In conversion or processing plants yellowcake is converted to uranium hexafluoride (UF6), commonly referred to as 'hex'. Hex gas is used for the next step in the nuclear fuel chain: the enrichment process.

There are only a few commercial conversion facilities currently operating in the world. These plants are in the US (ConverDyn, Metropolis, Oklahoma), Canada (CAMECO, Port Hope, Ontario), France (Comurhex, Pierrelatte, Malv?si), the UK (BNFL, Springfields, Preston) and the Russian Federation. Conversion plants that are closed down are for example, Sequoyah Fuels (Gore, Illinois, US) and le Bouchet (France).

Conversion process

The conversion process from yellowcake to UF6 includes the following steps:

Refining
1. Removal of several impurities from yellowcake, in different steps
2. Conversion of yellowcake (U3O8) to UO3. Nitric acid is added to yellowcake to produce a uranyl nitrate solution. Impurities are removed. Water is evaporated. The left over concentrated uranyl nitrate hexahydrate is thermally decomposed to uranium trioxide (UO3).

Reduction
3. Pulverisation of UO3 into a fine powder
4. Reduction from UO3 with hydrogen gas into UO2

Hydrofluorination
5. Reaction of UO2 with hydrogen fluoride (HF) to form UF4
6. Calcination of UF4 to remove all water

Fluorination
7. Reaction of UF4 powder with fluorine gas to produce UF6 gas

Distillation
8. Filtering UF6
9. Crystallising UF6
10. Liquifying UF6.

UF6 cylinderUnder moderate temperature and pressure, hex is a liquid. The liquid UF6 is drained into specially designed steel shipping cylinders. If the cylinder is cooled, hex becomes a white crystalline solid and is shipped in this form. Cylinders with UF6 are transported by truck and/or boat to enrichment plants.

A single 14 tonne cylinder of UF6 has the energy equivalent of 900,000 barrels of oil.

Risks for workers

As with mining and milling, the primary risks associated with conversion are chemical and radiological. Risks from atmospheric effluents include the spills of uranium hexafluoride and hydrogen fluoride into the air. This can lead to possible inhalation hazards. In the above conversion process extremely corrosive and pyrophoric chemicals are used that may cause fire and explosion.

Waste

The waste from conversion facilities is mainly sludge that is usually drained into sludge pools. These sites are a hazard to the environment. Groundwater and surface water flows away untreated. Surface creep, the movement of the top soil layer, and tension cracks increase the chance of erosion from a site with subsequent contamination of the surrounding environment.

Contaminated water from processing sites is often collected in water-collection ponds and treated, but still remains toxic. In Canada these water-collection ponds frequently overflow.