An introductory course in nuclear physics. Some basic knowledge and skill in mathematical analysis (integration, power series expansions, ordinary and partial differential equations) as well as in numerical computation. The aim of this course is threefold: a description of the basic principles of nuclear engineering (fuel cycles, reactor types, etc ...), the understanding of the fundsamental concepts (cross sections, phase space, neutron fluxes and currents, criticality, etc...) and the development of a model (the multigroup-diffusion model) allowing to perform reactor computations. By reactor computations, one generally refers to the determination of the conditions that have to be satisfied in order to produce energy in steady-state, the space dependence of the energy production inside the reactor and the time evolution of the energy production if the steady-state conditions are no longer met. The MECA2600 course is adapted from the reference "Nuclear Reactor Analysis", by J.J. Duderstadt and L.J. Hamilton (John Wiley, 1976), chapters 1 to 6.