Technology and Operation - Kernkraftwerk Gösgen

From the 1970s up to the 1990s, the KKG
bought natural uranium on the international
market and also obtained it through a partnership
with a US mining company. Instead of
natural uranium, it is also possible to use the
uranium and plutonium recovered during
spent fuel reprocessing as an energy source
for fuel assemblies.
Uranium enrichment
Natural uranium is a mixture of uranium-238
(99.28 %), fissionable uranium-235 (0.71 %)
and a very small amount of uranium-234. Today,
light water reactors use uranium fuel
containing about 4 to 5 % uranium-235. The
process involved in increasing the uranium-
235 concentration of natural uranium to the
concentration required in reactor operation is
called enrichment. Various isotope separation
techniques have been developed for the
enrichment of natural uranium. Only the gas
diffusion technique and gas centrifuge technology
are used on a commercial scale, both
of which require uranium in a gaseous form
(UF6).
The enrichment of uranium can also be
achieved by mixing it with other higher enriched
uranium. This blending process, which
gives the typical enrichment levels required
for light water reactors, is employed in Elektrostal’s
fuel fabrication plants in Russia. To
manufacture fuel pellets, uranium from spentfuel
reprocessing with a residual enrichment
of less than 1 % uranium-235 is blended with
uranium from Russian stocks which has an enrichment
of 20 to 30 %. Since 2000, the KKG
has been using fuel assemblies made from reprocessed
uranium which are fabricated in
Russia under licence from the fuel supplier
Areva NP. This is sparing on resources and
Nuclear fuel cycle
contributes to reducing stocks of military uranium.
By using reprocessed uranium, the KKG
is able to make savings of some 180 tons of
natural uranium each year.
Fuel assembly fabrication
Following enrichment, the uranium hexaflouride
(UF6) is converted into uranium dioxide
(UO2), which is the starting material for
fuel pellets. These ceramic pellets are inserted
into Zircaloy cladding tubes, which
are welded so that they are gas-tight. Two
hundred and five such fuel rods are made up
into a fuel assembly. The enrichment level of
the KKG fuel assemblies is between 4.5 and
around 5 % uranium-235. Fuel assemblies of
this type can achieve average burn-ups of 55
to 65 megawatt days per kilogram.
Uranium can be replaced by plutonium as a
primary energy source. Mixed oxide (MOX)
fuel assemblies contain a mixture of uranium
dioxide (UO2) and plutonium dioxide (PuO 2).
The uranium carrier material is depleted, i.e.
it contains virtually no uranium-235. The
added plutonium is obtained during the reprocessing
of spent fuel assemblies and is itself
a mixture of several plutonium isotopes.
The external appearance of a MOX fuel assembly
does not differ from that of a uranium
fuel assembly.
Plutonium is bred in a light water reactor
through the conversion of uranium-238. In a
conventional uranium fuel assembly, plutonium
thus makes a contribution of some
40 % to the power generated. In a reactor
core with one third MOX fuel assemblies, the
contribution of the plutonium to the reactor
power can even be as high as some 60 %.
The reprocessing of around 400 tons of KKG
fuel assemblies gave rise to some four tons
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