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Review

Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste

by
Sergey V. Yudintsev
1,
Michael I. Ojovan
1,2,* and
Victor I. Malkovsky
1
1
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry of the Russian Academy of Sciences, Staromonetny Lane, 35, 119017 Moscow, Russia
2
Department of Materials, Imperial College London, London SW7 2AZ, UK
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(2), 70; https://doi.org/10.3390/jcs8020070
Submission received: 17 December 2023 / Revised: 31 January 2024 / Accepted: 8 February 2024 / Published: 10 February 2024
(This article belongs to the Section Composites Applications)
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Abstract

:
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 (T1/2) from 284 days (144Ce) to 90 years (151Sm), including 147Pm (T1/2 = 2.6 years), 154Eu (T1/2 = 8.8 years), and 155Eu (T1/2 = 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 (T1/2 = 2.14 × 106 years), 239Pu (T1/2 = 2.41 × 104 years), 240Pu (T1/2 = 6537 years), and 242Pu (T1/2 = 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 (Cax(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.

1. Introduction

For the third year in a row, the IAEA has revised the forecast for the growth of nuclear energy in the world [1]. It is now expected that in 2050, the nuclear power plant capacity will be 890 GW in the high version and 458 GW in the low version, up from the current value of 369 GW. Compared with the 2020 forecast, the upper limit is increased by 178 GW (24%); compared with 2022, the high and low estimates increased by 2% and 14%. This correlates with estimates of economic growth, and additional impetus for nuclear energy development comes from concerns about climate change [2,3]. In November 2023, the European Parliament classified nuclear energy, along with 15 other technologies, as a “clean” technology, which will stimulate its further development. At the recent UN climate conference COP-28 in the UAE (30 November–12 December 2023), 22 countries, including the USA, Canada, France, Japan, the Republic of Korea, Sweden, and United Kingdom committed to increasing their nuclear power capacity in 2050 year 3-fold compared with 2020. 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 now; that is, from 130 to 200 thousand tons 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—LWR, BWR, VVER [4].
Sustainable operation of nuclear energy will require (1) the provision of resources and (2) the development of methods for the effective and safe management of radioactive waste, including the most dangerous high-level radioactive waste (HLW) [5]. Both problems can be solved by the transition to a two-component nuclear power plant with slow and fast neutron reactors operating in a closed nuclear fuel cycle (NFC) mode with reprocessing of SNF [6]. This will allow fissile materials (U, Pu) to be involved in the NFC and creates the basis for the rational management of HLW by separating chemically similar elements into fractions that differ in half-lives, radiotoxicity, etc. One of these fractions consists of rare-earth elements (REEs) and minor actinides (Am, Cm), which can be co-extracted from HLW by improved PUREX, TRUEX, DIAMEX, UREX [7,8,9,10,11,12,13,14,15]. There are three known ways of solving the MA problem: (1) heterogeneous transmutation in fast neutron reactors after extracting the REEs and MAs, partitioning the MAs and REEs to separate Am and Cm; (2) combustion of MAs in molten salt reactors; (3) immobilization of the REE–MA fraction into stable matrices (waste forms) for disposal. Based on technological feasibility and efficiency, the third option for processing MAs by immobilization followed by disposal is the most preferable. Some glasses (La-borosilicate, LaBS), polyphase or monophase ceramics, and glass–crystalline composites with capacious and stable phases (artificial minerals), such as pyrochlore, zirconolite, britholite, brannerite, monazite, etc., are considered appropriate waste forms (matrices) of actinides.
Publications rarely consider the heating of matrices by radiogenic heat and its effect on the properties of materials. One of the consequences of radiogenic heating is the possibility of inducing spontaneous crystallization of the vitreous matrix (devitrification) and its transformation into a glass–ceramic. Here, we examine the available data on the content of REE—MA fractions in the SNF and HLW, their half-lives and radiogenic heat release during the decay, and the possibility of forming stable crystalline phases that immobilize MAs caused by the radiogenic heating of vitrified HLW.

2. REE and MA Contents in SNF and Reprocessing HLW

During the fission of actinides in a nuclear reactor, REEs are formed in large quantities; their share among fission products (FP) in the composition of spent fuel is about 30% [16,17]. The overwhelming majority of these elements are represented by stable isotopes or nuclides with such a long half-life that they can be considered stable (Table 1).
A small proportion of the REEs in SNF (HLW) are represented by short-lived isotopes (half-life T1/2 up to 10 years: 144Ce, 147Pm, 154Eu, 155Eu) and medium-lived radionuclides (T1/2 less than 100 years: 151Sm). The decay of these REE radionuclides makes the main contribution to the heat released by SNF in the first few years after its removal from a nuclear reactor [18,19] (Table 1 and Table 2, Figure 1).
Subsequently, the role of REEs quickly diminishes, and Cs, Sr, and trans-uranium actinides begin to play an increasingly important role (Table 2). After about 60 years, the contribution of the two groups of isotopes, Cs-Sr and actinides, will become equal; then, actinides will begin to make the main contribution to the heat released by SNF.
The ratio of the amounts of REEs and MAs (Am, Cm) in SNF is biased in favor of REEs; depending on the burnup of the fuel and its storage time, their content (Table 3) is 90–95 wt.% REE versus 10–5 wt.% of actinides (Am, Cm). The amount of 241Am during SNF storage increases due to decay of the short-lived precursor 241Pu. The heat released by the REE–MA fraction in accordance with its composition (Table 1, Table 2, Table 3, Table 4 and Table 5) is first determined by the decay of short-lived REEs, and then by 244Cm, 241Am, and, to a lesser extent, 243Am.

