[en] The recycle of ²³³U for the refabrication of fuel elements for a high-temperature gas-cooled reactor will require an assessment of chemical specifications for impurities in the uranyl nitrate product stream from the fuel reprocessing facility. The behavior of thorium, as an impurity in this simulated process stream, was investigated under equilibrium conditions during full loading of carboxylic acid cation exchange resins with uranium. Mathematical correlations of the data indicate a complex metathetical reaction for thorium distribution between the two phases. Activity coefficient ratios for thorium distribution were calculated by a simple empirical relationship with thorium concentrations in the aqueous phase over the nitrate ion concentration range of approx. 0.2 to 2.0 N. Estimates of thorium behavior and its separation from uranium can be calculated from these values and derived constants for the reaction system. In all tests, thorium was preferentially absorbed on the resin with separation factors, relative to uranium concentrations, greater than unity