The Nucleus
353
Some neutrons may be lost from the surface of the active zone. In this
connection, it is noted that the fission-effectiveness of the neutrons is proportional
to the volume (i.e. l
3
) whereas the surface losses are proportional to the surface
area (i.e. l
2
). Hence, the relative surface losses can be decreased by increasing
the active volume, and the size at which the chain reaction is just self-sustaining
is known as the critical volume, and the corresponding mass of the active material
is known as the critical mass.
In addition, there are other sources of neutron losses, such as absorption by
the impurity in the moderator. These losses must be carefully controlled, for
example, by sing moderators which are nearly free of impurities. The essential
requirement of continued chain reactions is that the number of fission neutrons
remaining after taking into account all the losses, must be greater than the initial
neutrons which induced the fission.
Moderators
The role of a moderator is to slow down the neutrons without absorbing them.
Elementary considerations show that maximum energy is transferred to the
target if the target mass is equal to the projectile mass.
The ideal moderator would have been hydrogen. Unfortunately, hydrogen
can capture a neutron according to the reaction p (n, γ) d. More suitable
moderators are the deuteron d (nucleus of deuterium), graphite (C) or beryllium
(Be). About 25 collisions are adequate to thermalize 2 MeV neutrons in heavy
water (D 2 O), and about 100 collisions in C or Be.
Control Rods
The number of fission neutrons available for the controlled chain reaction, after
taking into account the various losses, must be slightly greater than the neutrons
which caused the initial fission. To produce sustained, stable chain reactions,
the excess neutrons must be removed and controlled. This is usually done by
inserting what are known as control rods into the core of the reactor. These rods
are made of an element with a large cross-section for neutron capture.
The element often used in control rods is cadmium which has a very large
capture cross-section for thermal neutrons,
113
Cd (n, γ)
114
Cd. The insertion of
these rods decreases the reactivity of the reactor whereas withdrawal increases
the reactivity. It is important to observe that the response of the chain reaction
to fluctuations in neutrons, is slow, because of the delyed neutrons produced in
fission (see Example 6, Sec. 9.9). This permits the use of the control mechanism
with a time delay of about 1 min.
Coolant
The heat generated in the active region of the reactor is carried away by a heatcarrying agent, usually water or an alkali metal such as sodium (the agent should
353
Some neutrons may be lost from the surface of the active zone. In this
connection, it is noted that the fission-effectiveness of the neutrons is proportional
to the volume (i.e. l
3
) whereas the surface losses are proportional to the surface
area (i.e. l
2
). Hence, the relative surface losses can be decreased by increasing
the active volume, and the size at which the chain reaction is just self-sustaining
is known as the critical volume, and the corresponding mass of the active material
is known as the critical mass.
In addition, there are other sources of neutron losses, such as absorption by
the impurity in the moderator. These losses must be carefully controlled, for
example, by sing moderators which are nearly free of impurities. The essential
requirement of continued chain reactions is that the number of fission neutrons
remaining after taking into account all the losses, must be greater than the initial
neutrons which induced the fission.
Moderators
The role of a moderator is to slow down the neutrons without absorbing them.
Elementary considerations show that maximum energy is transferred to the
target if the target mass is equal to the projectile mass.
The ideal moderator would have been hydrogen. Unfortunately, hydrogen
can capture a neutron according to the reaction p (n, γ) d. More suitable
moderators are the deuteron d (nucleus of deuterium), graphite (C) or beryllium
(Be). About 25 collisions are adequate to thermalize 2 MeV neutrons in heavy
water (D 2 O), and about 100 collisions in C or Be.
Control Rods
The number of fission neutrons available for the controlled chain reaction, after
taking into account the various losses, must be slightly greater than the neutrons
which caused the initial fission. To produce sustained, stable chain reactions,
the excess neutrons must be removed and controlled. This is usually done by
inserting what are known as control rods into the core of the reactor. These rods
are made of an element with a large cross-section for neutron capture.
The element often used in control rods is cadmium which has a very large
capture cross-section for thermal neutrons,
113
Cd (n, γ)
114
Cd. The insertion of
these rods decreases the reactivity of the reactor whereas withdrawal increases
the reactivity. It is important to observe that the response of the chain reaction
to fluctuations in neutrons, is slow, because of the delyed neutrons produced in
fission (see Example 6, Sec. 9.9). This permits the use of the control mechanism
with a time delay of about 1 min.
Coolant
The heat generated in the active region of the reactor is carried away by a heatcarrying agent, usually water or an alkali metal such as sodium (the agent should