The key task in the solidification of high-level waste (HLW) into a magnesium potassium phosphate (MPP) compound is the immobilization of mobile cesium isotopes, the activity of which provides the main contribution to the total HLW activity. In addition, the obtained compound containing heat-generating radionuclides can be significantly heated, which increases the necessity of its thermal stability. The current work is aimed at assessing the impact of various methodological approaches to HLW solidification on the thermal stability of the MPP compound, which is evaluated by the mechanical strength of the compound and its resistance to cesium leaching. High-salt surrogate HLW solution (S-HLW) used in the investigation was prepared for solidification by adding sorbents of various types binding at least 93% of ¹³⁷Cs: ferrocyanide K-Ni (FKN), natural zeolite (NZ), synthetic zeolite Na-mordenite (MOR), and silicotungstic acid (STA). Prepared S-HLW was solidified into the MPP compound. Wollastonite (W) and NZ as fillers were added to the compound composition in the case of using FKN and STA, respectively. It was found that heat treatment up to 450 °C of the compound containing FKN and W (MPP-FKN-W) almost did not affect its compressive strength (about 12–19 МPa), and it led to a decrease of high compressive strength (40–50 MPa) of the compounds containing NZ, MOR, and STA (MPP-NZ, MPP-MOR, and MPP-STA-NZ, respectively) by an average of 2–3 times. It was shown that the differential leaching rate of ¹³⁷Cs on the 28th day from MPP-FKN-W after heating to 250 °C was 5.3 × 10⁻⁶ g/(cm²∙day), however, at a higher temperature, it increased by 20 and more times. The differential leaching rate of ¹³⁷Cs from MPP-NZ, MPP-MOR, and MPP-STA-NZ had values of (2.9–11) × 10⁻⁵ g/(cm²∙day), while the dependence on the heat treatment temperature of the compound was negligible.

The sorption behavior and spatial microdistribution of Cs-137, Sr-90 and Am-241 onto the surface of a fractured rock sample from the R-11 borehole of the exocontact zone of the Nizhnekansky granitoid massif were studied. The sorption efficiency of the fractured sample increases in the row of Sr < Cs < Am, where americium is the most retained radionuclide. Based on the image processing of radiograms and scanning electron microscopy data, the mineral relative sorption efficiency (RSE) values were determined to quantify the contribution of the mineral phases of the fractured sample to radionuclide retention. It was established that cesium and strontium are predominantly retained in cracks filled with secondary mineral chlorite. Zeolite is a less effective sorbent with respect to cesium and strontium. Americium sorption is uniform over the whole surface of the fractured sample, with close RSE values for all mineral phases (RSE ~1). The behavior of cesium in heterogeneous and monomineral systems of crushed mineral phases of quartz, biotite and zeolite NaA imitating minerals of the fractured rock sample R-11 was determined. It was shown that the fraction of the sorbed cesium in a heterogeneous system of two mineral phases—biotite and quartz—was larger than the sum of sorption values for the same separated mineral phases. Based on the models of radionuclide sorption on illite–smectite minerals, it was able to estimate the depth of the penetration of solution into the fractured rock sample R-11. The variations of penetration depths for solutions of specific radionuclide into the fractured rock sample were established.

The sorption properties of mineral phases that constitute the rocks of the Yeniseysky area of the Nizhnekansky granitoid massif toward Np, Pu, Am, Sr, and Cs were studied under conditions of underground radioactive waste disposal. As a result of sorption experiments, sorption kinetic curves were obtained, and the distribution coefficients (K_d) of radionuclides on mineral phases were determined. The resulting data made it possible to evaluate the differences in the sorption properties of minerals towards the studied radionuclides. For each radionuclide, with the exception of americium, it was possible to establish the most effective mineral phases in terms of sorption: for Np(V)—hydroxyapatite and calcite, for Pu(V)—hydroxyapatite and magnetite, for Sr(II)—biotite and hydroxyapatite, for Cs(I)—biotite and muscovite. Americium was sorbed quantitatively on all mineral phases, except for biotite and quartz.

Methodical approach to evaluation of the relative efficiency of various rock minerals in radionuclide sorption is suggested. The approach involves sorption experiments on the surface of polished rock...

The influence of corrosion products of metal structures of a repository on the sorption behavior of long-lived radionuclides in the environment of the host crystalline rocks should be taken into account during modeling aimed at confirming the safety of deep disposal of radioactive waste (RW). This study focuses on the influence of iron(II/III) on the sorption and spatial distribution of ²³⁷Np, ²³⁹Pu, and ²⁴¹Am on the minerals of a fractured gneiss sample from the Yeniseisky site of the Nizhnekansky massif (Krasnoyarsk krai). Kinetic dependencies and quantitative parameters were obtained for actinide sorption in the presence of iron in model solutions, including solution affected by contact with steel ST3. It was shown that the presence of iron (oxy)hydroxide increases the rate of actinide sorption by the rock, whereas ionic iron species have practically no effect on this process. Digital radiography demonstrated that the preferential actinide retention phase was formed by iron(III) precipitation on the calcite surface. The Raman spectrum of this phase corresponds to the iron-bearing mineral lepidocrocite.