Reactor Theory (Neutron Characteristics)DOE-HDBK-1019/1-93NEUTRON FLUX SPECTRUMRev. 0Page 33NP-02Figure 2 Prompt Fission Neutron Energy Spectrum for Thermal Fission of Uranium-235NEUTRON FLUX SPECTRUMThe number of neutrons that exist at a given energy level varies. A plot of eitherthe fraction of neutrons or the neutron flux at a given energy versus the energylevel is called a neutron energy spectrum. The neutron energy spectrum varieswidely for different types of reactors.EO 4.1STATE the average energy at which prompt neutrons areproduced.EO 4.2DESCRIBE the neutron energy spectrum in the followingreactors:a.Fast reactorb.Thermal reactorEO 4.3EXPLAIN the reason for the particular shape of the fast,intermediate, and slow energy regions of the neutron fluxspectrum for a thermal reactor.PromptNeutronEnergiesThe neutrons produced by fissionare high energy neutrons, andalmost all fission neutrons haveenergies between 0.1 MeV and10 MeV. The neutron energydistribution, or spectrum, may bestbe described by plotting thefraction of neutrons per MeV as afunction of neutron energy, asshown in Figure 2. From thisfigure it can be seen that the mostprobable neutron energy is about0.7 MeV. Also, from this data itcan be shown that the averageenergy of fission neutrons is about2 MeV. Figure 2 is the neutronenergy spectrum for thermalfission in uranium-235. The valueswill vary slightly for othernuclides.
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