Thermal and Fast Breeder Reactor Neutron Spectra

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NEUTRON FLUX SPECTRUM DOE-HDBK-1019/1-93 Reactor Theory (Neutron Characteristics) NP-02 Page 34 Rev. 0 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.


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