Abstract: Fissionable fuels and/or fertile breeder materials for nuclear reactors crised of fissionable or potentially fissionable materials having evenly distributed throughout certain metal oxides, such as aluminum oxide and niobium oxide, exhibiting the capacity to retain products formed by nuclear fission as stable, nonvolatible compounds in the nuclear material. The fuel element is combined with an effective amount of oxide that is relatively nonvolatile, has a low neutron-capture cross-section and forms such stable, nonvolatile compounds with the solid products resulting from nuclear fission.