undertaking a site selection process to identify an appropriate Geological Disposal Facility as recommended by the Committee on Radioactive Waste Management.