3. Effect of Radiogenic Heat on the Vitreous Matrix of the REE–MA Fraction

Stable elements or U and Th with a long half-life and low heat release are usually used in the laboratory synthesis and study of high-level waste matrices. For trivalent MAs (Am, Cm), REEs serve as simulants (Table 6); most often, Nd [22,23]. This approach is based on the proximity of the radii of Nd3+, Am3+, and Cm3+ [24]. It is justified when studying the solubility and isomorphic capacity of phases, the coordination of cations in the structure, and the distribution of elements between crystalline phases and glass.
The similarity in the behavior of rare-earth elements (La, Nd) and MA (Am, Cm) in glass–crystalline matrices has been confirmed experimentally [25,26,27]. However, studies rarely consider the effect of the temperature factor—heating caused by the radiogenic heat. The use of high-capacity matrix compositions allows for a more efficient use of the space of future disposal facilities, which will reduce the specific costs of radioactive waste disposal. High short-lived radionuclide contents will cause heating of the matrix, which can change its properties. A few works [28,29,30,31] show that the temperatures of HLW matrices can reach several hundred degrees Celsius and persist for tens and hundreds of years. Due to the tendency of glass to crystallize at high temperatures, heating will have the greatest effect on vitreous matrices. If stable crystalline MA-containing phases are eventually formed, this will improve the immobilizing properties of the matrix. The phases comprise compounds of REEs and MAs with the structure of zirconolite, britholite, pyrochlore, brannerite, and monazite. The first four phases are characteristic of silicate glass–ceramics, and monazite appears during the crystallization of phosphate glasses. In essence, we are talking about the targeted crystallization of glass to obtain glass–ceramic compositions with increased stability. The idea of such transformations is considered in papers [32,33].
The advantages of glass–crystalline HLW matrices have been noted in many publications [32,33,34,35,36,37,38,39]. Considering the possible temperatures due to radiogenic heat, partial crystallization of glass with the formation of mineral-like phases of REEs and actinides that are stable in water seems real. We have considered two scenarios for heating the vitreous matrix of REE and MA due to radiogenic heat: (1) for the REE–MA fraction, where REEs are considered to be stable elements; (2) for the REE fraction with the presence of radioactive isotopes. This makes it possible to estimate the temperatures of REE–MA matrices containing actinide and rare-earth radionuclides. Assessments of the heating of the matrix with rare-earth elements are of importance in connection with their possible release at SNF reprocessing, including the “dry” regeneration of SNF in molten salts [40,41,42,43,44] with subsequent immobilization of the so called “lanthanide fraction” in the glasses. This REE fraction of SNF reprocessing will have the following approximate composition (in wt.%): 38 Nd2O3, 24 CeO2, 12 La2O3, 11 PrO2, 8 Sm2O3, 5 Y2O3, 1 Eu2O3, and 1 Gd2O3. First, we will consider the change over time in the heat release and temperature of the matrix of the REE–MA fraction under the assumption that the REE isotopes are stable; then, we will carry out the calculation only for the REE fraction, taking into account short-lived radionuclides. The basics of the calculations are presented in [31].

4. Estimation of Temperature of the REE–MA Fraction Matrix When REEs Are Stable

The half-lives of 239Pu and 237Np are long (tens of thousands and millions of years), while short-lived 238Pu (88 years) and 241Pu (14 years) quickly decay to 234U (246,000 years) and 241Am (431 years), so Am and Cm become the main source of radiogenic heat in SNF, and they also determine the heat released by the REE–MA fraction. It is assumed that the REE–MA fraction contains 95 wt.% REE (stable) and 5 wt.% MA, including 3.5% 241Am (T1/2 = 432 years), 1% 243Am (7370 years), 0.45% 244Cm (18 years) and 0.05% 245Cm (8500 years). Taking into account their heat release (W/kg)—241Am, 114.7; 243Am, 6.4; 244Cm, 2841.8; 245Cm—5.8 [21]—the initial heat released by the fraction can be attributed to two isotopes: 244Cm (~78%) and 241Am (~22%). It was assumed that the vitrified REE–MA fraction is loaded in a container with a diameter of 0.2 m and placed in a deep borehole in granites in the interval from 3 to 5 km; between the walls of the container with vitrified HLW and the borehole, there can be a layer of sorption bentonite buffer 10 cm thick or the buffer is absent. The parameters used in the calculations are summarized in Table 7.
The changes over time in the intensity of the calculated heat released and temperatures in the center and on the surface of the block are shown in Figure 2. For the first few decades, their values change little, but then they quickly decrease following the decay of radioisotopes. Three years after loading the container into borehole, the temperatures in the center/on the surface of the block are 386/358 °C, and after 30 years, their values will be 201 and 190 °C. The presence of a bentonite buffer slightly increases the temperature (by 10 °C) due to its thermal conductivity being lower than that of the rock. To these values, it is necessary to add the increase in rock temperature due to the geothermal gradient, which, at a depth of 3–5 km, will be 100–150 °C. Thus, the temperature of a matrix with waste can reach 450–550 °C. An increase in the concentration of the REE-MA fraction in the glass or diameter of the block will cause its further increase.

