Composite Forms in the System RЕЕ2O3 – ZrO2 – TiO2 for Minor Actinides (Am, Cm) and REEs Immobilization

1. Introduction

Over the past 65 years, the concentration of CO₂ in the Earth's atmosphere has increased by a quarter – from 315 ppm in 1958 to 420 ppm in 2023 and continues to grow. By March 2024, its level had increased by another 5 ppm, and this is the largest jump in the entire history of observations. A similar concentration of CO₂ in the air was reached 3-5 million years ago, when the temperature was higher by 2-3 °C, and the sea level was 10-20 meters higher than today. The increase in the amounts of climate-active gases is caused by the combustion of fossil fuels and transport. A huge contribution to environmental degradation is made by particulate matter, up to 2.5 micrometres in size, which causes up to 9 million premature deaths per year worldwide [1,2,3]. Low-carbon energy plays a key role in combating air pollution and global warming due to greenhouse gases [4,5]. Along with renewable sources, this also includes nuclear energy [6]. By 2026, the share of low-carbon generation in the world will reach 50%, with about a quarter of it coming from nuclear power plants [7]. The contribution of nuclear power to achieving carbon neutrality should increase by 2050, with the total capacity of nuclear power plants amounting to 890 GW(e) in the high and 458 GW(e) in the low scenario, compared to the current 369 GW(e), which is higher of former estimations [5]. Nuclear power is statistically the cleanest source of energy by the CO₂ emissions and the safest along with wind and solar power compared with coal, oil, gas, biomass and hydropower (Figure 1).

In November 2023, the European Parliament has categorized NE, along with 15 other technologies, as a “clean” technology. In December 2023, at the 28th UN Climate Conference (COP28, UAE), 22 countries, including the United States, Canada, United Kingdom, France, Japan, and South Korea, committed to triple the capacity of nuclear power plants by 2050 compared to 2020. It is expected that the share of nuclear generation in Russia will increase to 25% by 2045 from the current 20%. Investments in nuclear energy are growing worldwide; in many countries, they are either close to or have exceeded the costs of constructing hydrocarbon-fuelled power plants [9]. The implementation of these plans may be however hampered by a shortage of uranium and the problem of handling high-level waste (HLW). Based on the growth in the capacity of nuclear power plants and under the condition of an open fuel cycle with the disposal of spent nuclear fuel (SNF), their operation will require 2–3 times more uranium than nowadays: or 130–200 thousand tonnes per year. This threatens the rapid depletion of available natural uranium resources if the basis of nuclear energy in the future continues to be only thermal neutron reactors — light water reactors (LWR) [10]. These problems are solved by the transition to a two-component nuclear power system with thermal and fast neutron reactors with a closed nuclear fuel cycle - that is, the reprocessing of SNF, the separation of fission products, U and Pu from HLW, which are in demand for nuclear power generation, as well as minor actinides (Np, Am, Cm), which pose the greatest environmental hazard [11,12,13,14,15].

There are two ways of handling long-lived actinides and fission products [15,16]. The first is to separate them from HLW and then irradiate with neutrons for transmutation into stable or short-lived isotopes. However, its implementation encounters great technical difficulties and requires enormous costs [17]. The other method involves incorporating radionuclides into highly stable matrices and disposing them of [18,19,20,21,22,23,24]. In this case, the safety of HLW disposal facilities will be ensured by a bentonite-based sorption buffer, a corrosion-resistant container and robust HLW matrix. Additional barrier is the geological environment due to the low solubility of radionuclides in groundwater and their sorption by rocks around the disposal facility, and the slow flow of waters that carry radionuclides. In accordance with international standards [20] the preferred strategy for the management of radioactive waste is to contain it (i.e. to confine the radionuclides within the waste matrix, the packaging and the disposal facility) and to isolate it from the biosphere.

The current practice of isolating HLW involves its inclusion in borosilicate (B-Si) or alumina-phosphate (Al-P) glasses produced in electric furnaces or by induction melting [15,25,26,27,28,29,30,31,32,33,34,35,36,37]. The advantages of these glasses include a proven production technology and the ability to include many radioactive and stable isotopes. Their disadvantages include a low HLW load (5–20 wt.%), where the main part is made up of stable isotopes (REEs, Zr, Mo). The dry residue of liquid HLW from the reprocessing of LWR spent nuclear fuel contains up to 4 wt.% actinides and 50 wt.% fission products, most of which are stable [27,38]. In addition, the glasses can crystallize over time with the possible formation of more soluble phases, and when they come into contact with groundwater, colloids with high migration in the geological environment are formed.

Partitioning of HLW will allow actinides to be included in the most durable and thermodynamically stable crystalline matrices which might be components of integral composite systems containing both crystalline and vitreous wasteforms [25,26,27,28,37,39,40]. When selecting potential materials, phases with high loading in respect of the waste are firstly identified, then their corrosion and radiation resistance are studied, and finally the possibility of their remote industrial production is assessed.

We have considered possible crystalline matrices for immobilization of the REE – minor actinides (Am, Cm) fraction. By the size of the cation (Am, Cm)³⁺ are close to Nd³⁺, therefore the Nd₂O₃ – ZrO₂ – TiO₂ system is of interest, which contains (Zr,Nd)O_2-x and Nd₂(Zr,Ti)₂O₇ with the fluorite or pyrochlore structure, Nd₂Ti₂O₇ of the perovskite type, as well as phases Nd₄(Ti,Zr)₉O₂₄ and Nd₂TiO₅, which have no natural structural analogues. This analysis was carried out based on literature data and the results of our own studies. The latter were mainly published in Russian journals and are little known to the world scientific community.

2. Separation of HLW for Groups of Radionuclides Isolation

The PUREX process (extraction of Pu and U) is the basis of SNF reprocessing; it can be supplemented by operations to extract other elements, including trivalent REE and minor actinides. The TRUEX, TODGA, and DIAMEX techniques allow for the extraction of REE, Am, and Cm; they have already been tested on real HLW [41,42,43,44,45,46,47]. Both the proportion of the REE-actinide fraction in SNF and the ratio of REE and actinides in it depend on the fuel composition, burnup, and storage time of SNF before reprocessing. The burnup of LWR reactor fuel reaches 45 GW×day per tonne; in the future, it is expected to be increased to 55 GW×day per tonne [42,48]. As the burnup increases from 30 to 45 GW×day, the REE content in a tonne of SNF increases from 10.2 to 13.9 kg, and the sum of Am and Cm – from 0.52 to 0.87 kg [41,49]. A tonne of SNF with a burnup of 60 GW×day contains 800 g Am (63% ²⁴¹Am, 37% ²⁴³Am), 150 g Cm (90% ²⁴⁴Cm, 8% ²⁴⁵Cm, 1% each of ²⁴³Cm and ²⁴⁶Cm) and about 20 kg of REE. With an increase in SNF burnup to 70 GW×day, the Am content in a tonne of SNF will exceed 1 kg, and the amount of REE will be 23 kg (Table 1).

