## 2 Materials and Methods
### Methods  
The phase composition of the obtained samples was identified by the X-ray diffraction (XRD) method using a diffractometers Ultima-IV (Rigaku, Tokyo, Japan) and Empyrean (Panalytical, Malvern, UK). The XRD data were interpreted using the Jade 6.5 program package (MDI, Livermore, CA, USA) with PDF-2 powder database and the HighScore Plus program. The microstructure of the samples was investigated by the scanning electron microscopy (SEM) using a microscopes Mira3 (Tescan, Brno, Czech Republic) and JEOL JSM-6480 LV (JEOL, Tokyo, Japan), the electron probe microanalysis of the samples was performed by energy-dispersive X-ray spectroscopy (EDS) using an analyzer X-Max (Oxford Inst., High Wycombe, UK).  
The degree of inclusion of chlorides in NAFP glass was studied using an X-ray fluorescence spectrometer (XRF) Axious Advanced PW 4400/04 (PANalytical B.V., Almelo, The Netherlands).  
The compressive strength of the samples of MPP compounds and ceramics based on bentonite clay was determined using a test machine Cybertronic 500/50 kN (Testing Bluhm & Feuerherdt GmbH, Berlin, Germany).  
Thermal stability of the samples, obtained after solidification of the spent electrolyte imitators, was investigated in accordance with the current requirements [26] for solidified HLW. This treatment includes heating to 450 \({}^{\circ}\)C for 4 h as simulation of thermal heating of materials due to the radioactive decay of alpha-emitters present in HLW. Earlier [21], it was shown that heat treatment at a temperature of \(\geq\)300 \({}^{\circ}\)C resulted in thermal decomposition of the formed FKCN. Thus, the thermal stability of the MPP compound was studied at 270 \({}^{\circ}\)C for 4 h. The samples of the MPP compound were preliminarily kept at 180 \({}^{\circ}\)C for 10 h for removal of bound water from MgKPO\({}_{4}\)\(\cdot\)6H\({}_{2}\)O-based compounds in a muffle furnace SNOL 30/1300 (AB UMCGA GROUP, Utena, Lithuania), as previously shown in [18].  
The radiation resistance of the obtained samples was determined by their irradiation on an electron accelerator with a maximum dose of 10\({}^{8}\) Gy (MPP-Z compound after heat treatment at 180 \({}^{\circ}\)C and NAFP glass) and up to 10\({}^{9}\) Gy for a month (ceramics based on bentonite clay), after which the phase composition (XRD) and compressive strength of the samples were investigated.  
The hydrolytic stability of the samples obtained was determined in accordance with the semi-dynamic standard GOST R 52126-2003 at 25 \(\pm\) 3 \({}^{\circ}\)C and 90 \(\pm\) 3 \({}^{\circ}\)C [27]. In accordance with the standard, monolithic samples with known open geometric surface area were placed in distilled water used as a leaching agent. Distilled water was periodically replaced after 1, 3, 7, 10, 14, 21 and 28 days. Leaching was carried out in an electrical drying oven 2B-151 (Medlabortehnika, Odessa, Ukraine) in a tightly closed PTFE container. The leached solution was decanted and the content of the components in the solution after leaching was determined by ICP-AES (Agilent 720 (Agilent Technologies Inc., Springyale, Mulgrave, Victoria, Australia) and iCAP-6500 Duo (Thermo Scientific, Waltham, MA, USA)) and ICP-MS (X Series2 (Thermo Scientific, Waltham, MA, USA)), respectively. The \({}^{137}\)Cs and \({}^{152}\)Eu content in the solution was determined by gamma-ray spectrometry on a spectrometer with a high-purity germanium detector GC 1020 (Canberra, Meriden, CT, USA); \({}^{239}\)Pu content in the solution was determined using an \(\alpha\)- spectrometer Alpha Analyst (Canberra, Meriden, CT, USA).  
The leaching rate LR, [g/(cm\({}^{2}\cdot\)day)], of the components was calculated by Equation:  
\[\text{LR}=\frac{\text{c}\cdot\text{V}}{\text{S}\cdot\text{f}\cdot\text{t}}\,\]  
where c-element concentration in solution after leaching, g/L; V-the volume of leaching agent, L; S-the sample surface, cm\({}^{2}\); f-element content in matrix, g/g; t-duration of the n-th leaching period between replacements of contact solution, days.