Speaker
Description
The reprocessing of spent nuclear fuel generates high-level radioactive wastes, containing actinide elements with mainly long-lived isotopes such as Th, U, Np, Pu, Am and Cm in different quantities and chemical forms. Borosilicate glasses remain the best matrix forms for the immobilization of radionuclides due to their excellent mechanical and structual properties, therefore they will be used to stabilize the high-level radioactive wastes for disposal in a geological repository. Understanding the effects of actinide incorporation to a borosilicate glass matrix is of great importance in view of waste management. Lanthanides were considered as minor actinide surrogates based on their very similar chemical properties. The local structures of Ce3+, Nd3+ and Eu3+ ions in borosilicate glass have been investigated by neutron diffraction and synchrotron radiation based techniques. The atomic parameters, such as bond lengths and coordination environments derived from neutron- and X-ray diffraction, in combination with Reverse Monte Carlo simulations show correlation with X-ray absorption fine structure data. The lanthanide ions are in the common network with the tetrahedral SiO4 and with the mixed trigonal BO3 and tetrahedral BO4 units. Second neighbor atomic pair correlations reveal that the Ce3+, Nd3+ and Eu3+ ions are accommodated in both Si and B sites, supporting that the borosilicate-matrix well incorporates lanthanide ions and is likely to similarly incorporate actinides, opening a way to radioactive nuclear waste immobilization of this group of elements in a borosilicate glass matrix. Microscopy and microanalysis provided information on the amorphous state and on the major elemental composition of the high lanthanide-concentration samples. Results indicate that no crystallization or phase separation was detected. Details of the structural characteristics of lanthanide-containing borosilicate glasses will be presented.