This lecture covers the problematic features of German pebble bed reactors, the preferred design of modular HTGRs with coated particle fuel, the HTR-PM twin unit under construction in China, the fuel handling system, the inherent safety of modular pebble bed reactors, operational limits of coated particles, and the planned modules in a typical PWR containment. It also discusses the physics of nuclear reactors, channel/pressure tube type reactors, the graphite moderated BWR, the RBMK core and fuel element, the Chernobyl accident sequence, and the deficiencies of the RBMK reactor.