Structural investigation of borosilicate glasses containing lanthanide ions

High level radioactive actinides are produced as a side product in reprocessing spent nuclear fuel, for which safe long-term-inert immobilizer matrices are needed. Borosilicate glasses are of great potential amongst the candidates of suitable inert materials for radioactive waste immobilization. Understanding the effects of actinide addition to a borosilicate glass matrix is of great importance in view of waste immobilization. Here we present structural studies of a simplified glass-matrix, - 55SiO₂·10B₂O₃·25Na₂O·5BaO·5ZrO₂ - upon adding lanthanide (Ln-)oxides: CeO₂, Nd₂O₃, Eu₂O₃, in two different concentrations 10% and 30w% each, to investigate the effects of lanthanides (Ln) taken as chemical surrogates for actinides. Neutron diffraction combined with of Reverse Monte Carlo simulations show that all investigated glass structures comprise tetrahedral SiO₄, trigonal BO₃ and tetrahedral BO₄ units, forming mixed ^[4]Si-O-^[3]B and ^[4]Si-O-^[4]B linkages. ¹¹B Magic Angle Spinning Nuclear Magnetic Resonance is indicative of simultaneous presence of trigonal BO₃ and tetrahedral BO₄ units, with spectral fractions strongly dependent on the Ln addition. Ln-addition promote the BO₃ + O^-→[BO₄]^- isomerization resulting in lower fraction of boron in BO₃, as compared to BO₄ units. Raman spectra, in full agreement with neutron diffraction, confirm that the basic network structure consists of BO₃/trigonal and SiO₄/BO₄ tetrahedral units. Second neighbour atomic pair correlations reveal Ce, Nd, Eu to be accommodated in both Si and B sites, supporting that the borosilicate-matrix well incorporates Ln-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.