Nuclear reactors play a key role in 21st Century energy production. This new book provides critical research in both fission and fusion energy production as well as the technology of the reactors. The role of nuclear data in Accelerator Driven Systems in order to reduce the cost for reaching a certain level of safety is presented and a detailed discussion of turbulent mixing and void drift that includes state-of-the-art models is given. Motivation for construction of the fusion reactors, including the laser fusion facilities and other related problems, are addressed. A brief history of magnetic confinement fusion power plant conceptual designs, focusing on tokamaks, is outlined in this book. Furthermore, the progress and state-of-the-art of principal aspects of fusion safety and environment are discussed. Since a high quality of vacuum integrity is required in large tokamak machines, leak detection systems are overviewed and a reasonable leak detection strategy is proposed.