The buildup of the important transmutation products in irradiated uranium was calculated. Significant quantities of such products are produced upon irradiation with pile neutrons, using an MTR geometrical configuration as reference. These quantities are further increased with subsequent recycle through power reactors. The nuclides are U236, U237, Np237, and Pu238. Variables included in this study were: irradiation levels of 6 × 1019 to 3 × 1021 n/cm2; effect of recycle in the range 1 to 400 cycles and infinite recycle (or steady state); initial fuel enrichment (where applicable) in the range of 0.5–3.0% U235; and the effect of fraction of U236 removed by a gaseous diffusion plant reconcentration of U235 in the range 0–100% removal. This last variable depends on the operational characteristics of the diffusion plant. The buildup of transmutation products may have many appreciable effects on the design and operation of fuel recycle. The decay time required will increase as a result of higher concentrations of U237; chemical separation plants may be required to separate Np237 as well as uranium, plutonium, and fission products; and the buildup of Pu238 in the plutonium product may create additional biological or handling problems. An important conclusion of this work is that all problems resulting from isotope buildup in the U235 buildup chain may be decreased in seriousness by approximately an order of magnitude with removal of about 25% of the U236 by re-enrichment in a gaseous diffusion plant.