The document discusses nuclear reaction cross-sections, which are measured in barns. The cross-section for fast neutrons on U235 is 1.27 barns, while for thermal neutrons it is 580 barns. Moderators like graphite and heavy water are used to slow fast neutrons from 2 Mev to thermal energies to sustain a chain reaction. Control rods made of materials like cadmium and lead absorb neutrons to control the power production rate.