Civil Engineering Reference
In-Depth Information
1.3 Uranium Enrichment
For present Light Water reactors the initial fuel of the core must be enriched from
the 0.72 % of U-235 of natural uranium to an enrichment of 4-5 % U-235
(depending on the fuel burnup) in U-235/U-238 uranium dioxide (UO
2
) fuel. This
is achieved first by chemical conversion of the uranium ores U
3
O
8
into uranium
hexafluoride, UF
6
, which is gaseous above a temperature of 55
C. This gaseous
UF
6
is enriched presently in essentially three different commercial enrichment
processes:
- gaseous diffusion process
- gas centrifuge process
- LASER enrichment process
The LASER enrichment process SILEX is deployed in a first commercial
enrichment plant in the USA. The gas centrifuge process is already used in large
scale plants in Russia, Europe, Japan, China and the USA. The earliest deployed
large scale gaseous diffusion enrichment plants are still in operation in the USA,
France and China. They will be replaced in the future by the more economical gas
centrifuge enrichment plants and probably LASER enrichment plants.
The production capacity of enrichment plants is measured in kg or tons separa-
tion work units (SWU). An LWR of 1 GW(e) power requires a reload of 25 tons
enriched uranium fuel with an enrichment of 4.4 % U-235. This requires 175 tons
SWU [
7
,
8
] (Table
1.2
).
After enrichment in U-235 the UF
6
will be treated chemically to become UO
2
.In
fuel fabrication facilities UO
2
powder will be pressed and sintered to UO
2
pellets of
about 1 cm diameter and 1 cm height. These pellets are filled into about 4 m long
Zircaloy (zirconium-aluminum alloy) tubes (fuel rods). The tubes are then filled
with helium and welded gastight on both ends. On the upper end of these fuel rods
an empty space of about 10 cm length remains where the fission gases can collect
during reactor operation. Fission gas pressure can rise then up to several MPa.
A number of countries with nuclear power plants operate also UO
2
fuel cycle
plants. In total there were 37 uranium mines, 22 uranium conversion plants,
13 uranium enrichment plants, 40 uranium fuel fabrication plants and 5 spent fuel
reprocessing plants commercially operating in the world in 2008 [
5
].
1.4 Spent Fuel Reprocessing
Commercial spent fuel reprocessing was deferred in the USA in 1982 for fear of
proliferation of plutonium. Later this decision was revised by the US government
but no commercial reprocessing industry developed in the USA up till now. Only
intermediate storage and direct spent fuel disposal were pursued. Germany and
Sweden did follow this example of the USA. Other countries like France, Great