During the storage of SNF, the composition of the REE-actinide fraction changes due to an increase in the proportion of ²⁴¹Am as a result of the decay of short-lived ²⁴¹Pu (half-life, T½ = 14 years) and a decrease in the proportion of ²⁴⁴Cm (T½ = 18 years). An increase in the storage period of SNF from one year to 30 years increases the ratio ²⁴¹Am/²⁴³Am from 1.3 to 12.1, and the proportion of actinides in the REE – MA mixture increases from 2% to almost 9% (Table 2).

Among the REE in SNF and HLW, the light elements of the cerium group with a large cation radius (from La to Sm) dominate, the content of which decreases in the sequence: Nd (about 39% of all REE) – Ce (23%) – La (12%) – Pr (10%) – Sm (8%) – Y (4%) and 4% Eu, Gd and Pm. Their isotopes are stable or have long half-lives and can be considered as stable, for example: ¹⁴²Ce (5.0×10¹⁶ years), ¹⁴⁴Nd (2.3×10¹⁵ years), ¹⁴⁷Sm (1.1×10¹¹ years), ¹⁵⁰Nd (6.7×10¹⁸ years) [53]. Only a few isotopes have a short half-life: 285 days (¹⁴⁴Ce), 2.62 years (¹⁴⁷Pm), 4.76 (¹⁵⁵Eu), 8.6 (¹⁵⁴Eu) and 90 years (¹⁵¹Sm). REE phases, especially Nd, are promising as matrices for the REE-actinide fraction, since it dominates among REE (40% of their total content) in SNF and HLW, and the radii of Nd³⁺ and REE-MA cations coincide and for coordination number (c. n.) VIII are equal to 1.09 Å [54]. Let us consider the Nd₂O₃ – ZrO₂ – TiO₂ system, first the edge parts, and then the entire three-component diagram.

ZrO₂ – TiO₂ System. There are oxides of the composition (Ti,Zr)O₂ (tetragonal structure of rutile), (Zr,Ti)O₂ (depending on the temperature, monoclinic – analogue mineral baddeleyite, or tetragonal) and ZrTiO₄ (analogue – mineral shrilankite) [55]. The content of REE in them is low and therefore they are not interesting as matrices for minor actinides. However, such phases are often found in experimentally obtained samples along with titanates and zirconates of REE. Further, for simplicity and convenience, we will use their abbreviated names: Z for ZrO₂, T in the case of TiO₂, ZT instead of ZrTiO₄ and so on.

Nd₂O₃ – ZrO₂ System. For the immobilisation of actinides, zirconate Nd₂Zr₂O₇, NZ with a pyrochlore structure and oxide (Zr_2-хNd_х)₂O_2-0.5х with a fluorite structure are promising, although it contains less REE-actinide fraction than pyrochlore (20 and 60 wt.%, respectively). The pyrochlore field is maximum at 1500 °C [56], and above 2200 °C it transforms into cubic oxide (Zr_2-хNd_х)₂O_2-0.5х. These phases are also considered as inert matrix fuel for the transmutation of actinides, for example Am [16,57,58,59].

Nd₂O₃ – TiO₂ System. In this system, the greatest number of compounds – potential REE – MA hosts are formed: Nd₂TiO₅, Nd₂Ti₂O₇, Nd₄Ti₉O₂₄, Nd₂Ti₃O₉ [60]. Replacing Nd₂O₃ and TiO₂ with N and T, the phase formulas can be written as: NT (Nd₂TiO₅), NT₂ (Nd₂Ti₂O₇), N₂T₉ (Nd₄Ti₉O₂₄), NT₃ (Nd₂Ti₃O₉). The most important in this system (Table 3) are the eutectic of rutile and N₂T₉ (reaction No. 1), incongruent (No. 2) and congruent (No. 5) melting of the NT₂ and N₂T₉ phases. The NT₃ phase with a perovskite structure is formed at 1200 ^oC according to the reaction No. 4: 0.2 N₂T₉ + 0.6 NT₂ = NT₃. Comparatively small differences in the structure of the Nd₂O₃–TiO₂ and La₂O₃–TiO₂ systems are observed (Table 3) [61,62], whereas in the Sm₂O₃–TiO₂ system, instead of the monoclinic phase, the Sm₂Ti₂O₇ compound with a cubic pyrochlore structure appears [63,64].

The REE₂Zr₂O₇ and REE₂Ti₂O₇ phases (REE = La, Nd) crystallize in a cubic (pyrochlore) or monoclinic (perovskite type) structure [65,66,67,68]. Pyrochlore is formed when the ratio of the radii of REE³⁺ and (Ti,Zr)⁴⁺ is in the range from 1.46 to 1.78 Å; in other cases, the fluorite structure of REE zirconates and the perovskite structure of titanates are stable. The REE₂Ti₂O₇ (Sm–Yb, Y) and REE₂Zr₂O₇ (La–Gd) phases have a pyrochlore structure. Depending on the REE type, the REE2TiO5 phases have an orthorhombic (La–Sm), cubic (Er–Lu, Sc) or hexagonal (Eu–Ho, Y) structure [69]. These features have a significant impact on the capacity of the phases with respect to the actinide components of the HLW.

3. Crystal – Chemical Features of Rare Earth Zirconates and Titanates

The content of REE and actinides is one of the important criteria for selecting a matrix for immobilization of the radionuclides of REE – MA fraction. Crystal chemistry and structural features of the phases formed in the REE₂O₃ – ZrO₂ – TiO₂ system were considered using the example of Nd³⁺ [70] and are briefly discussed below.

Nd₂Zr₂O₇ Phase has a cubic lattice (Figure 2a), space group Fd-3m. There are 8 formula units in the unit cell, the Nd polyhedron is a scalenohedron (distorted cube), contains 8 O atoms - six equidistant from Nd³⁺ and two at a greater distance. Zr cations are surrounded by six O atoms at the vertices of a trigonal antiprism (distorted octahedron). The structure of pyrochlore can be also described through interpenetrating frameworks of BO₆ and A₂X octahedra, it is derived from a fluorite-type lattice (space group Fm-3m).

