Reactor Theory (Neutron Characteristics)DOE-HDBK-1019/1-93NEUTRON MODERATIONRev. 0Page 23NP-02NEUTRON MODERATIONIn thermal reactors, the neutrons that cause fission are at a much lower energythan the energy level at which they were born from fission. In this type ofreactor, specific materials must be included in the reactor design to reduce theenergy level of the neutrons in an efficient manner.EO 2.12 DEFINE the following concepts:a. Thermalizationd. Average logarithmic energy decrementb. Moderatore. Macroscopic slowing down powerc. Moderating ratioEO 2.13 LIST three desirable characteristics of a moderator.EO 2.14 Given an average fractional energy loss per collision,CALCULATE the energy loss after a specified number ofcollisions.NeutronSlowingDownandThermalizationFission neutrons are produced at an average energy level of 2 MeV and immediately begin toslow down as the result of numerous scattering reactions with a variety of target nuclei. Aftera number of collisions with nuclei, the speed of a neutron is reduced to such an extent that it hasapproximately the same average kinetic energy as the atoms (or molecules) of the medium inwhich the neutron is undergoing elastic scattering. This energy, which is only a small fractionof an electron volt at ordinary temperatures (0.025 eV at 20C), is frequently referred to as thethermal energy, since it depends upon the temperature. Neutrons whose energies have beenreduced to values in this region (< 1 eV) are designated thermal neutrons. The process ofreducing the energy of a neutron to the thermal region by elastic scattering is referred to asthermalization, slowing down, or moderation. The material used for the purpose of thermalizingneutrons is called a moderator. A good moderator reduces the speed of neutrons in a smallnumber of collisions, but does not absorb them to any great extent. Slowing the neutrons in asfew collisions as possible is desirable in order to reduce the amount of neutron leakage from thecore and also to reduce the number of resonance absorptions in non-fuel materials. Neutronleakage and resonance absorption will be discussed in the next module.
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