5. Temperature of the REE–MA Fraction Matrix with Decaying REE

SNF and HLW from reprocessing contain short-lived isotopes of rare-earth elements (Ce, Pm, Sm, and Eu). Therefore, it is necessary to evaluate the possible contribution of their decay to the heating of the REE waste form. In the first years after SNF is unloaded from the reactor, short-lived REEs make a significant contribution to heat release (Table 2, Figure 1) and then the Cs-Sr group begins to dominate; after 70 years, this role passes to trans-uranium elements. By analogy with calculations for the MA matrix, we estimated the temperature of glass containing the REE fraction. In the first years, the decay of short-lived radionuclides of this group will determine the heat release and heating of the SNF and the vitreous matrix of the HLW. Short-lived REE isotopes are represented by 151Sm (T1/2 = 90 years), which decays to 151Eu (stable); 147Pm (2.6 years) becomes 147Sm (1.06 × 1011 years), then through α-decay, it transmutes into 143Nd (stable); 144Ce (284 days) → 144Pr (17 min) → 144Nd (stable); 154Eu (8.8 years) → 154Gd (stable); 155Eu (5 years) → 155Gd (stable). Taking into account the half-lives and contents of REE isotopes, the main contribution to the heat released by the REE mixture (Table 1 and Table 2) will be made by 147Pm and 154/155Eu. Let us assume that a tonne of SNF contains 15 kg of REEs (Table 3), which, during SNF reprocessing, enter liquid HLW and are then vitrified. With an REE concentration in glass equal to 30 wt.%, a tonne of vitreous matrix will contain 300 kg of REEs, and their amount in 1 m3 (at a glass density of 3000 kg/m3) will be 900 kg, which is 60 times higher than in a tonne of SNF. Taking this into account their heat release in SNF (Table 2), the change in the heat-release density of glass with 30 wt.% REE over time was determined as shown in Table 8. The very high volumetric heat release in the early years leads to extremely high temperatures of thousands of degrees Celsius (Figure 3). After 5–10 years of SNF storage, heat release due to the decay of short-lived REE radionuclides drops tens and hundreds of times (Table 8), so their contribution to matrix heating in 10 years will decrease to several tens of degrees.
The temperatures of a glass containing 50 wt.% REE from the pyrochemical processing of SNF were estimated [30]. Calculations were performed for a cylindrical block of glass with a diameter of 0.3–1.0 m; a residual amount of 0 to 20 rel.% of trans-uranium elements (TUEs) was allowed due to their incomplete extraction. The specific heat released by glass with a rare-earth element fraction depends on the degree of purification of this fraction from actinides and decreases with time. The heating temperature of a glass block increases with increasing diameter and TUE content but quickly decreases over time as REE isotopes decay (144Ce, T1/2 = 0.8 years; 147Pm, 2.6 years; 151Sm, 90 years; 154Eu, 8.8 years; 155Eu, 5 years). The initial temperature in the center of a block of glass with 50 wt.% REE varies from 200 to 900 °C, depending on the diameter (0.3–1 m) and the degree of purification from actinide content (80–100%), and quickly decreases after 3 years of storage.

6. General Patterns of Crystallization of Vitreous Matrices Containing HLW

Usage of glasses for the immobilization of HLW depends on two important parameters—the melting (Tm) and glass transition (Tg) temperatures, the latter being also known as the transition temperature at which the glass viscosity is 1013 Poise (1012 Pa/sec) [45,46,47,48,49]. The Tg value is determined by thermal analysis; it is not a constant value but weakly depends on the heating rate of the glass sample. The melting temperatures (Tm) of aluminum and iron phosphate glasses are 900–1000 °C, increasing to 1200–1300 °C for alkaline B-Si glasses and 1300–1500 °C for refractory aluminosilicate glasses. The ability of glass to crystallize depends on the rate of diffusion of elements and increases with decreasing viscosity. The maximum rate of devitrification occurs at temperatures 100–200 degrees above the glass transition temperature. To obtain large phase grains, heating is first carried out at 20–50 °C above Tg (Figure 4) to form crystallization centers (nucleation stage), and then the temperature is raised by 100–200 °C (growth stage of crystalline phases).
Heat-treatment schedules used to obtain a glass–ceramic are different for systems (A) with closely overlapping dependences of the nucleation and growth temperatures and those (B) with widely spaced temperatures of the maximum nucleation rate (Imax) and growth rate (Umax). For systems of type (B), only separate nucleation and growth of crystals is possible, whereas systems of type (A) can be processed by applying isothermal nucleation and growth or nucleation and growth under controlled cooling conditions [48,49].
Crystallization can begin after the melt is drained from the melter into the canisters due to residual heat [50,51]. Therefore, the cooling of the melt with HLW must be rapid in order to form a homogeneous glass. This is achieved by tempering it at a rate of 500 °C per hour, and when the glass cools at a rate of 50 °C per hour or less, crystalline phases appear [52]. The cooling rate is especially important near the glass transition temperature (Tg), below which the viscosity of the melt sharply increases, and it turns into glass. The Tg value depends on the composition of the glass and increases with melting temperature (Tm), amounting to 0.4–0.6 of the Tm value [53]. Heating accelerates devitrification due to a decrease in viscosity, which accelerates the diffusion of elements (Figure 4); this increases the number of crystallization centers and the rate of crystal growth. For Na-Al-P glasses (Tg being about 400 °C), the crystallization rate is maximum at ~500 °C; for Na-B-Si glasses (Tg ~550 °C), it is at 650 °C [35,45,52].
Typical Tg values for phosphate glasses with REE are in the range of 450–550 °C [53]. For B-Si glass matrices they increase to 550–600 °C; maximum values 650–790 °C, and even 870–900 °C, are typical of Al-Si glasses without boron [35,45,54,55,56,57,58,59]. The type of crystallization of glass during heating depends on the value of the Tgr = Tg/Tm ratio; in the range of Tgr = 0.48–0.61, volumetric crystallization occurs, and at Tgr from 0.59 to 0.70, surface crystallization occurs [58]. An increase in Tg causes an increase in thermal stability of the glass matrix and, with it, the content of heat-generating radionuclides within it; an increase of 200 °C will allow for a 60% increase in the acceptable heat released by waste (e.g., concentration of short-lived isotopes) without crystallization of glass. Therefore, the waste loading of the Na-B-Si matrix is 3–5 times higher than that of Na-Al-P glass: 18–25 and 3–5 wt.%, respectively. Crystallization of glass is possible at temperatures lower than Tg, but due to its high viscosity, it will occur much more slowly. Depending on the position of the maximum rates of nucleation and crystallization on the temperature axis, the formation of glass–ceramics occurs during separate heating of the glass in several stages with a large difference in these temperatures (Figure 4) or in a single-stage process, where nucleation and the growth of crystals occur at the same temperature [48,49].
In industrial production, borosilicate glasses with HLW, after pouring the melt into a container containing several hundred kilograms of glass, cool naturally at a rate of about 1 °C/min [16]. Thus, for French B-Si glass R7T7 (Tg = 510 °C), the temperature drops to 400°C within 24 h after pouring the melt into canisters and then slowly decreases [37]. In order to avoid any risk of crystallization during storage and geological disposal, the temperature of the glass in the canisters (being increased due to radioactivity) must remain below Tg so that the diffusion of atoms is negligible, which limits the amount of waste in the glass. Alternatively, this involves adapting its composition to increase the Tg of glass.
The choice of composition in the SiO2–Al2O3–B2O3 system with the weight ratio SiO2: Al2O3:B2O3 = 3:2:1 made it possible to include a 50 wt.% REE fraction—a mixture of La2O3, CeO2, Nd2O3, Sm2O3, Gd2O3, Y2O3 or a simpler simulant (Nd2O3 + Gd2O3)—into homogeneous glass [30,42]. This increases the Tg value to 770–797 °C, which is significantly higher than the values for typical B-Si glass compositions for nuclear waste [35]. The melting point of such glasses, about 1450 °C, is also around 200–300 °C higher.
The heating of the matrix is influenced by its composition and radionuclide content, the dimensions of the container, and the thermophysical properties of the matrix and host rocks. It is possible to select the parameters at which the temperature within tens of years will be close to the glass transition temperature (Tg), which, for Al-P and B-Si glasses, varies from 500 to 700 °C [35,52]. As noted, in the manufacture of glass–ceramics, glass is heated in two stages: nucleation (1) and grain growth (2) at a higher temperature. The rate of formation of crystallization centers is maximum at a temperature slightly above Tg, and the highest rate of grain growth is 100–300 °C above Tg. This approach is used in experiments on the production of glass–ceramics; first, by heating the homogeneous glass at a temperature T = (Tg + 20–50 °C) and then increasing the temperature for crystal growth [48]. The duration of glass processing to obtain glass–ceramics ranges from 1–2 h to a few tens of hours. As a result, the glass turns into a glass–ceramic with double protection against leaching, since the stable crystalline phase with MAs is enclosed in glass with a lower actinide content. Heating of the matrix as a result of the decay of radionuclides will continue for decades, which will inevitably cause partial crystallization of the glass. Let us consider the experimental data on glass crystallization with simulants.

