'
DOE-HDBK-1019/1-93
NUCLEAR CROSS SECTIONS AND NEUTRON FLUX
Reactor Theory (Neutron Characteristics)
NP-02
Page 6
Rev. 0
Introduction
Fission neutrons are born with an average energy of about 2 MeV. These fast neutrons interact
with the reactor core materials in various absorption and scattering reactions. Collisions that
result in scattering are useful in slowing neutrons to thermal energies. Thermal neutrons may
be absorbed by fissile nuclei to produce more fissions or be absorbed in fertile material for
conversion to fissionable fuel. Absorption of neutrons in structural components, coolant, and
other non-fuel material results in the removal of neutrons without fulfilling any useful purpose.
To safely and efficiently operate a nuclear reactor it is necessary to predict the probability that
a particular absorption or scattering reaction will occur. Once these probabilities are known, if
the availability of neutrons can be determined, then the rate at which these nuclear reactions take
place can be predicted.
Atom Density
One important property of a material is the atom density. The atom density is the number of
atoms of a given type per unit volume of the material. To calculate the atom density of a
substance use Equation (2-1).
(2-1)
where:
N
= atom density (atoms/cm )3
'
= density (g/cm )3
N
= Avogadro's number (6.022 x 10 atoms/mole)
A
23
M = gram atomic weight
