Optimization conditions for the separation of rare earth elements, americium, curium and cesium with HPLC technique

In order to validate the neutronic code calculations used in the nuclear field, it is necessary to quantify the isotopic composition of the nuclear irradiated fuels with high precision and accuracy. A separation step whose objective is to isolate the elements of interest in a fraction as pure as possible, is performed before mass spectrometry detection (MC-ICP-MS, TIMS), in order to eliminate isobaric interferences. In a first step, uranium and plutonium are separated from the fission products by ion exchange chromatography. Then, fission products are separated by high performance liquid chromatography (HPLC). A gradient separation with a silica sulphonated support as stationary phase and hydroxy-methyl butyric acid (HMB) as eluent solution provides a good degree of separation of the rare earth elements, Am, Cm and Cs. But, for a same manufacturer, the rate of sulphonated groups is variable from one column to another for a given production batch. For every replacement of column, it is necessary to adjust the separation conditions. Furthermore, the high radioactivity of the samples injected damages the stationary phase, dragging a regular renewal of the columns. The goal of this study is to determine the optimal separation conditions with a minimum of tests in order to limit the time assigned to this operation. A first Doehlert experimental design with pH and HMB concentration as variables has been implemented to optimize Gd, Eu separation, in isocratic initial conditions. A second one has been tested in order to separate the higher lanthanides and to fix the final conditions of the gradient separation. A last set of tests has been performed to characterize the cesium behaviour. Finally, the gradient conditions have been adjusted in order to perform the full separation of lanthanides, Cs, Am and Cm with nuclear facilities.