7. Experimental Studies of Crystallization of Glasses with REEs and Actinides

There are numerous works on the crystallization of glass matrices with HLW simulants to form glass–ceramics. Usually, in experiments, REEs, U, or Th are used to replace trans-uranium actinides (Np, Pu, Am, Cm). Crystallization products in silicate systems are represented by zirconolite ((Ca,Ln)(Zr,Ln)(Ti,Al)2O7) or britholite ((Ca,Ln)10(SiO4)6O2), and when phosphate glass is heated, monazite (Ca,Ln)PO4 is formed. The symbol Ln refers to individual REEs, primarily Nd, their mixtures, as well as the REE-MA fraction as a whole. These crystalline phases have long been studied as possible actinide matrices with the data on their radiation and corrosion resistance indicating promise for their use for this purpose [16,17,23,28,33,35,39,45,60,61,62]. The important thing is that they have natural analogues, including radioactive minerals containing REEs, U, and Th, with an age of many millions of years. Their study makes it possible to evaluate the behavior of matrices with similar artificial phases over very long time periods.
Crystallization of glasses with the formation of monazite, zirconolite, and britholite has been studied due to interest in them as potential matrices of REEs and MAs. Of greatest interest are those that provide information about the production conditions, properties of glasses, and their heat treatment modes, including composition and melting temperature, solubility of the REE, glass transition temperatures, composition of phases and glass, distribution coefficients of elements between them, corrosion resistance, etc.

7.1. Zirconolite Glass–Ceramics with REE

Data on silicate matrices with zirconolite are available from various studies [32,37,39,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79]. In a two-stage synthesis, zirconolite glass–ceramics are obtained by melting a mixture of oxides at elevated temperatures, and then the glass is subjected to heat treatment (annealing) in one or more stages to crystallize the zirconolite [68,69,70,71,72,73,74]. When heating B-Si or Al-Si glasses containing REEs, depending on the composition, zirconolite or britholite appears; with a high content of Zr and REEs in the glass, both of these phases are present. Additions of B2O3, CaO, or Na2O reduce the melting temperature of glass from 1400–1550 °C to 1200–1300 °C; as a result, the glass heating temperature for the crystallization of zirconolite and britholite is also reduced to 600–800 °C [63,73]. An increase in the glass heating temperature promotes the growth of larger grains of crystalline phases with actinides.
Crystallization leads to a redistribution of elements between the glass and formed phases. The value of the distribution coefficient (Kp) for REEs (Ce, Nd, Gd) between zirconolite and glass varies from 1.5 to 3, increasing from light REEs to heavy rare-earth elements and yttrium [63,64,65,69,70,71]. The distribution coefficient (Kp) is defined as the ratio of concentrations of the element (radionuclide) in the crystalline phase to that in the vitreous phase at equilibrium. This explains the rather slight decrease in the element content in the residual glass as zirconolite crystallizes [75,76,77,78,79]. In general, the structure of zirconolite is preferable for the inclusion of heavy REEs of the yttrium group with a smaller ionic radius compared with larger cations of light REEs of the cerium group. Moreover, it is the latter that dominates in the composition of SNF and HLW and forms the basis of the REE-MA fraction (La, Ce, Nd). The solubility of REEs in typical Na-Al-B-Si glasses is estimated at 15–25 wt.%. By modifying the composition, this can be increased to 30 wt.% and even 40–50 wt.% [43]. TiO2 and ZrO2 are required for the formation of zirconolite so, in this case, the solubility of REEs in glass will be lower.

7.2. Britholite Glass–Ceramics

The maximum REE content in the zirconolite is 40 wt.% [23], and in the britholite, it reaches 75 wt.% [80]. Therefore, the REE content in glass–ceramics with britholite will also be approximately two times higher than in matrices with zirconolite. There are numerous examples of britholite crystallization from the melt or by quenching and annealing glass doped with lanthanides (Figure 5) and minor actinides (Am) [25,26,27,32,36,37,75,80,81].
Distribution coefficient (Kp) values in the britholite–glass system for La, Ce, and Nd vary from 10 to 15 [80,81,82], while for Am, a Kp value of about 7–9 was established [25,26,27]. Considering the above, the residual glass of a glass–ceramic with britholite has a lower REE content than in glass–crystalline matrices with zirconolite. Since the stability of the glass phase is worse than that of the crystalline phases, this indicates a greater positive effect on the change in the immobilizing ability of the matrix in the case of britholite crystallization.

