What Does Hardening of Neutron Spectra Mean?

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SUMMARY

The "hardening" of neutron spectra refers to the upward shift in average neutron energy, resulting in an increased proportion of fast neutrons while the thermal neutron portion decreases. This spectral shift enhances the conversion of U-238 into Pu-239, facilitating greater fission production without the need for higher U-235 enrichment. A harder neutron spectrum indicates that neutrons are traveling at higher speeds on average, which is crucial for nuclear reactions.

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Can someone explain what "hardening" of a neutron spectra means?
 
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nuclear420 said:
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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