Can someone explain what "hardening" of a neutron spectra means?
Average neutron energy, or more particularly, the distribution of neutron energy shifts upward, or the fast neutron portion of the distribution increases while the thermal (low energy) portion of the neutron energy distribution decreases.
One can take advantage of spectral shift (shift in neutron energy spectrum) to increase the conversion of U-238 into Pu-239, which allows for more fission production without increasing the U-235 enrichment.
Put simply, a harder spectrum has the neutrons traveling at higher speeds (on average).
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