ИСТИНА

Molten salt reactors have been developing for the purpose of "transmutation" of minor actinides. The working body in such reactors is assumed to be a molten salt: a eutectic mixture of fluoride salts, either FLiNaK (lithium, sodium, potassium fluorides) or FLiBe (lithium and beryllium fluorides). At present, there are no simple and reliable technologies for immobilization of complex radioactive waste represented by a mixture of fission product fluorides. In this work, matrix samples of a eutectic mixture of FLiNaK with imitators of high-level waste (up to 30 wt.%) and natural raw bentonite were synthesized. The samples in the form of tablets were annealed at different temperatures (700 - 1000 °C), their phase composition has been determined. The high chemical resistance at 25 °C (cesium leaching rate < 10-5 g/(sm2∙day)) and high mechanical durability (> 9 MPa) at all selected annealing temperatures (650 - 900 °C) has been observed. It has been determined by X-ray diffraction analysis with in-situ heating and thermogravimetry with differential scanning calorimetry that the most significant changes in the structure of matrix samples occur at a temperature of ~700 °C. It has been found that fluorine binds to clay components at temperatures below 800 °C. The optimal temperature range for synthesis is found to be between 650-800 °C. It is shown that the addition of calcium compounds has a positive effect on the binding of fluoride ion, which leads to an increase in the temperature of the onset of HF release and the durability of the samples and reduces cesium leaching. The spark plasma sintering method allows to produce matrix samples with similar characteristics. In addition, this method allows for faster synthesis and is potentially suitable for obtaining massive samples. In conclusion, the results demonstrate the principal possibility of using materials based on bentonite clay for immobilization of fluoride radioactive waste.