The structure Nd₂Ti₂O₇ is derived from the perovskite-type structure (Figure 2b): TiO₆ octahedra, connecting at their vertices, form plates of 4 octahedra (about 12 Å) in the a and b directions, between which single-capped trigonal prisms of NdO₇ are located. Three-capped trigonal prisms of NdO₉ fill the cavities of the octahedral blocks. Two-capped prisms of NdO₈ are located inside and between the octahedral blocks. The structure of Nd₂Ti₂O₅ (Figure 2c) consists of edge-linked seven-vertex NdO₇ and chains of square pyramids of TiO₅, connected by vertices in the [010] direction. The most complex structure of Nd₄Ti₉O₂₄ is represented by a titanium-oxygen framework, and Nd polyhedra are located in its cavities (Figure 2d). Nd atoms occupy 3 positions: Nd(1) polyhedron – distorted square antiprism, Nd(2) – octahedron, Nd(3) – distorted square prism. The Nd(1)O₈ polyhedra, connecting by edges and vertices, form layers parallel to the (110) plane. The Nd(3) polyhedra are connected by edges with Nd(1) layers and form 17.5 Å thick blocks, in the channels of which Nd(2) octahedra are located.

The REE³⁺ cations are characterized by coordination numbers (c. n.) equal to VII (one-capped trigonal prism, truncated cube), VIII (cube, two-capped antiprism) and IX (three-capped trigonal prism). The coordination polyhedra of the smaller Zr⁴⁺ and Ti⁴⁺ cations have the shape of an octahedron (c.n. = VI), except for Nd₂TiO₅, c. n. of Ti⁴⁺ = V, and this polyhedron is a square pyramid. The low “solubility” of impurities corresponds to odd c. n. (VII and IX) in Nd–O polyhedra, and high solubility corresponds to even c. n. (VIII). This explains the wide field of the pyrochlore solid solution and the narrow composition fields of the remaining phases. Pyrochlore Nd₂(Zr,Ti)₂O₇ has a high capacity of the structure with respect to actinides: up to 20 at.% U enters the Zr⁴⁺ position, and from 10 to 20 at.% U and Th enter the Nd³⁺ position. Some of the Nd³⁺ ions in Nd₄Ti₉O₂₄ have c. n. = VIII, which is probably resulted in a higher content of impurities (Ca, U and Zr) in this phase compared to Nd₂Ti₂O₇ and Nd₂TiO₅ [70,71,72,73,74].

Phase diagrams of systems REE₂O₃–ZrO₂–TiO₂ depend on the REE³⁺ radius (Figure 3).

Replacement of Nd³⁺ by La³⁺ has a weak effect, whereas a decrease in the ionic radius from Nd³⁺ to Sm³⁺, Gd³⁺ and Y³⁺ leads to a more significant change [74,75,76,77]. The La₂O₃–TiO₂–ZrO₂ system also contains: La₂TiO₅ (LT), La₄Ti₃O₁₂ (L₂T₃), La₂Ti₂O₇ (LT₂), La₄Ti₉О₂₄ (L₂T₉), La₂Zr₂О₇ (LZ₂), ZrTiО₄ (ZT), ZrO₂ (Z), TiO₂ (T). Systems with Nd₂O₃ and La₂O₃ are similar in the set of phases, however, in the system with lanthanum, the pyrochlore region is smaller. When large ions La³⁺ and Nd³⁺ are replaced by Y³⁺, Y₂Ti₂O₇ (pyrochlore) is formed and a cubic oxide (Zr,Y)O_2-x with a fluorite structure appears. The structure of Y₂TiO₅ is cubic (sp. group Fm-3m), but Nd₂TiO₅ and La₂TiO₅ have orthorhombic symmetry (sp. group Pnma). A feature of Nd and La titanates is slight variations in composition, the Ti : REE ratios in them are close to the values in the formulas (Figure 3). At 1350 ^oC, the LT₂, L₂T₃ and LT phases contain less than 2 mol.% ZrO₂, the ZrO₂ content in L₂T₉ is higher and equals 4 mol.%, the content of La₂O₃ and ZrO₂ in La₂Zr₂O₇ reaches 35 mol.% and 69 mol.% (Table 4). The isomorphism of La³⁺ in ZrO₂ and ZrTiO₄ is limited to 1 mol.% La₂O₃. In (Ti,Zr)O₂ and (Zr,Ti)O₂, high contents of ZrO₂ and TiO₂ (12–14 mol.%) are found, Zr:Ti ratio in ZrTiO₄ varies from 1.4 to 0.9.

4. Actinide Waste Forms in the REE₂O₃ (Nd₂O₃) – ZrO₂ – TiO₂ System

Let us consider the issue of matrices for the REE – actinide fraction in the REE₂O₃ (Nd₂O₃) – ZrO₂ – TiO₂ system taking into account the data [65,78] and previous our results [62,70,71,72,73,74,79,80,81,82]. About 30 samples were synthesized by cold pressing and sintering (CPS) of the oxide mixture at 1400–1550 °C or by induction cold crucible melting (ICCM). The samples were analysed on a Rigaku D/Max 2200 X-ray diffractometer (XRD, Cu Kα) and in a JSM–5610LV scanning electron microscope with a JED–2300 energy-dispersive spectrometer (SEM/EDS). To determine corrosion resistance, their interaction with water and brines was studied at 90–240°C for 30–143 days. After the tests, the solutions were analysed for REE content; to determine the colloidal form, they were filtered through membranes with pore sizes of 450, 200, 100, and 25 nm. The compositions of the solution and its filtrates were analysed using inductively coupled plasma mass spectrometer (XII ICP-MS Thermo Scientific). The samples were irradiated with 1 MeV Kr²⁺ (flux density of 10¹² ions /cm²×s) on an accelerator at the Argonne National Laboratory in the USA at T = 50–1023 K. Some of the samples were irradiated with 4.5−5 MeV electrons at JSC SRC "RIAR" (Russia) with subsequent study of their structure and hydrolytic stability.

As before, we will use mineral names or designations for the phases based on their formulas, where Nd₂O₃, ZrO₂ and TiO₂ are replaced by N, Z and T. In the Nd₂O₃–TiO₂–ZrO₂ system there are [65,78] phases Nd₂(Ti,Zr)₂O₇ (NTZ) with a pyrochlore structure, TiO₂ (T), ZrTiO₄ (ZT) and tetragonal ZrO₂ (Z). Nd titanates are represented by: Nd₂TiO₅ (NT), Nd₂Ti₂O₇ (NT₂), Nd₂Ti₄O₁₁ (NT₄), Nd₄Ti₉O₂₄ (N₂T₉). In [60], the identity of Nd₂Ti₄O₁₁ and Nd₄Ti₉O₂₄ was proven and the phase Nd₂Ti₃O₉ (NT₃) was found. The largest field in the Nd₂O₃–ZrO₂–TiO₂ diagram is occupied by pyrochlore, smaller variations in composition are characteristic of Nd₄Ti₉O₂ (Figure 3a), which makes these phases promising hosts for the REE-actinide fraction. Below, data on samples prepared by CPS or ICCM routes composed of the pyrochlore or orthorhombic REE titanate are discussed.

