Thermal Neutron Spectrum

The neutron energy spectrum in LWRs is shown in Fig. 6.1. For thermal reactors such as LWRs, 80 % of all fissions are induced by thermal neutrons, and the released neutrons have energy as high as around 2 MeV. The energy spectrum for the high-energy region, primarily above several hundred keV, is determined by the fission energy spectrum. In the resonance energy region, which is from several eV to several hundred keV, neutrons lose their energy by progressively colliding with moderator, and the resonance energy spectrum gives the smooth 1/E curve; however, some irregularity can occur because of the resonance absorption of 238U. In the thermal energy region, which is below the scission energy (around 1 eV), neutrons are in thermal equilibrium, and the energy spectrum is given by an approximately Maxwellian distribution.