This lecture covers the concepts of chain reaction, multiplication factor, 4 and 6 factor formulae, η factor, and conversion (breeding) ratio in neutron chain-reacting systems. It also discusses fuel cycles, general considerations of reactor design, and types of nuclear reactors such as VVER-1000, RBMK, PWR, BWR, CANDU, MAGNOX, HTR-10, and BN-600. The lecture concludes with a summary of the multiplication factor and conversion ratio.