This lecture covers the analysis of heterogeneous reactors, focusing on the multiregion reactor, one-group method, analytical solution for flux, criticality condition, and reflector savings. It discusses the equations and boundary conditions for a reactor with different regions, the infinite slab reactor with a reflector, and the monoenergetic neutrons in multiplying media. The critical reactor equation, slab reactor system, and material buckling are also explained. The lecture concludes with a summary of reflector savings and the solution for a slab reactor, emphasizing the criticality condition and the impact of reflectors.