ABSTRACT
This chapter discusses how fission neutrons, which are initially produced with an average kinetic energy of about 2 MeV, are converted into thermal or epithermal neutrons with a kinetic energy of less than 1 keV. Many materials can serve as neutron moderators, and ideally, the moderating material should be the same material as the coolant. For a variety of reasons, fast reactors use different coolants than thermal reactors do, and in general, most reactor coolants tend to fall into three broad categories. These include: thermal water reactor coolants; gas reactor coolants; and fast reactor or liquid metal coolants. Reactor coolants can be classified as fluids, liquid metals, and gases. Early on, three liquid metals were considered as fast reactor coolants; they were liquid sodium, liquid potassium, and liquid mercury. Liquid sodium has a very high boiling point, and so reactor coolant loops can be operated at very high temperatures and low pressures without boiling the coolant.
