Nuclear fuel is a research concept and material science domain focused on substances engineered to sustain controlled nuclear fission or fusion reactions for energy generation. This field investigates the physical, chemical, and nuclear characteristics of fissile or fusile materials, their performance and behavior under extreme reactor environments including high temperatures and intense radiation, and their complete lifecycle from production and utilization through to reprocessing and disposal. The significance of nuclear fuel lies in its unparalleled energy density, its critical role in nuclear power generation, and the complex associated challenges concerning reactor safety, radioactive waste management, and nuclear non-proliferation.