Immobilization of spent electrolyte–radioactive waste (RW) generated during the pyrochemical processing of mixed nitride uranium–plutonium spent nuclear fuel is an acute task for further development of the closed nuclear fuel cycle with fast neutron reactors. The electrolyte is a mixture of chloride salts that cannot be immobilized directly in conventional cement or glass matrix. In this work, a low-temperature magnesium potassium phosphate (MPP) matrix and two types of high-temperature matrices (sodium aluminoironphosphate (NAFP) glass and ceramics based on bentonite clay) were synthesized. Two systems (Li_0.4K_0.28La_0.08Cs_0.016Sr_0.016Ba_0.016Cl and Li_0.56K_0.40Cs_0.02Sr_0.02Cl) were used as spent electrolyte imitators. The phase composition and structure of obtained materials were studied by XRD and SEM-EDS methods. The differential leaching rate of Cs from MPP compound and ceramic based on bentonite clay was about 10⁻⁵ g/(cm²·day), and the rate of Na from NAFP glass was about 10⁻⁶ g/(cm²·day). The rate of ²³⁹Pu from MPP compound (leaching at 25 °C) and NAFP glass (leaching at 90 °C) was about 10⁻⁶ and 10⁻⁷ g/(cm²·day), respectively. All the synthesized materials demonstrated high hydrolytic, mechanical compression strength (40–50 MPa) even after thermal (up to 450 °C) and irradiation (up to 10⁹ Gy) tests. The characteristics of the studied matrices correspond to the current requirements to immobilized high-level RW, that allow us to suggest these materials for industrial processing of the spent electrolyte.

The search for matrices and technological solutions for the reliable immobilization of volatile radionuclides and high-level waste (HLW) components is an actual radiochemical problem. Methods of obtaining of sodium alumino-iron phosphate (NAFP) and iron phosphate (FP) glass composite materials synthesized at temperatures of 450–750 °C, their structure and hydrolytic stability were investigated in this paper. The structure of the samples was studied by XRD and SEM-EDS. It was shown that, in the case of FP materials, the phase composition varies depending on the synthesis temperature, while NAFP materials have a complex multiphase composition at all crystallization temperatures. It has been established that the samples of the obtained glass composite materials have a high hydrolytic stability. At the same time, FP material obtained at 650 °C are the most stable, which makes this medium-temperature method of synthesis promising for the immobilization of volatile HLW components.

Magnesium potassium phosphate (MPP) matrix MgKPO₄ × 6H₂O is a known material for radioactive waste (RW) conditioning; magnesium oxide MgO is used in the classical method of matrix synthesis. The aim of the work was to investigate the possibility of using the widely available natural mineral raw material dolomite, incorporating mixed magnesium and calcium carbonate, for the production of this matrix. To control the quality of the samples obtained, their phase and elemental compositions, microstructure, density, volumetric and apparent porosity, compressive strength, and hydrolytic stability were studied. As a result, it was found that using dolomite powder pre-calcined at 720 °C for 1.5 h, samples of matrix whose properties meet the regulatory requirements for solidified RW were obtained. Thus, a new, cost-effective method of synthesis of the MPP matrix has been demonstrated to solve the RW solidification problem.

Among the many radiochemical problems, the search for new materials and technologies for the immobilization of radioactive waste remains relevant, and the range continues to change and expand. The possibility of immobilizing the spent chloride electrolyte after the pyrochemical processing of the mixed uranium-plutonium spent nuclear fuel of the new fast reactor BREST-OD-300 on lead coolant into glass-composite phosphate materials synthesized at temperatures of 650–750 °C was studied. The structure of the obtained samples was studied using XRD and SEM/EDS methods. It has been shown that the spent electrolyte simulator components create stable mixed pyrophosphate phases in the glass composite structure. The materials were found to have high hydrolytic stability. This indicates the promise of using phosphate glass composites as materials for the reliable immobilization of the spent electrolyte.