Separation by extraction chromatography of neptunium and plutonium from process flows of a reprocessing plant for HTR fuel elements

Closing the Th-U fuel cycle, the recovered material should be purified from Np and Pu in the course of reprocessing. For this purpose, an extraction chromatograph process was deloped and tested with simulated reprocessing end streams containing Np and Pu. Chromatographic compounds of the LEWEXTREL-type were investigated which consisted of a polystyrene matrix loaded with alamine 336 (95% tri-n-octylamine). Regarding the loading capacity of the compound and the mass transfer between the phases, the best results were obtained with LENEXTREL 35//37,5 (linked with 35 weigth- % of divinylbenzene, loaded with 37,5% of alamine 336) using a particle size of < 10-4 m. The compound was proved to be stable against radiolytic decomposition up to 106 rad. Further irradiation decreased the overall capacity of the resin without changing the kinetic properties so that the compound can be used up to a radiation dose of 108 rad. The desorption of Np and Pu was accomplished using complexing agents. Most efficient elutriants were mixtures of 0.1 M HNO3 with critic or tartraric acid. Based on the parameters determined in the study, a separation unit was laid out which can be connected with a pilot reprocessing plant for spent Th-U fuel of a 2 kg/d throughpout. With this unit, solutions containing Th and U are purified from Np and Pu providing DF-values in the range of 102 up to 104. In-line monitoring of the process will not be needed assuming constant mass transfer properties of the separation compound and considering the decrease of the resin capacity when the column is packed. (orig./RB)