4.1. Samples of the Composition (REE)₂(Zr,Ti)₂O₇ with a Pyrochlore Structure

To check the pyrochlore region boundaries, samples whose composition points lie on the REE₂Zr₂O₇ – REE₂Ti₂O₇ line (Figure 4) were studied. They were obtained by sintering for 5 h at 1400 °C (Zr : Ti ≤ 1) or 1550 °C (Zr : Ti > 1). The batch charge was prepared from TiO₂, ZrO₂ and REE₂O₃ in quantities corresponding to the formula REE₂Zr_2–xTi_xO₇, “x” varies from 0 (sample T0) to 2 (T20) with a step of 0.1 or 0.2. The proportions of REE oxides corresponded to the contents in the SNF and were, in wt% as follows: 11.8 La₂O₃, 23.0 Ce₂O₃, 10.7 Pr₂O₃, 38.9 Nd₂O₃, 8.1 Sm₂O₃, 1.3 Eu₂O₃, 1.5 Gd₂O₃, 4.7 Y₂O₃. Up to x = 0.8 (T0 – T8), the samples consist of pyrochlore; at a higher titanium content, monoclinic REE titanate with a perovskite structure appears. This phase becomes the main one at x > 1.2, i.e. in samples T12 – T20. The content of total REE oxides in it is approximately 10 % higher than in pyrochlore – 65 and 55 wt%, respectively (Table 5).

Samples P1 and P2 (Figure 4) were obtained by the induction cold crucible melting (ICCM) method; neodymium (P1) or a mixture of lanthanum, cerium and gadolinium (P2) were waste simulants. The synthesis was carried out in air; a zirconium ring was placed in the batch for melting. After the melt was formed and the batch was loaded, it was melted for 0.5 h, then the setup was turned off for cooling and forming a ceramic block. The composition of sample P1 (formula Nd₂Zr_1.5Ti_0.5O₇) lies in the pyrochlore field (Figure 4). According to XRD and SEM/EDS data, it is composed of pyrochlore, the composition of which varies within the sample (Table 6). Light areas are enriched in Zr (on average 41.5% ZrO₂) compared to dark ones (36.6%), the Nd₂O₃ contents are similar.

The composition of sample P2, wt.% is 14.6 TiO₂, 22.4 ZrO₂, 14.9 La₂O₃, 15.0 Ce₂O₃, 33.1 Gd₂O₃, formula (La_0.5Ce_0.5Gd)TiZrO₇ and is located at the edge of the pyrochlore field. Pyrochlore dominates in it (85–90%), REE titanate is present (Figure 4). From the centre to the edges of the pyrochlore grains, the concentration of zirconium and lanthanum decreases (Table 6), and that of gadolinium and titanium increases. Pyrochlore is enriched in ZrO₂ and Gd₂O₃ but contains less La₂O₃ and Ce₂O₃ relative to REE titanate. REE titanate is located between the pyrochlore grains, which indicates its late formation. Another phase of the composition, wt.%: 18.7 TiO₂, 5.1 ZrO₂, 26.7 La₂O₃, 15.0 Ce₂O₃, 13.5 Gd₂O₃, 18.6 SiO₂, 2.4 Fe₂O₃ by the ratio of REE: (Ti + Zr + Fe) : Si is close to the mineral perrierite (La,Ce)₂Ti₂Si₂O₁₁. It is formed due to the dissolution of the crucible coating, which explains the localization in the marginal parts and the presence of Si and Fe.

4.2. Samples of the Composition (REE)₂(Zr,Ti)₂O₇ with a Pyrochlore Structure

Samples with the (REE)₄Ti₉O₂₄ phase containing 48 wt.% of the REE-actinide fraction were obtained by sintering or melting–crystallization. The points of the bulk compositions of the samples lie near or in the field of this phase at the Figure 3a. Sample RT-1 (Figure 5, Table 7) composed of REE titanate (light) and titanium oxide with a rutile structure (dark), was obtained by sintering for 4 h at 1375 °C of a mixture, wt.%: 52 TiO₂, 45 Nd₂O₃, 3 Sm₂O₃ (formula Nd_3.75Sm_0.25Ti₉O₂₄). Several samples (IM-1, IM-2, RT-2, MPM-1) were obtained by the ICCM method at 1500 °C. They are composed of orthorhombic titanate and rutile, the proportion of which depends on the composition of the batch and varies from 10 to 50% (Figure 5). Rutile contains 3–8 wt.% ZrO₂, where Zr⁴⁺ replaces Ti⁴⁺ due to close radii (0.72 Å and 0.61 Å). The formula of REE titanate is calculated for 24 O^2-, the sum of Ti⁴⁺ and Zr⁴⁺ is close to 13 (Table 7). It contains up to 3 wt.% ZrO₂. Analysis of the samples in a high-resolution electron microscope (Figure 5f) confirmed the orthorhombic symmetry (sp. Gr. Fddd) and the unit cell parameters, Å: “a” = 14.0, “b” = 35.5, “c” = 14.6.

Samples obtained via melting have larger grain sizes than those obtained by sintering. The melting temperatures of samples with orthorhombic titanate are about 1500 °C. This is 100–250 °C lower than that of Ti-Zr pyrochlore, which simplifies the synthesis.

5. On the Pseudo-Ternary Nature of the Nd₂O₃ – ZrO₂ – TiO₂ System

The Nd₂O₃ – ZrO₂ – TiO₂ system was previously considered as ternary, however, in the case of partial reduction of Ti⁴⁺ (TiO₂) to Ti³⁺ (Ti₂O₃), it becomes quaternary. A sample of the composition, mol.%: 21 Nd₂O₃, 16 ZrO₂ and 63 TiO₂ was synthesized by melting in a glass-carbon crucible [82]. According to the data [65,78], it should consist of Nd₂Ti₂O₇ and ZrTiO₄, however, according to the results of X-ray diffraction and SEM/EDS studies, the sample is composed of two Nd titano-zirconates of different colours and compositions (Figure 6, Table 8) and rutile. Electron back scatter diffraction (EBSD) confirmed the structure of pyrochlore for the light phase and zirconolite for the dark one (Figure 6). When the melt interacts with glass-carbon crucible, the reaction occurs: 2Ti⁴⁺O₂ + C = Ti³⁺₂O₃ + CO. Ti³⁺ ions serve as charge compensators during the exchange: Nd³⁺ + Ti³⁺ → Ca²⁺ + Ti⁴⁺, which leads to the formation of zirconolite (ideally CaZrTi₂O₇), in which Ca²⁺ is fully replaced by Nd³⁺. Based on the composition of the phases (Table 8), their formulas were calculated with Ti⁴⁺ only and taking into account Ti³⁺. Reducing conditions are necessary for Ti³⁺ appearance, but only Ti⁴⁺ is stable at the synthesis in air, and the system is ternary.

