Uranium (U) is abundant in some parts of the
world. Natural uranium is composed mainly of U-238 (about
99.28%) and
a small portion of U-235 (about 0.71%) and a minute portion
of U-234. U-235 is a fissile
nuclide,
i.e. a nuclide with a high probability of inducing fission
after absorbing a low energy neutron (thermal
neutron), whereas U-238 is not fissile. U-238 can induce
fission after absorbing a high energy neutron (fast neutron)
and then forming the Plutonium 239 (Pu-239), which is a fissile
nuclide. Pu-239 would produce fission in the reactor in
the
same way as U-235 and contributes in supplying part of the
energy for power generation.
In general, low enriched uranium fuel is used in commercial
type PWR with light water as the coolant
and moderator.
The nuclear fuel in the reactor is originated from natural uranium but the percentage of U-235 in the fuel must be increased to 3 to 5% above that of natural uranium (i.e. 3 to 5 % enrichment)
for use in the PWR in order to achieve a sustainable nuclear
fission. The enriched uranium fuel in the form of uranium
dioxide for the PWR is fabricated into a few hundreds of small
fuel pellets, which are stacked up, enclosed and sealed in
a zirconium alloy tube of about 3.8 metres in length and 9.5
mm in diameter (the fuel rod). The fuel rods then assembled
in clusters to form the fuel assemblies for loading into the
reactor core. The new fuel assembly has very low radioactivity
and can be handled and transported safely. There are altogether
157 fuel assemblies with 264 fuel rods per assembly in the
reactor core at GNPS
and LNPS.
