Reactor Theory (Neutron Characteristics)
DOE-HDBK-1019/1-93
NEUTRON FLUX SPECTRUM
Rev. 0
Page 33
NP-02
Figure 2 Prompt Fission Neutron Energy Spectrum for
Thermal Fission of Uranium-235
NEUTRON FLUX SPECTRUM
The number of neutrons that exist at a given energy level varies. A plot of either
the fraction of neutrons or the neutron flux at a given energy versus the energy
level is called a neutron energy spectrum. The neutron energy spectrum varies
widely for different types of reactors.
EO 4.1
STATE the average energy at which prompt neutrons are
produced.
EO 4.2
DESCRIBE the neutron energy spectrum in the following
reactors:
a.
Fast reactor
b.
Thermal reactor
EO 4.3
EXPLAIN the reason for the particular shape of the fast,
intermediate, and slow energy regions of the neutron flux
spectrum for a thermal reactor.
Prompt Neutron Energies
The neutrons produced by fission
are high energy neutrons, and
almost all fission neutrons have
energies between 0.1 MeV and
10 MeV. The neutron energy
distribution, or spectrum, may best
be described by plotting the
fraction of neutrons per MeV as a
function of neutron energy, as
shown in Figure 2. From this
figure it can be seen that the most
probable neutron energy is about
0.7 MeV. Also, from this data it
can be shown that the average
energy of fission neutrons is about
2 MeV. Figure 2 is the neutron
energy spectrum for thermal
fission in uranium-235. The values
will vary slightly for other
nuclides.