6. Behavior of the Matrices with REE – Actinide Fraction in Hot Aqueous Solutions

Along with the waste capacity, a very important characteristic of actinide matrices is their resistance to leaching. A number of standard tests have been developed to determine this parameter [25,26,28,36,39,83]. The behaviour of pyrochlore has been studied in static and dynamic tests [64,66,68,73,84,85,86,87]. The normalized leaching rate of U and REE from titanate pyrochlore under near-neutral conditions is 10^-4 – 10^-5 g/(m² × day), it increases by an order of magnitude in alkaline (pH > 9) or acidic solutions (pH < 5). This is 3 orders of magnitude lower than leaching from HLW glass matrices under similar conditions [88,89]. The rates of actinide and REE leaching from zirconate pyrochlores are lower than from titanate ones. For pyrochlore La₂Zr₂O₇ in static tests in water and alkaline solution (pH = 10) at 90–150 °C, the values for La are below 10^-4 g/m² day, and for Zr are equal to 10^-6 g/(m² × day) [90,91,92]. The rate of plutonium leaching from titanate pyrochlores before their amorphization (MCC-1, water, 90 ^oC, 14-28 days) varies from ~10^-2 to 10^-4 g/(m² × day). The spread of values is probably due to the presence of other phases in the samples, in addition to pyrochlore. After amorphization, the rate of Pu leaching increases to 7×10^-2 – 9×10^-3 g/(m² × day). Leaching of Cm from pyrochlore (Gd,Cm)₂Ti₂O₇ after amorphization increased from 10^-2 to 2×10^-1 g/(m² × day) [89,93,94]. For pyrochlore (Gd,Cm)₂(TiZr)O₇ the values of 1.7×10^-2 and 4.7×10^-2 g/(m² × day) were obtained [94]. Amorphization increases the rate of actinide leaching by 3–10, rarely 50 times [66,87,89,93,94,95,96,97]. Therefore, in runs with short-lived actinides (²⁴⁴Cm), rate of actinide leaching increases due to phase amorphization, as well as an increase in the acidity of the solution to pH = 4 due to its radiolysis.

Studies of the corrosion resistance of the REE₂Ti₂O₇ and REE₄Ti₉O₂₄ phases are rare [64,73,79,80,98,99]. The leaching rates of La and Nd (95–240 °C, water, brine, 30–143 days) are close to the values for pyrochlore and vary from 10^-3 to 10^-5 g/(m² × day), which corresponds to the dissolution of a matrix layer a few microns thick in a year. Taking into account the studies [66,72,73,80], we conclude that pyrochlore and Nd titanates are resistant to corrosion in hot waters. Incongruent dissolution with a higher leaching rate of REE and actinides compared to Ti and Zr is typical for them. This leads to the formation of a surface layer, tens of nm thick, enriched with these elements, which complicates leaching of actinides [64,92,96]. After 40 days of contact of the pyrochlore with a solution of 0.5 M CaCl₂ + 0.5 M NaCl (T = 200 ^oC), no changes were detected in the SEM (Figure 7).

7. Behavior of the Matrices with REE – Actinide Fraction Under Irradiation

The sources of radiation in the HLW forms are β- and α-decay, γ-radiation, spontaneous fission of actinides (Table 9 and Table 10). The disordering of the crystalline structure is mainly due to the formation of α-particles and heavy recoil nuclei during actinide decay. α-particles (He²⁺ with an energy of 4.5–5.5 MeV) account for up to 98% of the decay energy. At the end of a range of 10–20 μm, they collide with hundreds of atoms, knocking them out of their original position (Table 10). Recoil nuclei with an energy of 70–100 keV collide with 1–2 thousand atoms on a path of 10–40 nm. The role of beta particles, gamma radiation, and neutrons in this aspect is small. Influence of spontaneous fission of heavy actinides increases starting from ²⁴⁴Cm, but this effect is weak due to their low content.

The synthesis of actinide matrices is labour-intensive and complicated process due to the need for protection from radiation, as well as the high cost of Pu, Am, Cm isotopes. To study radiation damage, simulation techniques are typically used – irradiation with neutral or charged particles [25,26,28,50,66,68,69,85,86,93,97,99,101,102,103,104,105,106,107,108,109,110,111]. They allow for a very short time (minutes) to achieve radiation doses that a matrix with real HLW will accumulate over many thousands of years. The most common method for studying the radiation resistance of actinide matrices is irradiation with heavy ions. The goal of such works is to determine the critical radiation dose (D_cr), leading to amorphization of the structure, and the critical temperature (T_cr), above which amorphization does not occur at any doses. The higher the first value and the lower the second, the more resistant the phase structure is to radiation. The main advantages of irradiation with ions are: short time; no induced activity, which simplifies the study of matrices; critical doses and temperatures of all phases in the sample can be determined simultaneously; disordering is observed in an electron microscope during irradiation. Matrices are usually irradiated with 1 MeV Kr^+/2+, in which case the amorphization doses in units of displacements per atom (dpa) are close to the values during the decay of actinides [66,68,87,88,89,93,95,97,101].

In the REE₂O₃–ZrO₂–TiO₂ system, the phases with the pyrochlore structure have been intensely studied [66,95,97,101,102,103,104,105], there is a lot of data on the behaviour of REE₂TiO₅ and REE₂Ti₂O₇ under irradiation [69,87,97,101,102,103,104,105,106,107,108,109,110,111], and for REE₄Ti₉O₂₄ there are scarce data [111]. For the A₂B₂O₇ phases (A = La–Lu, B = Ti, Zr), the critical temperature decreases (the resistance to radiation increases) upon transition from REE phases with the perovskite structure to compounds with the pyrochlore structure [103,104,105]. We studied samples consisting of pyrochlore only (T0), as well as pyrochlore and monoclinic titanate (T15, T18, T20), orthorhombic REE titanate and rutile (IM-2, IM-9), which compositions are given in the Table 5, Table 6 and Table 7. When irradiated with 1 MeV Kr²⁺ to a dose of 25×10¹⁴ ion/cm², the structure of the zirconate phase is transformed into the fluorite lattice (Figure 8), and for the monoclinic perovskite phase, amorphization is already observed at 2.5×10¹⁴ ion/cm². This proves the resistance of zirconate pyrochlore (T0) to irradiation, zirconate-titanate pyrochlore (T15) has intermediate values of D_cr, low resistance is typical for monoclinic and orthorhombic rare earth titanates (Figure 9). For specific type of irradiation, T_cr is constant (Figure 9), and D_cr increases with rise of temperature, since the amorphous state is unstable. Heating leads to healing of defects, so the higher doses of irradiation are required to disorder the atoms in the lattice with increasing temperature.

