## 4 Conclusions  
In this work, three types of matrices (MPP matrix, NAFP glass, ceramics based on natural bentonite clay) with the addition up to 20 wt% of the spent electrolyte imitator LiCl-KCl were synthesized. The phase composition of the samples, their mechanical, hydrolytic, thermal and radiation stability were characterized. The results of this work indicate that the studied matrices and methods of their synthesis provide reliable characteristics that comply with the regulated requirements for immobilized RW: mechanical compressive strength \(\geq\)9 MPa; radiation resistance (without changes in mechanical strength and structure at an absorbed dose of up to 10\({}^{8}\)-10\({}^{9}\) Gy); and leaching rate (for all components \(\leq\) 10\({}^{-5}\) g/(cm\({}^{2}\cdot\)day)). Each of the materials has its own advantages and individual disadvantages. Thus, a low-temperature MPP matrix is easily synthesized at room temperature, but its strength is dramatically decreased under thermal stress. The glass forms a homogeneous system, demonstrates low leaching rates, but under thermal loads, the release of individual mineral phases on the surface is observed. A ceramic matrix based on bentonite clay is able to withstand both thermal loads and high doses of radiation, but the method for synthesizing such a matrix requires complex operations. As the result, the fundamental possibility to immobilize and include up to 20 wt% HLW from pyrochemical processing of MNUP SNF into the matrix was confirmed. The choice of the optimal matrix depends on the final option of the technology for purifying the electrolyte from radionuclides. Different options will result in generation of RW of various levels of radiation hazard; therefore, it will determine methods of their immobilization. In the case of intermediate level waste and, accordingly, a low probability of radiation heating produced by immobilized radionuclides, the MPP matrix seems to be optimal, while both glass and bentonite ceramics are promising for HLW. Figure 9: Kinetic curve of the leaching rate of components of the ceramics based on bentonite clay. Further investigations will be aimed at testing various modifiers for the studied matrices, which would improve their characteristics. Conceptualization, S.A.K., S.S.D. and A.V.M. ; methodology, S.A.K., S.S.D., S.E.V. and V.G.P. ; validation, S.A.K., S.S.D. and A.V.M. ; formal analysis, S.A.K., S.S.D., A.V.F., K.Y.B. and A.V.M. ; investigation, A.V.F., A.V.M. and K.Y.B. ; resources, S.E.V. and V.G.P. ; writing--original draft preparation, S.A.K., A.V.F. and A.V.M. ; writing--review and editing, S.E.V. and V.G.P. ; visualization, S.A.K., S.E.V. and V.G.P. ; supervision, S.E.V., V.G.P., B.F.M. ; project administration, S.E.V. and B.F.M. ; funding acquisition, S.E.V. and B.F.M. All authors have read and agreed to the published version of the manuscript. The work was supported by the Ministry of Science and Higher Education of Russia (grant agreement No.075-15-2020-782). Data Availability Statement: Data sharing not applicable. The authors thank [PERSON], [PERSON] (Lomonosov Moscow State University; Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry of Russian Academy of Sciences) and [PERSON] (Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Academy of Sciences) for the opportunity provided to use diffractometers of Ultima-IV X-ray (Rigaku) and Empyrean (Malvern Panalytical); [PERSON] and [PERSON] (Laboratory of Methods for Investigation and Analysis of Substances and Materials, GEOKHI RAS); for performing the ICP-AES and ICP-MS analysis, [PERSON] and [PERSON] (Meteoritics laboratory, GEOKHI RAS) for the analysis of samples by SEM with EDS. The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. * [PERSON] et al.