A study of the mechanical and thermophysical properties of crystal matrices for the immobilization of high-level wastes

Objectives. The aim of the study was to confirm the compliance of the mechanical and thermophysical properties of titanate-zirconate mineral-like matrices intended for immobilization of the rare-earth-actinide fraction of high-level waste (HLW) with pyrochlore structures (Nd₂ZrTiO₇) and orthorhombic titanate of rare earth elements (Nd₄Ti₉O₂₄+TiO₂) with the Russian requirements for the final forms of radioactive waste sent for disposal. With regard to fractionated radioactive waste, this type of matrix is preferable when compared with conservative aluminophosphate and borosilicate glasses. This is due to larger capacity, and a better level of chemical, thermal, and radiation resistance.

Methods. The synthesis of mineral-like matrices was carried out by remelting a granular precursor consisting of mineral-forming metal oxides and a solution imitating the rare earth-actinide fraction of HLW in an induction furnace with a cold crucible. The thermal diffusivity was determined by the laser flash method. The heat capacity of the matrix samples was measured by differential scanning calorimetry. Ultimate flexural and compressive strengths were determined using universal test machines. The elastic moduli (Young’s) were measured by the acoustic method. The temperature coefficients of linear expansion were determined using a high-temperature dilatometer.

Results. The ultimate strength of the matrices (Nd₂ZrTiO₇) and (Nd₄Ti₉O₂₄+TiO₂) was found to be 150–179 and 20.6–57.8 MPa in compression and bending respectively. Young’s moduli vary from 3.7 ∙ 10⁷ to 2.15 ∙ 10⁸ kN/m². With an increase in temperature from 50 to 500°C, the values of thermal conductivity have a pronounced tendency to decrease from 1.71 to 0.91 W/(m∙K). The temperature coefficients of linear expansion increase from 6.96 ∙ 10⁻⁶ to 1.01 ∙ 10⁻⁵ K⁻¹ in the same temperature range.

Conclusions. Comprehensive studies of titanate-zirconate mineral-like matrices show that their mechanical and thermal properties in certain cases significantly exceed the minimum requirements of regulatory documentation for the final forms of HLW.

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