Reactor Theory (Neutron Characteristics)
NEUTRON FLUX SPECTRUM
Neutron Flux Spectrum Summary
Prompt neutrons are born at energies between 0.1 MeV and 10 MeV. The
average prompt neutron energy is about 2 MeV.
Fast reactors have a neutron energy spectrum that has the same shape as the
prompt neutron energy spectrum.
Thermal reactors have a neutron energy spectrum that has two pronounced
peaks, one in the thermal energy region where the neutrons are in thermal
equilibrium with the core materials and another in the fast region at energies
where neutrons are produced. The flux in the intermediate region (1 eV to
0.1 MeV) has a roughly 1/E dependence.
The neutron flux spectrum for the fast energy region of a thermal reactor has a
shape similar to that of the spectrum of neutrons emitted by the fission process.
The reason for the 1/E flux dependence at intermediate energy levels in a
thermal reactor is due to the neutrons' tendency to lose a constant fraction of
energy per collision. Since the neutrons lose a greater amount at the higher
energies, the neutrons tend to "pile up" at lower energies where they lose less
energy per collision.
The neutron flux spectrum for the slow region of a thermal reactor contains a
peak at the energy where the neutrons are in thermal equilibrium with the atoms
of the surrounding material.
The important information in this chapter is summarized below.