Civil Engineering Reference
In-Depth Information
Therefore, the U-235/U-238 fuel in Light Water Reactor cores is mixed with
materials of low mass number (moderator) in order to slow down the fission
neutrons having high kinetic energy by a number of elastic and inelastic collisions
to kinetic energies in the range
1 eV (Fig.
2.3
). This is most effective if the
enriched uranium fuel is put in cylindrical rods which are arranged in e.g. a square
grid. This lattice of fuel rods arranged with certain distances must be cooled by a
flowing coolant which can be identical with the moderator as in case of water in
Light Water Reactors. The fission neutrons after a few collisions with the fuel atoms
then fly with high velocity into the surrounding water/moderator. They are slowed
down by collisions within a short distance to so-called thermal energies of 0.025 eV
(Fig.
2.5
). The neutrons are then in thermal equilibrium with the relatively low
kinetic energies of the water molecules. Another advantage of the deceleration of
the neutrons in the surrounding water is given by the fact that the probability is
lower that the neutrons can be captured in the resonance region of U-238 inside the
fuel rods (Fig.
2.4
)[
1
-
5
].
After the neutrons are thermalized within the moderator region they migrate
back by diffusion processes into the fuel rods. As they have lower kinetic energies
now also the microscopic fission cross sections are much higher (in the 0.025 eV
energy range) than those for fission neutrons. The consequences are more fission
reactions. Also the ratio between fission and capture reactions becomes more
favorable in the fuel.
An optimum volume ratio between moderator and fuel for the grid of fuel rods is
found around 2 for light water (H
2
O). This optimal volume ratio can be achieved by
adaption of the distance between the fuel rods. For heavy water (D
2
O) as a
moderator this optimal volume ratio is about 20 and for graphite as a moderator it
is found to be around 54 [
1
-
5
].
Light water (H
2
O) has a higher microscopic capture cross section than heavy
water or pure graphite. Consequently it is possible to build and operate nuclear
power reactors with natural uranium (0.72 % U-235 enrichment) if heavy water or
graphite are used as moderator. In fact the first reactor used natural uranium as fuel
and graphite as moderator. With light water as moderator in Light Water Reactors
the uranium fuel must be enriched to 3-5 % in U-235 (depending on the burnup of
the uranium fuel). Structural materials which must be used for the design of the
reactor core should also have low microscopic capture cross sections. Light Water
Reactors, therefore, use an alloy of zirconium and aluminum (Zircaloy) for the
cladding of the fuel rods and grid spacers of the fuel elements [
1
-
4
].
Uranium dioxide (UO
2
) with its high melting point (2,865
C) and its good
irradiation properties in the neutron field of the reactor core is used as fuel in LWR
cores.
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