Neutron Moderation and Resonance Absorption

In thermal reactors which use thermal neutrons like LWRs, high-speed neutrons (with average energy about 2 MeV) are required to lower their energy from the fission energy range to the thermal range with as few collisions as possible to induce the next fission. Therefore, when we consider a chain reaction, it is important to understand not only the neutron moderation process from a fast neutron to a thermal neutron, and also what kind of energy spectrum distribution the neutron has. The process that fast neutrons become thermal neutrons is called the slowing-down process. In this chapter, we consider the neutron slowdown to thermal neutrons with energies of about several eV and the energy distribution of neutrons (neutron energy spectrum). Moreover, we consider the resonance capture reaction because as neutrons slow down from fast to thermal energy, a large part of the energy region is within the resonance capture range. In addition, we only consider neutron slowing down in an infinite medium with no space dependence for similarity.