For the phase of composition A_1.94Ti_2.00O_6.92 (A is a mixture of REE as in HLW): D_cr = 2.5×10¹⁴ Kr²⁺/cm², T_cr = 900 K. Critical doses (T = 298 K) of the phases Nd_3.96(Ti_8.92Zr_0.12)O_23.94 and REE_3.95(Ti_8.71Zr_0.34)O_24.01 (REE = 0.43La + 0.92Ce + 0.30Pr + 1.63Nd + 0.25Sm + 0.10Eu + 0.09Gd + 0.23Y) are determined as 3×10¹⁴ Kr²⁺/cm² (0.2 dpa), their T_cr is 900 K. Similar values were obtained for monoclinic titanates: for La₂Ti₂O₇ the critical dose is defined as 2.16×10¹⁴ Kr²⁺/cm², and the critical temperature, T_cr = 840 K.

A sample consisting of Nd₄(Ti,Zr)₉O₂₄ and rutile (Ti,Zr)O₂ was obtained by the ICCM method [99] and irradiated with electrons beam (energy 4.5-5 MeV, beam power 20 kW, current 17 mA) to a dose of 5×10⁹ Gray. Irradiation did not affect the cell parameters of the neodymium titanate and rutile phases (Figure 10), but the leaching rate of Nd and Ti from the irradiated sample with water on the 28th day increased from 10^-5 to 10^-4 g/(m² × day).

When irradiated with 1 MeV Kr²⁺ ions, the critical temperatures of the Nd phases change from 135 and 685 K for the pyrochlores Nd₂Zr₂O₇ and Nd₂Ti_0.8Zr_1.2O₇ to 918 K for the perovskite Nd₂Ti₂O₇ and 1200 K for Nd₂TiO₅ [108]. At a disposal facility temperature of 300–350 K (in mines) or 400–550 K (in deep boreholes), so only pyrochlores with a Zr : Ti ratio > 1 will retain crystallinity, while the remaining compositions will be amorphized due to actinide decay. In this case, the actinide leaching rate increases by only an order of magnitude. Radiation has a direct (amorphization) and indirect (increase in acidity during radiolysis, heating) effects on the properties of actinide matrices and their behaviour in the disposal facility. The influence of amorphization on leaching is usually less than one order of magnitude. The acid pH of the solution is quickly neutralized by interaction with the container materials and the bentonite-based buffer. Temperature causes an increase in the leaching rate, but slows down the process of disordering the structure, preserving the original properties of the matrix. In general, radiation resistance is significantly less important for the selection of matrices than their corrosion resistance. Deep groundwater is represented by brines, so the leaching resistance of matrices must be studied under conditions close to these settings. Calculations were made of the thermal field of a borehole repository for the REE-actinide fraction. The estimates have considered the container diameter, waste content, and holding time before disposal [53]. Minor actinides (Am, Cm) decay will result in heating of the waste forms up to 300–500 ^oC for decades, while REE decay has a rather short-term effect on the matrix heating. With a waste content of 30 wt.% in the immobilising matrix, the temperature increase in 10 years will be less than 40 °C.

8. On the Synthesis of Matrices in the REE₂O₃ (Nd₂O₃) – TiO₂ – ZrO₂ System

A material with the best properties will not become a matrix of real HLW unless it can be effectively obtained in the required volumes remotely. Given the high radiation, the availability of industrial waste forms production technology is especially important [15,28,29,30,31,32,33,34,35,36,37,39,86,112,113,114,115,116]. Due to its absence, the ceramics Synroc and the super-calcine proposed in the 1970s did not find practical application [25,26,28,114]. Only now, 50 years later, the technology for producing Synroc by hot pressing is close to implementation at a radiochemical plant in Australia [115]. The technologies already tested on real actinide-containing materials (waste) include melting in electric furnaces or induction crucibles, and sintering [15,37,39,86,116]. Melting has been used for over 40 years to vitrify high-level waste [25,26,27,28,29,30,31,32,33,34,35,36,37,112,113], sintering – for the synthesis of fuel with Pu, Np, Am – MOX, REMIX, MNUP [117,118,119]. Cold pressing and sintering route was developed to produce Synrocs and pyrochlore-based ceramics for weapons-based plutonium [15,26,28,39]. Hot pressing method was proposed to produce Synroc ceramics [25,114,115].

Melting methods are more preferable above the industrial technologies, as they are less sensitive to the quality of the batch and do not require the use of pressure. In the 1970s, induction melting technology was proposed for vitrification of high-level waste [25,26,27,28,29,30,31,32,33,34,35]. Its advantages are contactless energy supply and skull melting under active hydrodynamic conditions. In Russia, research has been conducted for over 20 years on the synthesis of induction melting in a cold crucible, ICCM of vitreous matrices and ceramics, including Synroc, zirconolite, pyrochlore, murataite, brannerite, etc. [80,81,86,99,120,121]. Abroad, the ICCM method is widely used to obtain ceramic and glass-ceramic forms with HLW simulants [28,30,39,122,123,124,125]. It allows obtaining matrices of the REE-actinide fraction at temperatures up to 2200 °C, although high temperatures require large energy costs. The REE zirconates are refractory phases, the liquidus temperatures are above 2000 °C. In titanate systems, the melting temperatures are lower: for La₂Ti₂O₇ and Nd₂Ti₂O₇, they are 1790 and 1650 °C, but for Nd₄Ti₉O₂₄ and the eutectic 85 mol.% Nd₄Ti₉O₂₄ - 15 mol.% TiO₂ they are below 1500 °C (Table 3), which makes ICCM a promising for their manufacture.

