[en] The disposal of excess ²³³U as waste is being considered. Because ²³³U is a fissile material, a key requirement for processing ²³³U to a final waste form and disposing of it is the avoidance of nuclear criticality. For many processing and disposal options, isotopic dilution is the most feasible and preferred option to avoid nuclear criticality. Isotopic dilution is dilution of fissile ²³³U with nonfissile ²³⁸U. The use of isotopic dilution removes any need to control nuclear criticality in process or disposal facilities through geometry or chemical composition. Isotopic dilution allows the use of existing waste management facilities that are not designed for significant quantities of fissile materials to be used for processing and disposing of ²³³U. The amount of isotopic dilution required to reduce criticality concerns to reasonable levels was determined in this study to be approximately 0.53 wt % ²³³U. The numerical calculations used to define this limit consisted of a homogeneous system of silicon dioxide (SiO₂), water (H₂O), ²³³U and depleted uranium (DU) in which the ratio of each component was varied to learn the conditions of maximum nuclear reactivity. About 188 parts of DU (0.2 wt % ²³⁵U) are required to dilute 1 part of ²³³U to this limit in a water-moderated system with no SiO₂ present. Thus for the U.S. inventory of ²³³U, several hundred metric tons of DU would be required for isotopic dilution