7.3. Monazite Glass–Ceramics

Phosphate glass–ceramics with monazite are described in [32,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98]. Industrial immobilization of HLW in B-Si glass in the world has been carried out since the 1970s; in Russia, since the 1980s, an Al-P composition has been used for this purpose. To date, the total volume of vitrified waste can be estimated at 40,000 tons, including 32,000 tons of borosilicate glass and 8000 tons of phosphate glass [5]. Phosphate glasses are of interest from the point of view of their easier transformation into a glass–ceramic matrix under radiogenic heating. Monazite glass–ceramics were obtained by melting an oxide–phosphate charge in alundum crucibles in an electric furnace at 1000 °C, followed by quenching and heating of the glass for 4 h in air at 500 °C. The calculated composition of sample AF-1 (wt.%) is as follows: 21.0% Na2O, 17.0% Al2O3, 50.0% P2O5, 10.0% Ln2O3 (Ln = La + Ce + Nd), and 5.0% fission products (MoO3, ZrO2, BaO) and corrosion products (Cr2O3, Fe2O3, NiO, MnO). The samples are composed of glass and an REE phosphate with the structure of monazite (Figure 6), which is formed in the melt due to the excess of REE content over solubility, estimated at 2–3 wt.% [5,52,97,98].
Monazite grains are up to 20 μm in size and form intergrowths. Partitioning of REEs is biased in favor of monazite, and the distribution coefficient (Kp) between monazite and glass is 30 (Table 9). Heating at 500 °C leads to the formation of Na-Al-P phases around the monazite grains (Figure 6 a,b). The spherulites practically do not contain any REEs.
Alumina phosphate (SAP) and alumina–iron phosphate (SAIP) glasses with REEs and actinides (U, Np, Pu, Am) have been studied [88,93,94,95]. Their compositions, in mol.%/wt.%, are as follows: 40/24.3 Na2O, 20/20.0 Al2O3, 40/55.7 P2O5 (SAP); and 40/23.0 Na2O, 10/9.5 Al2O3, 10/14.8 Fe2O3, 40/52.7 P2O5 (SAIP). The REE content varied from 1 to 20 wt.%, and from less than 1 wt.% (Np, Pu, Am) to 50 wt.% (U) for actinides. The glass was obtained by melting for 0.5–6 h at 1000–1300 °C and pouring onto a plate for tempering or cooling according to a regime similar to cooling a block in a 200 L container at the “Mayak” Production Association. When the glass contains up to 5 wt.% REE, monazite does not form. When it is slowly cooled, phosphates of Na, Al (or Fe-Al), and REEs (monazite) appear, and the rate of the leaching of elements increases 5- to 10-fold to 10−6 g/(cm2 × day) [93]. Actinide simulators (REEs) are concentrated in the monazite during crystallization; their distribution coefficient between monazite and residual glass is 45–70. The phase composition of glass matrices with REEs (La, Ce, Eu, Gd) and their structure and stability in water have been determined [94,95]. The introduction of up to 5 wt.% lanthanide oxides (Ln) into glasses does not cause crystallization during quenching (except for La-doped glass) and has a small effect on their structure and hydrolytic stability. At slow annealing, SAP glasses crystallize with the formation of Al-P, Na-Al-P, Ln-P (monazite), and Na-Ln-P phases, and additional oxide Gd2O3 in the case of Gd.
By melting for 1 to 6 h at 1000 or 1200 °C, alumina phosphate (SAP) and alumina–iron phosphate (SAIP) glasses with 9 wt.% REE were obtained. Part of the melt was quenched, and the rest was cooled in the furnace. Tempered glasses are homogeneous, while those cooled in a furnace, in addition to glass, contain Al, Na-Al, and rare-earth phosphate (monazite). Experiments (water; solid fraction 0.071–0.125 mm; 90 °C; 7 days) revealed an increase in the rate of leaching from partially crystallized glass of 5- to 10-fold compared with tempered glass. The inclusion of 9 wt.% REE does not impair the stability of glasses in water with respect to Na, Al, and P. For annealed glasses, leaching rates increase by 2–40-fold due to the formation of Al and Na-Al phosphates, but REE leaching remains low due to the occurrence of monazite. If glass crystallizes, monazite will retain REEs and prevent the properties of the matrix from deteriorating with time. Leaching rates on the 30th day are equal: 3.5 × 10−8, 3.4 × 10−9, and 6.3 × 10−9 g/(cm2 × day) for Np, Pu, and Am, respectively. The higher leaching rates for Np than for Pu and Am are probably due to its presence, at least in part, as the NpO2+ ion [88].
According to [95], SAP (Tm = 1000 °C) and SAIP (Tm = 1200 °C) glasses with 10 wt.% REE do not contain crystalline phases, and only the sample with La contains traces of monazite (LaPO4). When annealed, the REE-free SAP glass partially crystallizes, releasing AlPO4 and β-Na6Al3(P2O7)3. In glass with La2O3, the main phases are AlPO4 and monazite. In the sample with Ce2O3, no crystalline phases other than AlPO4 were found. The introduction of CeO2 leads to the appearance of cerianite, monazite, and Na3Al2(PO4)3. Annealed samples with Pr and Nd oxides contain AlPO4, monazite, and traces of Na3Al2(PO4)3. Na3REE(PO4)2 orthophosphate was found in samples with Sm, Eu, and Gd. The content of REE oxides in the glass does not affect the leaching of Na, Al, Fe, or P. The REE leaching rates are 10−7–10−6 g/(cm2 × day). Tempered SAIP glasses are less soluble than SAP, and annealed samples are more stable than tempered ones due to the inclusion of REEs in monazite. After annealing, the yield of Na and P from SAP glass increases by 2- to 15-fold, but the leaching rates of Al, Fe, and REEs do not change.
Glass–ceramics containing up to 20 wt.% oxides (REEs) were obtained [94] by melting at 1250 °C and quenching or slow cooling to room temperature. Samples produced by heating SAP glass are composed of glass, AlPO4, and monazite. Iron-containing glass–ceramics consist of glass, monazite, and Na-Al-Fe phosphate. Reducing the cooling rate of glass leads to the appearance of more crystalline phases. The leaching rates of Na, Al, Fe, and P are 10−5–10−7 g/(cm2 × day), and below 10−5 g/(cm2 × day) for REEs.
The structure and properties of two Na-Al-(Fe)-P glasses with a 10 wt.% mixture of (La0.57Ce0.36Nd0.93)O3 were studied [97]. Samples were prepared by melting for 1 h at 1200 °C and quenching. The solubility of REEs in glasses, in wt.%, are as follows: 0.8–0.9 La2O3, 0.7–0.8 CeO2, 1.9–2.0 Nd2O3, for a total of 3.5 wt.%. The main proportion of REEs is found in monazite, which forms grains up to 10 microns in size. The Kp values between monazite and glass are equal to 17–20 (Ce), 17–21 (La), and 13–17 (Nd).
The possibility of producing a Fe-P glass–ceramic with monazite (Ce,La,Nd)PO4 by melting–quenching was tested [91]. The mixture, loaded with up to 15 mol.% of REE imitator, was melted for 2 h at 1200 °C, and the melt was quenched into glass and annealed for 1 h at 450 °C to relieve stress. Differential thermal analysis was used to determine (Table 10) the glass transition temperatures (Tg), onset of crystallization (Tr), and melting temperatures (Tm). The Tg and Tr values firstly increase with an increase in the content of the actinide simulator (REE) and then decrease due to the formation of monazite (Ce,La,Nd)PO4 followed by depletion of the residual glass in these elements. Thermal resistance of glass to crystallization correlates with the value of the ratio (Tr–Tg):(Tm–Tr); the higher it is, the more resistant the glass when heated. The resistance of glass to crystallization is also characterized by the ratio α = Tr/Tm; at α ≥ 0.6, high thermal resistance is noted. The resulting glass–ceramics are resistant to corrosion, and their leaching rate in water is ~10−4 g m−2 day−1. The optimal waste content in the matrix was determined to be 15 mol.% (22 wt.%).

