The nuclear fuel cycle can be broken down into what is generally known as the ‘front end’ and ‘back end’ operations. The front end covers the operations from the mining of uranium to the manufacture of fuel assemblies for loading into the reactors. The back end covers the operations concerned with spent fuel that leaves reactors.
The raw material to make nuclear fuel is uranium ore. The main sources are found in North America, Australia, South Africa and Central Asia. The ore contains about 1.5% uranium but some deposits are much richer. The ore is first ground and purified using chemical and physical processes to yield a dry powder of natural uranium oxide known as uranium ore concentrate, or UOC. The historical name for UOC was “yellowcake” because the early concentrates were typically yellow in colour.
UOC is transported worldwide from the mining areas to conversion plants that are located in North America, Europe and Russia. It is first chemically purified and then converted by a series of chemical processes into natural uranium hexafluoride (Hex), which is the form required for the following enrichment stage. The natural Hex produced from the conversion of UOC is a very important intermediate in the manufacture of new reactor fuel. There is a very large commercial trading in it that involves international transport.
Hex is routinely transported by road, rail or sea, or more commonly, by a combination of modes.
Although Hex is a low specific activity material there would be a chemical hazard in the unlikely event of a release because it produces toxic by-products on reaction with moist air.
The valuable isotope of uranium that splits in a nuclear reactor is U-235, but only around 0.7% of naturally occurring uranium is U-235. This is increased to the level required, about 3-5% for light water reactors, either by a gaseous diffusion process or in gas centrifuges. Commercial enrichment plants are in operation in the USA, Western Europe and Russia, which gives rise to international transport of Hex between conversion and enrichment plants.
Depleted Hex, the residual product from the enrichment process, has the same physical and chemical properties as natural Hex and is transported using the same type of cylinders.
Uranium dioxide powder derived from Hex of less than 5% enrichment is also a low specific activity material The enriched Hex is first converted into uranium dioxide powder which is then processed into pellets by pressing and sintering. The pellets are stacked into zirconium alloy tubes that are then made up into fuel assemblies for transport from the fabrication plant to the reactor site. Fuel fabrication plants are located in many countries across the world.
The fuel assemblies are some 4m long. They are transported in specially designed robust steel packages. The design and configuration of packages during transport is arranged so that a nuclear chain reaction does not occur.
