Nuclear energy is a large-scale, dispatchable, low-carbon source of electricity and heat, but its sustainability depends on more than operational carbon emissions. This chapter examines nuclear energy across reactor physics, fuel-cycle chemistry, radioactive-waste management, safety, proliferation governance, lifecycle environmental performance, economics and emerging reactor technologies. It reviews fission and fusion fundamentals; compares light-water, Generation IV and small modular reactor concepts; and evaluates spent-fuel management and deep geological disposal. Evidence from Chernobyl and Fukushima is distinguished from evacuation and wider social consequences using United Nations scientific assessments. Lifecycle greenhouse-gas emissions are compared with other electricity technologies, while water use, public acceptability, safeguards and intergenerational equity are treated as complementary sustainability dimensions. Current deployment data and net-zero scenarios situate nuclear power within a portfolio that also includes renewable generation, storage and demand flexibility. The chapter concludes that nuclear power can contribute to low-carbon energy systems under appropriate technical, financial and governance conditions, but its benefits do not remove the need for rigorous safety, waste-management, safeguards and project-delivery institutions.