Abstract
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.
Highlights
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
The Ln elemental content of the Matrix-Ce10, Matrix-Nd10 and Matrix-Eu10 glasses obtained by X-ray fluorescence (XRF) agreed with the nominal weighed-in concentrations within relative 5%
Maximum conversion of BO3 into BO4 units is produced by Ce ions, a result which is found by both neutron diffraction and 11B nuclear magnetic resonance (NMR) studies
Summary
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. A plethora of the useful glass properties stem from those of the network formers which constitute the bulk of the glass These properties, are considerably modified by the network modifiers, which exist as single ions amongst the cross-linked network (of Si-O-B in borosilicate glasses) reducing the relative number of strong bonds leading to lower melting point, lower viscosity and modified thermal and electrical properties. This effect is of particular importance from the point of view of chemical durability of a glass. That Ce is easier to reduce to its trivalent state at higher preparation
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