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.
Mining to produce uranium ore concentrate
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).
Conversion of uranium ore concentrate to
uranium hexafluoride
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.
Enrichment of uranium hexafluoride
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.
Fuel fabrication
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.