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Each day thousands of shipments of radioactive materials are transported around the world.
The front end covers all the operations from the mining of uranium to the manufacture of new fuel assemblies for loading into the reactors, i.e. the transport of uranium ore concentrates (UOC) to uranium hexafluoride conversion facilities, from conversion facilities to enrichment plants, from enrichment plants to fuel fabricators and from fuel fabricators to the various nuclear power plants.
The raw material to make nuclear fuel is uranium ore, the main sources of which are found in North America, Australia, Southern Africa and Central Asia. The ore typically 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 a low specific activity material and the radiological hazard is very low. It is normally transported in sealed 210 litre drums (an Industrial Package) in standard sea (ISO) freight containers. These can be transported by road, rail or sea, and in many cases a combination of modes of transport is used. The UOC is transported to conversion plants for conversion into uranium hexafluoride (Hex).
UOC is transported worldwide from the mining areas to conversion plants located in North America, Europe and Russia. It is first chemically purified and then converted by a series of chemical processes into natural 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.
In the production process, large cylindrical steel transport cylinders some 1.25m (48″) in diameter, each holding up to 12.5 tonnes of materials are filled directly with Hex which can be liquid or gaseous depending on the manufacturing process. The Hex then solidifies inside the cylinder on cooling to room temperature. In storage and during transport the Hex material inside the cylinders is in a solid form. Natural Hex is also stored in these cylinders prior to being transported to an enrichment plant. 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 (fissions) 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. Enriched Hex is transported in smaller universal cylinders. These cylinders are some 76cm (30″) in diameter and are loaded in overpacks so that the packaging is resistant to crashes, fires, immersion and prevents chain reactions. The loaded overpacks are generally transported using ISO flat rack containers for transport to fuel fabrication 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 typically about 4m (12′) 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.