8. Synthesis of Glass–Ceramics by Heating a Mixture of Glass and Components of the Crystalline Phases

To obtain glass–ceramics, it has been proposed to sinter a mixture of glass, oxides of the crystalline phases, and waste at atmospheric [32,99,100] or elevated pressure, known as the hot-pressing method [45,49,101,102]. In this way, glass–ceramics with zirconolite (sintering for 0.5–1.5 h at 1000–1250 °C) and monazite were synthesized. In the second case, lanthanum metaphosphate glass was obtained by melting for 45 min at 1230 °C, which, after grinding, was pressed and sintered for 4 h at 1200 °C to obtain monazite glass–ceramics. In another variant, a mixture of HLW simulant and iron phosphate glass was pressed at a pressure of 30 MPa and sintered for 2–5 h at 500 °C to form glass–ceramics with monazite. As already noted, it is possible to select such contents of REE–MA waste fractions and block sizes so that, due to radiogenic heat, the temperature of the matrix is at 600–700 °C for several years. However, these methods require additional stages, which complicates the synthesis of highly radioactive materials and makes this process difficult to apply in the actual industrial production of these matrices.
Therefore, the main attention is paid to simpler methods of obtaining matrices by partial crystallization of the melt or by heating the tempered glass. This approach can use equipment that is already being used for the vitrification of HLW [90,91]. It is an optimal method for phosphate glass–ceramics with monazite due to its lower melting point compared with B-Si glasses.

9. Assessment of the Possibility for Deep-Borehole Disposal of REE–Actinide Matrices

The fight against climate change and energy geopolitics are leading to an increasing number of countries seeking to develop nuclear power. In this regard, the management of HLW is becoming increasingly urgent. In the coming years, it is planned to commission an SNF mine-type disposal facility in Finland, designed for 9000 tons of spent fuel.
However, long periods (30–50 or more years) from the start of searching for sites for a mine disposal facility to the start of its construction [103] make the alternative option of HLW disposal in deep boreholes attractive [104,105,106,107,108,109,110,111]. The first nuclear-waste storage facility in deep boreholes can be created within five to ten years [105], which is an order of magnitude shorter than the construction time of traditional mine storage facilities.
Deep underground HLW disposal facilities can be of the mine type with a depth of about 0.5 km, in the form of vertical boreholes (3–5 km), or with their horizontal ending at a depth of 1–2 km. The advantages of placing waste packages in boreholes are noted for small-diameter containers with various HLW, including long-lived radionuclides. For deep-borehole disposal, waste forms with the following characteristics are preferred: low volume and package size, high specific activity, and/or high concentration of long-lived radionuclides, including fissile materials. In addition to better isolation due to a greater disposal depth than a shaft option, the borehole repository has the advantages of (i) cost efficiency; (ii) faster pre-operation, production and closure phases; and (iii) modularity. Of particular concern is the possibility of migration of long-lived actinides (Am, Pu, Np) in the form of colloids. Deep waters are Na-Ca brines with high ionic strength; this will cause colloids to aggregate into larger particles, settle out, and be retained by rocks, limiting the migration of radionuclides. In a deep-borehole repository, the temperature of the HLW matrix can reach significant values due to radiogenic heat and the natural thermal background of the host rocks due to the geothermal gradient.
The preferred types of HLW forms for placement into deep wells [108,109,110,111,112,113,114,115] are waste with high heat release, including chlorides and fluorides of Cs and Sr; cesium–strontium fractions; actinide-containing waste in a stable matrix, including Pu-containing waste; and the REE–actinide fraction of HLW. The advantages of the borehole disposal of HLW over mines include the following: long-term safety due to the large burial depth and extremely low solubility of actinides under reducing conditions; economical access to rocks with high insulating properties; lower infrastructure requirements and significantly smaller surface area; shorter terms of construction and placement of HLW; significantly lower cost; the possibility of creating waste in close proximity to the place of waste generation; extremely low probability of unauthorized access to radioactive materials; minimal control after HLW placement and storage-facility closure. In addition, the high salinity of the deep waters complicates the formation of a colloidal form of radionuclide transfer and the development of convection due to the heat released by HLW [115]. Field-research programs for deep-borehole disposal projects have been repeatedly discussed in the literature [111].
Promising matrices (waste forms) of REEs and MAs are considered to be the glasses of B-Al-Si, Ca-B-Al, Al-P, and Fe-P composition, as well as glass–ceramics with crystalline phases of zirconolite, britholite, and monazite [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,49,59,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,112,113,114,115,116,117,118,119]. The content of the REE-MA fraction in such matrices can reach 30–50 wt.%, and very deep wells with placement of the HLW forms at a depth of 3 to 5 km are optimal for their disposal.