9. Discussion

Prevention of negative ecological impact of nuclear power waste is a fundamental scientific task and a practical problem. The main threat is associated with high-level waste containing long-lived actinides and fission products. Two approaches have been proposed to solve this problem: P&T (partitioning and transmutation) or P&C (partitioning and conditioning) [58,125]. The P&C approach is being developed by physicists and radiochemists; it is based on the separation of radioisotopes and their conversion into stable and short-lived isotopes upon neutron irradiation. This scheme includes the separation of HLW onto radionuclides or their groups, transmutation in a fast neutron flux, and disposal of the remaining waste. The first information on transmutation dates back to 1964 [126,127] and concerned fission products (⁹⁰Sr, ¹³⁷Cs). Transmutation of actinides in thermal water-cooled reactors (LWR), was proposed in 1972 [128], but later fast neutron reactors and subcritical installations – accelerators were chosen for this purpose as more promising [129,130,131]. The alternative approach (P&C) is being developed by geologists (mineralogists, geochemists), materials scientists, and some radiochemists; it involves the inclusion of waste in matrices and disposal. This method is more economical, since it requires less isotope separation and there is no need to build very complex expensive installations for deep partitioning of HLW, fuel fabrication, multiply irradiation and SNF reprocessing.

The purpose of transmutation is to reduce the environmental hazard of SNF and HLW [11,12,41,43,59,132,133,134,135,136,137,138,139,140,141] in order to achieve radiation (radioecological) equivalence of HLW and uranium ore used to manufacture nuclear fuel as quickly as possible. Calculations have shown that extraction and transmutation of 99.9% of actinides reduces this time to 100 – 300 years, which eliminates the need to construct deep geological disposal facilities. The obvious mistake of this calculations is connected with proposal on complete dissolution of HLW and uranium ore in water and the subsequent entry of radionuclides into the human body [142]. The shortcomings of the radiation equivalence concept are discussed in [143,144]. Many studies have noted the difficulties in implementing industrial separation of radionuclides, MA loaded fuel production, its irradiation in fast reactors, SNF reprocessing to extract actinides again [41,43,58,125,135,136,137,138,139,141,145,146,147].

For the transmutation of minor actinides, installations with accelerators [131,148,149,150] and molten salt reactors [151,152,153,154] are proposed. The latter do not need the actinides separation, but their problems are associated with high cost and lack of operating experience, the requirements of isotopically pure ⁷Li and to remove fission products, high corrosive activity of the fluoride melt etc [153]. It can be assumed that the announced ambitious task of implementing large-scale transmutation in the first half of the 21st century [149] will not be realized. Due to the need for large volumes of scientific and experimental design work in the field of creating new types of reactors, fuel and materials, the use of this technology is shifted to the distant future [17]. Another problem with this approach is that in this case it will be necessary to deal with large volumes of highly active materials for many years, which creates a high risk of accidents, irradiation of personnel and the population. Therefore, the question of using transmutation comes down to the choice between risks and consequences for current (transmutation) and future (disposal of HLW) generations.

The possibility of geological isolation of actinide-containing waste is beyond doubt. There are over two thousand uranium deposits with an age of many millions of years and total resources of over 10 million tonnes [155]. Numerous data prove the absence of uranium migration in reducing environments, which determines the preservation of uranium ores as well as uranium and thorium retention in natural vitreous and crystalline materials for hundreds of millions of years [50,66,85,87,101,156]. Such environments are realized at depths of the first hundreds of meters. Arguments in favour of long-term conservation of radionuclides include the results of studies of radioactive minerals and ores, laboratory and field experiments, thermodynamic calculations and computer modelling. The world has already accumulated 30-35 thousand tonnes of vitrified HLW from the reprocessing of SNF (the main volumes are stored in France, Russia, UK, and US) [7,15,37,157], which will be placed in underground disposal facilities. In countries with an open nuclear fuel cycle without reprocessing of SNF, its geological disposal is also assumed. The protective properties of the geological environment are due to the low solubility of radionuclides in water and the high sorption capacity of rocks. This is facilitated by the slow movement of underground waters – carriers of radionuclides. A contribution to safety will be provided by sorptive bentonite buffer swelling in water, corrosion-resistant container, and durable matrix immobilising nuclear waste. Its main characteristics are high loading for waste, low leaching rate in water, and the possibility of remote fabrication on an industrial scale.

Extraction and precipitation methods are used to separate liquid HLW onto fractions [13,17,24,38,41,42,43,44,45,46,47,52,125,130,139,157]. In France, a pilot reprocessing of 15 kg of SNF was conducted with the separation of the REE-actinides [158]. In Russia, a plant for the separation of the Cs-Sr fraction was in operation from 1996 to 2007, and of the REE-actinides since 1999 [159]. From 1996 to 2003, 1630 m³ of HLW were processed there and 1.73 million Curies of the REE-actinide fraction (Am, Cm) were separated by precipitation of oxalate.

During the year of operation of a LWR with a capacity of 1000 MW, 20 tonnes of SNF are formed. A plant for the reprocessing of 800 tonnes of irradiated fuel generates about 4000 m³ of liquid HLW per year, one m³ of such waste containing up to 90 g of Am and Cm. A fleet of reactors with a total capacity of 100 GW(e), as in the USA or Western Europe, produces up to 3 tonnes of minor actinides per year – almost equal amounts of Np and the sum of Am and Cm; the annual accumulation of the REE-actinide fraction during the reprocessing of SNF will be about 30 tonnes. In France, 350 – 400 billion kWh of electricity are produced per year at nuclear power plants [160]. This leads to an annual production of SNF containing up to 300 kg of Am and 150 kg of Cm [43], and the mass of the REE-actinide fraction will be about 10 tonnes. In 2022, Russian NPPs generated more than 220 billion kWh of electricity [161], or ~60% of that produced in France.

In the case of reprocessing of all discharged SNF and fractionation of HLW, up to 6 tonnes of REE-actinide fraction will be formed per year. Its immobilisation will require 10–20 tonnes (2–4 m³) of matrix with 50–25 wt.% REE-actinide fraction. According to estimates, a cluster of horizontal boreholes drilled from a vertical wellbore, known as SuperLAT technology (Figure 11) will accommodate up to 1000 t of SNF [162] or 500 t of HLW.

Placing solidified actinide waste in deep disposal facilities is obviously less dangerous than injecting liquid radioactive waste (LRW), which has been practiced in Russia for about 60 years [164,165,166]. This technology involves injecting it into porous collectors at depths of 180 to 500 m, isolated from aquifers by clays. The total volume of LRW by 2014 exceeded 60 million m³, the activity was 7×10¹⁹ Bq or ≈2 billion Curie. The actinide content in water at a distance of 100–150 m from the injection wells decreases by 10⁴ – 10⁶ times compared to the initial values, i.e. the bulk of the radionuclides was quickly precipitated.

