Splitting of a heavy nucleus into two smaller nuclei (fission products), usually from a neutron impact, accompanied by the emission of neutrons, radiation and a large amount of heat. The energy released in the form of heat is the underlying principle of nuclear power generation.

Fragments of heavy nuclei produced by nuclear fission (breakdown of uranium 235 or plutonium 239 nuclei) or by subsequent radioactive decay of nuclides formed in this process. Fragments from the fission process and their daughter products are called "fission products". In reprocessing plants, fission products are separated by solvent extraction after the fuel has been dissolved in nitric acid; they are concentrated by evaporation, stored for conditioning by vitrification, and poured into stainless steel canisters.

A fuel assembly is made of fuels rods containing uranium oxide pellets — the fissile material — held together in a metal frame, or “skeleton”, usually made of zirconium alloy.

All of the operations involved in making nuclear fuel. These operations are primarily uranium mining, milling, conversion and enrichment; fuel fabrication; spent fuel reprocessing and recycling of recovered fissile materials; and waste management. The fuel cycle is said to be "closed" when spent fuel is reprocessed and fissile materials recovered by reprocessing are recycled. In the "open" or "once-through" cycle, the spent fuel is stored or disposed of once it has been discharged from the reactor.

A bundle of fuel rods filled with uranium pellets. A reactor core holds from 100 to 200 fuel assemblies, depending on the type of power plant.