10. Requirements for the Selection of HLW Matrices and Their Fractions

The nuclear waste forms must have certain properties and meet a number of criteria [46,61,112,120,121,122,123,124,125,126]; namely, high stability and tolerance to radiation effects, including transfer into new elements, ballistic effects, ion and electronic excitation, gas bubble and volumetric swelling effects, depending on the types of radionuclides; and a high waste loading (usually 20–35 wt.%) to reduce repository volumes. Waste forms must be chemically flexible and contain a mixture of radionuclides and associated waste elements. Matrices must be resistant to dissolution in water to minimize the release of radiotoxic substances, especially long-lived actinides. The manufacturing process must be simple and feasible using proven methods and without the use of vacuum or a special atmosphere. It is advisable to have natural mineral analogues in order to be confident in the reliability of the forecast of their long-term behavior. All these requirements are met by glass–ceramics with zirconolite, britholite, and monazite phases.

11. Use of Self-Heating of HLW for Deep Disposal

There are other proposals for using the heat of radioactive decay; for example, disposing highly radioactive waste by melting rocks and plunging them deep into the Earth’s core and even mantle [127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145]. The source of radiogenic heat capable of heating the surrounding rocks above 1200 °C may be spherical or cylindrical containers with a diameter of 0.1 m to 1 m containing actinides [137], HLW or SNF [138,139]; individual radioisotopes, for example 14 kg of 137Cs or 0.5 kg 60Co with a total activity of 3.85 × 1018 Bq [136]; or activity at the surface of 1017 Bq/m2 [140]. The need to use containers made of a very refractory and expensive material (tungsten) is the least of the problems that arise with this approach. In another scenario, a container with HLW or SNF is placed into a pre-drilled hole filled with material with a lower melting point than the surrounding rocks [141,142,143,144,145,146], which facilitates their dipping. However, such proposals have not gone beyond theoretical calculations, which should also account for changes within the waste form [147,148].

