## 3 Results and Discussion
### Mechanical, Thermal and Radiation Resistance of Materials  
The results of tests on determination of compressive strength of the MPP compound samples obtained by solidification of electrolyte imitator solution of composition Li\({}_{0.4}\)K\({}_{0.28}\)La\({}_{0.08}\)Cs\({}_{0.016}\)Sr\({}_{0.016}\)Ba\({}_{0.016}\)Cl and containing 23 wt% zeolite are presented in Figure 3. It was established that with increasing the chloride content in the compound the compressive strength of the samples increases by 2-3 times. This parameter reaches about 24 MPa at the maximum chloride content (12 wt%) in the compound. These values are higher than the regulatory requirements for solidified RW (not less than 9 MPa) [26]. It was noted that when 23 wt% zeolite is added to the compound, the compressive strength of the samples increases and is about 40 MPa (Figure 3). To determine the thermal and radiation resistance of the MPP compound with immobilized RW imitator solution, the samples were first heated at 180 \({}^{\circ}\)C for 6 h to remove bound water from the compounds. The compressive strength of MPP-Z compound decreases after heat treatment of samples at 450 \({}^{\circ}\)C for 4 h. The decrease in the compressive strength of the compound is probably associated with an increase in the porosity of the sample due to the removal of bound water, as shown in [30], and a transformation of the crystalline structure of the MPP compound into an amorphous one, as shown in [31]. It has to be noted that \begin{table}
\begin{tabular}{c c c c} \hline
**Element** & **Calculated** & **SEM-EDS** & **XRF** \\ \hline Na & 15.4 & 14.2 & 15.2 \\ P & 20.7 & 21.7 & 20.1 \\ Fe & 9.3 & 8.7 & 10.0 \\ Al & 4.5 & 5.4 & 4.8 \\ Cs & 0.3 & 0.2 & 0.3 \\ K & 1.6 & 1.8 & 1.6 \\ Li & 0.4 & - & - \\ La & 1.7 & 1.7 & 2.3 \\ Sr & 0.2 & 0.5 & 0.4 \\ Ba & 0.3 & 0.3 & 0.4 \\ Cl & 5.4 & 1.9 & 1.9 \\ O & 40.2 & 42.4 & 41.6 \\ Si & - & 1.2 & 1.4 \\ \hline \end{tabular}
\end{table} 
Table 2: The elemental composition (wt%) of NAFP glass samples containing spent electrolyte imitator. Figure 3: Compressive strength of MPP compound containing spent electrolyte imitator. the compressive strength of the studied compounds meets the regulatory requirements for solidified RW and is not less than 9 MPa (Figure 3) [26]. High values of the compressive strength ensure radiation safety when handling solidified radioactive waste (transportation, reloading, placement in a storage facility). At the same time, zeolite exhibits reinforcing properties, allowing the maintaining of the strength of the compound at the required level, even after high temperature treatment of samples, as was shown earlier in [19]. It was found that the surface of NAFP glass samples after heat treatment at 450 \({}^{\circ}\)C for 4 h is inhomogeneous and consists of several phases (Figure 4a). This is probably due to the crystallization process that began on the sample surface, which is characteristic of glasses at (480-630) \({}^{\circ}\)C [32]. Several phases with different contents of structure-forming elements (glass phases (Phase #1, Figure 4a, Table 3) and a mixture of sodium-iron-aluminum pyrophosphates (Phase #2, Figure 4a, Table 3), as shown earlier in [22]) as well as a phase containing sodium and chlorine (Phase #3, Figure 4a, Table 3) were found. The transition zone of degradation of the surface of NAFP glass is shown in Figure 4b, the thickness of the altered surface layer was about 2-3 microns. The data on the compressive strength of the ceramics based on bentonite clay, including after heat treatment at 450 \({}^{\circ}\)C for 4 h and irradiation by electrons, are presented in  
Figure 4: SEM image of (**a**) NAFP glass and (**b**) cleavage surface of the sample after heat treatment at 450 \({}^{\circ}\)C.