Reactor Theory (Neutron Characteristics)
DOE-HDBK-1019/1-93
NEUTRON MODERATION
Rev. 0
Page 23
NP-02
NEUTRON MODERATION
In thermal reactors, the neutrons that cause fission are at a much lower energy
than the energy level at which they were born from fission. In this type of
reactor, specific materials must be included in the reactor design to reduce the
energy level of the neutrons in an efficient manner.
EO 2.12 DEFINE the following concepts:
a. Thermalization
d. Average logarithmic energy decrement
b. Moderator
e. Macroscopic slowing down power
c. Moderating ratio
EO 2.13 LIST three desirable characteristics of a moderator.
EO 2.14 Given an average fractional energy loss per collision,
CALCULATE the energy loss after a specified number of
collisions.
Neutron Slowing Down and Thermalization
Fission neutrons are produced at an average energy level of 2 MeV and immediately begin to
slow down as the result of numerous scattering reactions with a variety of target nuclei. After
a number of collisions with nuclei, the speed of a neutron is reduced to such an extent that it has
approximately the same average kinetic energy as the atoms (or molecules) of the medium in
which the neutron is undergoing elastic scattering. This energy, which is only a small fraction
of an electron volt at ordinary temperatures (0.025 eV at 20 C), is frequently referred to as the
thermal energy, since it depends upon the temperature. Neutrons whose energies have been
reduced to values in this region (< 1 eV) are designated thermal neutrons. The process of
reducing the energy of a neutron to the thermal region by elastic scattering is referred to as
thermalization, slowing down, or moderation. The material used for the purpose of thermalizing
neutrons is called a moderator. A good moderator reduces the speed of neutrons in a small
number of collisions, but does not absorb them to any great extent. Slowing the neutrons in as
few collisions as possible is desirable in order to reduce the amount of neutron leakage from the
core and also to reduce the number of resonance absorptions in non-fuel materials. Neutron
leakage and resonance absorption will be discussed in the next module.
