a substance containing one or more fissile nuclides capable of maintaining a chain reaction in a reactor; also a substance containing one or more fertile nuclides that can be transmuted into such fissile nuclides, aine, milles saab toimuda tuumaahelreaktsioon. Tuumareaktorites kasutatakse harilikult uraani ja plutooniumi isotoope 233/92U, 235/92U ja 239/92Pu. Looduses leidub neist ainult 235U (primaarne tuumkütus), teisi (sekundaarseid) toodetakse tuumareaktorites nn tuumatoorainest
