Environmental Engineering Reference
In-Depth Information
Control rods
Coolant out
Coolant in
Coolant flow
Fuel rods
Thermal shield
Core barrel
Pressure vessel
Support plate
Figure 6.1
Schematic of nuclear reactor with water as coolant/moderator.
Moderators
are used to slow the energetic neutrons that evolve from the fission reaction,
yielding low-energy neutrons, also called thermal neutrons. This increases the probability for the
neutrons to be absorbed in another fissile nucleus, so that the chain reaction can be propagated.
Moderators contain atoms or molecules whose nuclei have high neutron scattering and low neutron
absorption characteristics. Typical moderators are light water (H
2
O), heavy water (D
2
O), graphite
(C), and beryllium (Be). The light or heavy water moderators circulate around the fuel rods.
Graphite or beryllium moderators constitute a block into which fuel rods are inserted. For example,
the original Fermi “pile” consisted of a graphite block into which metallic uranium fuel was inserted.
The Chernobyl-type reactors also use graphite as a moderator.
Control rods
contain elements whose nuclei have a high probability of absorbing thermal
neutrons, so that they are not available for further splitting of fissile nuclei. In the presence of
control rods, the chain reaction is controlled or stopped altogether. Typical control rods are made
of boron (B) or cadmium (Cd).
The chain reaction inside the reactor is governed by the
neutron economy coefficient k
. Under
a steady state, the number of thermal electrons is invariant with time,
dn
/
dt
=
0, and
k
=
1. The
reactor is then in a
critical
condition. When
k
1, it is
supercritical
. A nuclear reactor becomes critical when control rods are lifted out of the core of the
reactor to a degree where more than one neutron released by the fission of a fissile nucleus survives
without being absorbed by the control rods. The position of the control rods determines the power
output of the reactor. Monitoring the critical condition in a nuclear reactor while varying the output
is quite complicated. Generally, nuclear power plants are run at full load, providing the base load
of a grid. Running the plant at full load is also more economical.
Once the remaining fuel in the rods cannot sustain the rated capacity of the plant, even with
complete withdrawal of the control rods, the fuel rods need to be replaced. This occurs every
2-3 years.
Heat must be constantly removed from the reactor. Heat is generated not only by the fission
reaction, but also by the radioactive decay of the fission products. Heat is removed by a
coolant,
which can be boiling water, pressurized water, a molten metal (e.g., liquid sodium), or a gas (e.g.,
<
1, the reactor is
subcritical;
when
k
>
