This paper presents calculated values of the extrapolation distance for the water-reflected 235U-H2O-Zr ternary system. This extrapolation distance, together with previously published critical buckling data, permits the ready determination of critical dimensions for all possible compositions of this system. Prior to this study, only limited data were available for the extrapolation distance for the 235U-H2O binary system, and no data existed for the ternary system. A quantitative determination of the extrapolation distance was achieved utilizing, in a unique manner, nuclear codes developed for reactor design purposes. Accuracy of the results has been confirmed at compositions for which experimental data are available. The extrapolation distance was found to be essentially independent of the shape of the system but strongly dependent upon composition. A single diagram that presents critical buckling and extrapolation distance as a function of composition has been developed. With this diagram it is possible to determine critical dimensions for a given shape and composition and optimum conditions for criticality. As an important practical example, the minimum critical limits for optimally water-moderated cylindrical arrays of 235U-Zr fuel elements are presented as a function of fuel-element length and composition.