The very low migration ability of actinides in underground water at reducing conditions, which are already dominant at depths of the first hundreds of meters, is evidence in favour of geological isolation of actinide wastes [125,167,168]. Programs for the geological disposal of HLW exist in many countries, including Russia [22,23,24,167,168,169,170,171,172]. The main contribution to human exposure will be made not by actinides, but by long-lived fission products (⁷⁹Se, ⁹⁹Tc, ¹²⁹I) due to their weak sorption by rocks and high migration rates in the geological environment [168,171]. Along with the traditional mine storage facilities for SNF and solidified HLW ultimate disposal [18,19,20,21,22,23,24,172], more and more attention is being paid to a deep boreholes – vertical, with a depth of 3 to 5 km [173,174,175], or initially vertical, and then horizontal at the depths of 1–3 km [162,163,176,177,178].

10. Concluding Remarks

Two technologies have been proposed for handling actinide-containing fractions: transmutation or immobilization. The purpose of transmutation is to reduce the amounts of long-lived radionuclides by irradiation in nuclear reactors or accelerators. The first works on this topic devoted to fission products are about 60 years old [126], 50 years ago this method was proposed for actinides [128]. Practically at the same time the study of crystalline matrices for HLW began [25]. The principles of transmutation are developed theoretically [41,137,179], and experiments on irradiation of targets with actinides have been conducted [140]. Practical implementation of the approach faces a lot of technical difficulties and high costs. Reducing the amounts of radionuclides requires multiple radiation of actinides-loaded fuel [43]. Due to increased amounts of heavy actinides, in the first turn ^244,245Cm [179] the irradiated targets will posses with high radioactivity, neutron fluxes and temperature, which complicates reprocessing and production of new fuel. For these purposes, fast neutron reactors are needed, in which the actinide fission dominates over neutron capture. Reasonable doubts on the benefits of minor actinides transmutation are discussed in [180]. As can be seen, many questions concerning the technical feasibility of effective transmutation of minor actinides, raised more than 30 years ago [181], are still relevant. Extraction and accumulation of heavy minor actinides (Am, Cm) with low (tens kg) critical masses threatens the nuclear weapons non-proliferation [182,183].

Another approach to solving the problem of long-lived radionuclides is to incorporate them into stable matrices for placement in deep geological disposal facilities. Its feasibility is demonstrated by the study of natural analogues of matrices – minerals that have retained actinides (Th,U) and REEs with properties similar to minor actinides for millions of years [50,85,87,184,185,186]. The presence of geological environments with a very low actinide migration is demonstrated by data on the preservation of U and Th deposits, their behaviour in the natural nuclear reactor Oklo (Gabon), studies on the solubility and sorption of actinides in laboratories and in natural (field) tests. This is supported by data on volcanic rocks confining uranium and thorium for over 140–145 million years [156].

Let us summarize the analysis of the phases of the REE₂O₃–ZrO₂–TiO₂ system as applied to the problem of REE – actinide fraction immobilisation. It is known that Nd³⁺ serves as an analogue of trivalent actinides (Am, Cm), therefore the Nd₂O₃–TiO₂–ZrO₂ system is perspective for searching hosts for REE–MA. It contains phases Nd₂(Zr,Ti)₂O_7-x, Nd₂TiO₅, Nd₂Ti₂O₇, Nd₂Ti₃O₉ and Nd₄Ti₉O₂₄, containing from 48 to 73 wt.% Nd₂O₃. Pyrochlore Nd₂(Zr,Ti)₂O_7-x and titanate Nd₄Ti₉O₂₄ are of most interest as matrices: their stability fields (impurity content) are larger than those of other Nd titanates. With an excess of titanium, an additional phase of rutile (TiO₂) is formed, as a result of which the melting temperature decreases to 1450 – 1500 °C, which facilitates synthesis. Unlike pyrochlore, the Nd₄Ti₉O₂₄ compound has no analogues in nature. The content of Nd₂O₃ (actinide imitator) in it varies from 48 wt.% (only Nd₄Ti₉O₂₄) to 35 wt.% (70% Nd₄Ti₉O₂₄ and 30% rutile). Leaching rate of actinides and REE from Ti-Zr pyrochlore and REE titanates matrices in a wide range of temperatures (50–240 °C), compositions and pH of solutions (water, brines) varied in the range 10^-3 – 10^-5 g/(m² × day) and decreased with increasing interaction time. This is due to the formation of a thin surface layer enriched in zirconium and titanium, which prevents the release of REE and actinides into solution. As for the other types of matrices [187] the leaching rate in the first days is determined by their solubility, and then, due to formation of an alteration surface layer the mode changes to diffusion exchange with low velocity.

Phases of the Nd₂O₃ – TiO₂ – ZrO₂ system are considered as matrices for isolating Pu and REE-actinide fraction [62,65,68,188]. In addition to Ti-Zr pyrochlore, Nd₄Ti₉O₂₄ is of interest due to its higher content of impurities (Ca²⁺, Zr⁴⁺, U⁴⁺) compared to Nd₂TiO₅ and Nd₂Ti₂O₇. With an excess of titanium, rutile (TiO₂) is formed, which reduces the melting temperature by 200–300 °C, which facilitates the synthesis of such a composite matrix.

Durable stable crystalline phases hosting long-lived radionuclides might be components of integral composite systems containing both crystalline and vitreous nuclear waste forms. There is a notable trend to use durable composite materials containing both crystalline and vitreous phases in nuclear waste immobilisation practice. The content of crystalline phases is now being increased in the glass formulations used at nuclear waste vitrification [116,189,190,191], which is schematically shown by the red arrow originating in the left side of the diagram (Figure 12). It is notably that the first industrial plant which uses hot pressing in Australia is now designed to synthesize composite materials with Synroc-type crystalline and mineral-like phases distributed within a glass [115,191,192,193,194].

Matrices of HLW are currently supposed to be placed at depths of 500–700 m in mine-type disposal facilities or in super-deep (3 to 5 km) boreholes. For the REE-actinide fraction, the borehole option is preferable; in this case, due to the increase in temperature with depth, the rocks can be heated to 200 °C and even higher values [195]. High temperature slows down the radiation destruction of the crystalline structure of actinide hosts and reduces the effect of amorphization on the leaching of radionuclides. Both the large distance to the surface and low velocity of water movement will also serve as important safety factors for the HLW disposal.

Progress in the field of methods for separating HLW [47], their incorporation into matrices [39,193] and recent achievements on geological disposal of the spent nuclear fuel and solidified high-level waste [23,195,196,197] allows us to expect a solution to the problem of handling the REE – minor actinides already in the coming decades. The use of transmutation for this purpose appears to be essentially more complex technological approach, required significantly more financial resources and time for implementation [17].