NEUTRON FLUX SPECTRUM
Reactor Theory (Neutron Characteristics)
Figure 3 Comparison of Neutron Flux Spectra for
Thermal and Fast Breeder Reactor
Thermal and Fast Breeder Reactor Neutron Spectra
The spectrum of neutron energies produced by fission varies significantly from the energy
spectrum, or flux, existing in a reactor at a given time. Figure 3 illustrates the difference in
neutron flux spectra between a thermal reactor and a fast breeder reactor. The energy
distribution of neutrons from fission is essentially the same for both reactors, so the differences
in the curve shapes may be attributed to the neutron moderation or slowing down effects.
No attempt is made to thermalize or slow down neutrons in the fast breeder reactor (liquid metal
cooled); therefore, an insignificant number of neutrons exist in the thermal range. For the
thermal reactor (water moderated), the spectrum of neutrons in the fast region (> 0.1 MeV) has
a shape similar to that for the spectrum of neutrons emitted by the fission process.
In the thermal reactor, the flux in the intermediate energy region (1 eV to 0.1 MeV) has
approximately a 1/E dependence. That is, if the energy (E) is halved, the flux doubles. This
1/E dependence is caused by the slowing down process, where elastic collisions remove a
constant fraction of the neutron energy per collision (on the average), independent of energy;
thus, the neutron loses larger amounts of energy per collision at higher energies than at lower
energies. The fact that the neutrons lose a constant fraction of energy per collision causes the
neutrons to tend to "pile up" at lower energies, that is, a greater number of neutrons exist at the
lower energies as a result of this behavior.