Moderator and Reflector Materials
Moderators have attributes of the structural materials. They have adequate mechanical strength, thermal and irradiation stability, fabricability, and corrosion resistance. In a thermal reactor the nuclear requirements are the same for moderators and reflectors. The high cost of heavy water and its importance as a reactor moderator have generated active competition for the best production methods. Carbon in the form of graphite has the unique distinction of being the first reactor moderating material ever used. The intrinsic properties of reactor-grade graphite, impermeable graphite, or pyrolytic carbon serving as moderator-reflector structure material in the thermal reactor are its low neutron absorption cross section and its good moderating properties. The compatibility problems of a graphite moderator are almost entirely those arising from reaction with the coolant. The most popular and promising gases for cooling reactors are air, carbon dioxide, and helium.