The structural and chemical properties of zirconolite (ideally CaZrTi₂O₇) as a host phase for separated REE–actinide-rich wastes are considered. Detailed analysis of both natural and synthetic zirconolite-structured phases confirms that a selection of zirconolite polytype structures may be obtained, determined by the provenance, crystal chemistry, and/or synthesis route. The production of zirconolite ceramic and glass–ceramic composites at an industrial scale appears most feasible by cold pressing and sintering (CPS), pressure-assisted sintering techniques such as hot isostatic pressing (HIP), or a melt crystallization route. Moreover, we discuss the synthesis of zirconolite glass ceramics by the crystallization of B–Si–Ca–Zr–Ti glasses containing actinides in conditions of increased temperatures relevant to deep borehole disposal (DBD).

The fluid transfer of radionuclides in the geologic medium is considered under conditions when radionuclides are contained in fluids not only as solutes but also as colloids. The effect of colloidal transport of radionuclides on the rate of spreading of radioactive contamination in an underground medium is estimated, with assessment of this effect in mathematical models describing the transport of radionuclides by subsurface water. For this purpose, the exchange of radionuclides between subsurface water, colloid, and an immobile solid phase is considered, taking into account the precipitation of colloidal particles on both the immobile solid phase and other colloidal particles and their recurrent mobilization into the liquid phase. It is noted that, in real colloidal transfer, the heterogeneity of the geologic medium and colloidal particles in subsurface water is of great importance. The known models of colloidal transfer of radionuclides are evaluated on the basis of the analysis performed.

The current policy of managing high-level waste (HLW) derived in the closed nuclear fuel cycle consists in their vitrification into B-Si or Al-P vitreous forms. These compounds have rather limited capacity with respect to the HLW (5–20 wt%), and their properties change over time due to devitrification of the glasses. Cardinal improvement in the management of HLW can be achieved by their separation onto groups of elements with similar properties, followed by their immobilization in robust waste forms (matrices) and emplacement in deep disposal facilities. One of the possible fractions contains trivalent rare-earth elements (REEs) and minor actinides (MAs = Am and Cm). REEs are the fission products of actinides, which are mainly represented by stable isotopes of elements from La to Gd as well as Y. This group also contains small amounts of short-lived radionuclides with half-lives (T_1/2) from 284 days (¹⁴⁴Ce) to 90 years (¹⁵¹Sm), including ¹⁴⁷Pm (T_1/2 = 2.6 years), ¹⁵⁴Eu (T_1/2 = 8.8 years), and ¹⁵⁵Eu (T_1/2 = 5 years). However, the main long-term environmental hazard of the REE–MA fraction is associated with Am and Cm, with half-lives from 18 years (²⁴⁴Cm) to 8500 years (²⁴⁵Cm), and their daughter products: ²³⁷Np (T_1/2 = 2.14 × 10⁶ years), ²³⁹Pu (T_1/2 = 2.41 × 10⁴ years), ²⁴⁰Pu (T_1/2 = 6537 years), and ²⁴²Pu (T_1/2 = 3.76 × 10⁵ years), which should be immobilized into a durable waste form that prevents their release into the environment. Due to the heat generated by decaying radionuclides, the temperature of matrices with an REE–MA fraction will be increased by hundreds of centigrade above ambient. This process can be utilized by selecting a vitreous waste form that will crystallize to form durable crystalline phases with long-lived radionuclides. We estimated the thermal effects in a potential REE–MA glass composite material based on the size of the block, the content of waste, the time of storage before immobilization and after disposal, and showed that it is possible to select the waste loading, size of blocks, and storage time so that the temperature of the matrix during the first decades will reach 500–700 °C, which corresponds to the optimal range of glass crystallization. As a result, a glass–ceramic composite will be produced that contains monazite ((REE,MA)PO₄) in phosphate glasses; britholite (Ca_x(REE,MA)_10-x(SiO₄)₆O₂) or zirconolite ((Ca,REE,MA)(Zr,REE,MA)(Ti,Al,Fe)₂O₇), in silicate systems. This possibility is confirmed by experimental data on the crystallization of glasses with REEs and actinides (Pu, Am). The prospect for the disposal of glasses with the REE–MA fraction in deep boreholes is briefly considered.