This book provides a clear and objective understanding of nuclear power's role in a low-carbon, renewables-heavy energy system. It situates nuclear energy as a potential long-term electricity stabiliser, taking into account ongoing climate concerns, the growing dominance of wind and solar, renewed global interest in nuclear power, and recent large-scale grid disruptions.The book explores one of today's central challenges: decarbonising electricity systems at scale while maintaining reliability. Although wind and solar power are expected to contribute significantly, their variability and weather dependence limit their ability to fully displace fossil fuels. Energy storage and demand management offer partial solutions, but during prolonged periods of low renewable output, persistent gaps become clear, bringing renewed attention to the role of nuclear power within a low-carbon energy mix.Nuclear Power in the 21st Century provides a rigorous examination of present and future nuclear technologies. It explains the factors that limit their operational flexibility and assesses the strengths, weaknesses, and future prospects of both fission and fusion designs. It presents practical approaches for configuring nuclear plants to provide additional electricity when renewables underperform for extended periods, while addressing major concerns, including safety, cost, and radioactive waste with a balanced and evidence-based perspective.Timely and analytically grounded, this book is an essential read for policymakers, engineers, researchers, and anyone seeking a realistic understanding of the future of energy.