A portable nuclear fuel cartridge comprising a unitary support structure and a plurality of nuclear fuel assemblies that collectively form a nuclear fuel core. The nuclear fuel core is integrated into the unitary support structure to collectively form a self-supporting assemblage than can be lifted as a single unit. In another aspect, the invention is a method of fueling and/or defueling a nuclear reactor utilizing a nuclear fuel cartridge that is loaded and/or unloaded from the nuclear reactor as a single unit. In another aspect, a nuclear reactor core is provided that comprises a nuclear fuel core comprising; a plurality of first nuclear fuel assemblies, each of the plurality of first nuclear fuel assemblies having a first transverse cross-sectional configuration; and a plurality of second nuclear fuel assemblies, each of the plurality of second nuclear fuel assemblies having a second transverse cross-sectional configuration that is different than the first transverse cross-sectional configuration.