12. Conclusions

The choice of glass–ceramics for the immobilization of HLW was proposed in the 1970s—20 years later than glass and ceramic forms [32,34,45,49]. Their formation occurs as a result of the controlled crystallization of a melt or glass in one or several stages. The distinction between uncontrolled (undesirable) and controlled (desirable) glass crystallization with an emphasis on HLW immobilization at high waste loadings has been reviewed [32]. To immobilize HLW, it is desirable to minimize the number of processing steps, so glass–ceramics are usually produced from a melt, where the nucleation and growth of crystals occur simultaneously during cooling or at an intermediate stage of exposure, rather than as a result of quenching the melt into glass and reheating it, as in the process of producing inactive commercial glass–ceramics. In the case of controlled nucleation and growth, the starting point is a single-phase melt or tempered glass, where crystals are formed by cooling from the melt or heating from the glass. If crystallization will occur upon cooling, it is important to design a system in which the nucleation and crystal-growth temperatures overlap. In another version of the synthesis, crystals are formed at high temperature—for example, during hot isostatic pressing (HIP) or high-temperature sintering—and its starting materials are usually solid mixtures of HLW powders and glass, or glass-forming oxides. The final product consists of crystalline phases embedded in intergranular glass, as demonstrated in glass–ceramics with U- or Pu-containing pyrochlore, apatite for wastes with high fluorine or chlorine content, and zirconolite with Pu. To obtain monazite glass–ceramics, glass powder from lanthanum metaphosphate is mixed with an HLW simulant (oxides of Nd, Zr, La, Ce, Fe, Mo) and heated to 1200 °C, then cooled in a switched-off furnace, obtaining monazite and the ZrP2O7 phase in the residual glass [84]. To suppress the formation of undesirable phases of ZrP2O7 or FePO4, it was proposed to add B2O3 or TiO2 in the initial batch [148,149,150,151].
To select the optimal regimes for glass–ceramic formation, data on melting, transition, and glass crystallization temperatures have great importance. For known HLW vitreous matrices, the glass transition temperatures range from 400 to 700 °C; the minimum values are characteristic of phosphate compositions, and the maximum values are typical of boron–aluminosilicate glasses. Since the growth of phase grains in glass begins when they are heated to 100–150 °C above the Tg, intensive crystallization of glass matrices with the formation of stable phases of radionuclides will occur in the range of 500–800 °C, which corresponds to the calculated temperatures under the influence of radiogenic heat. The heating was calculated for a matrix of the REE-MA fraction consisting of 95% REEs (all isotopes were considered stable) and 5% MAs, of which 3.5% was 241Am (T1/2 = 432 years), 1% was 243Am (7370 years), 0.45% was 244Cm (18 years), and 0.05 was 245Cm (8500 years). Taking into account the heat released by isotopes (W/kg) is as follows: 241Am—115, 243Am—6, 244Cm—2842, 245Cm—6; the initial heat released by the matrix is mainly due to 244Cm (~78%) and 241Am (~22%). Although the bulk of REEs are found in SNF and HLW ) as stable isotopes, some of them are radioactive with half-lives of 0.8–9 years (144Ce, 147Pm, 154,155Eu) or 90 years (151Sm). During the first few years, the contribution of REE decay to the heat released by SNF exceeds not only the share of heat release by actinides but also of such fission products as 90Sr and 137Cs. Over time, heat generation due to the decay of REEs decreases, and after 10 years, it can be neglected. Therefore, the decay of REEs will affect the heating of the REE-MA matrix for a rather short time. When the matrix contains 30 wt.% of an REE-MA fraction, the temperature increase due to the decay of REEs in a year is estimated at 100 °C, and after 10 years, it will be less than 40 °C. Storing SNF before reprocessing will reduce the content and heat release by the shortest-lived isotopes of fission products, which include REEs and platinum-group metals. So, in 10 years, the heat released by SNF due to the decay of trans-uranium actinides and alkali and alkaline earth metals will decrease by 2- to 3-fold; for REEs, it will decrease by 50- to 60-fold, and for noble metals, by almost 300-fold. Before reprocessing, SNF is stored for 5–10 years and the effect of REE decay on the temperature can be neglected.
However, in the case of an ultra-deep borehole repository, it is necessary to take into account that the temperature at depths of 3–5 km increases by 90–150 °C due to the geothermal gradient. In combination with radiogenic heat, the temperature of the matrix with waste can be 500–800 °C for decades, which will cause crystallization of the glass with the appearance of stable phases of REEs and MAs—monazite in the phosphate system and zirconolite and britholite in the borosilicate system. The reality of this process is confirmed by data from numerous experiments on the partial crystallization of glasses with REEs and actinides. The distribution coefficients of REEs and actinides between crystals and residual glass are maximum for monazite in the phosphate system and britholite in silicate compositions and range from 10 to 30, which causes a decrease in their content in the residual glass during crystallization. The Kp of the elements between zirconolite and glass are an order of magnitude lower, so a significant part of the elements will be in the residual glass and, at exposure to water, can be leached from the glass–crystalline matrix.
Advantages of boreholes for high-level heat-generating waste disposal, including that of the REE-MA fraction, are as follows [106,110,115,121]: long-term safety due to the large depth of disposal; economical access to rocks with high insulating properties; low infrastructure requirements and small area of ground structures; short construction time, waste loading and sealing of the storage facility; the possibility of creating waste near the site of waste generation; low probability of unauthorized access; minimal control after the facility is closed; high water salinity, which prevents convection due to heat released by HLW; low solubility of actinides under reducing conditions; and instability of the colloids in brines.
Disadvantages and limitations associated with high temperatures include increased corrosion rates of the engineered barriers—the container, the bentonite buffer, and the matrix itself. It is critical to understand the thermodynamics and kinetics of phase separation and crystallization in HLW glasses in order to design glass–ceramics with the desired crystalline phase [49]. A major challenge in the design and development of waste glasses/glass–ceramics is their compositional complexity. In general, glass–ceramics with HLW include more than 30 different elements, including alkali, alkaline earth, multivalent transition and noble metals, as well as rare-earth elements and actinides, glass-forming oxides (Si, B, P, Al) (which complicates the nucleation and growth of various phases), and undissolved noble metals. TiO2 and ZrO2 in glass melts act as nucleating agents, so it is important to understand the distribution of elements in the crystalline phases, their size and morphology, as well as the chemistry of the residual glassy phase, in order to select and create durable glass–ceramic waste forms. Thus, traditional methods for studying the nucleation and growth from glasses may not be applicable to potentially heterogeneous industrial glass–ceramic systems that are produced in large ingots and thus cooled with different thermal profiles at different locations.
Due attention must be paid to the chemical resistance of glass–ceramics toward nuclear waste. Its stability in a geological repository is determined by the crystalline phases and chemical composition of the residual glass phase, as well as by the geochemical conditions of the surrounding rocks. Therefore, it is important to study the compositional dependence of crystallization behavior and its impact on the durability of the waste form. There is high interest in glass–ceramic forms of problematic waste with a high content of poorly soluble components, such as heat-generating radionuclides, for waste from new types of nuclear reactors, as well as in the case of a higher waste load than traditional borosilicate glasses. For the immobilization of nuclear waste and the long-term thermal and chemical stability of HLW forms, crystallization can be useful, while the uniformity of structure during vitrification of HLW is considered a requirement for glassy materials [152,153,154]. Additional experiments and theoretical calculations are needed to test the possibility of crystallizing glass matrices under the influence of radiogenic heat and transforming them into stable glass–crystalline and crystalline forms of actinides, including the REE–MA fraction.
The concept of applying radiogenic heat to transform glass into glass–crystalline matrices has undeniable advantages over other approaches to immobilize actinide waste. One of the most important arguments in favor of this decision is the possibility of using well-developed routes for vitrification of the HLW using ceramic melters or inductively heated crucibles [16,32,33,34,35,36,37,38,39,40,41,45,48,49,80,81,95,116,126,155]. Both the technologies have been successively applied for many years at an industrial scale at radiochemical plants around the world [156,157,158].

Author Contributions

Conceptualization, S.V.Y.; methodology, S.V.Y.; formal analysis, S.V.Y., M.I.O. and V.I.M.; investigation, S.V.Y., M.I.O. and V.I.M.; resources, S.V.Y.; data curation, S.V.Y., M.I.O. and V.I.M.; writing—original draft preparation, S.V.Y.; writing—review and editing, S.V.Y., M.I.O. and V.I.M. All authors have read and agreed to the published version of the manuscript.

Funding

The study was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation within the framework of a state assignment for the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry of the Russian Academy of Sciences.

Data Availability Statement

All data is available within the manuscript.

Acknowledgments

The authors are grateful to the reviewers for comments and reviews.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Contribution of decay of radionuclide groups to the heat released by spent fuel from a PWR reactor (burnup 60 GW × d/t) over 1 year to 109 years (see open access Refs. [18,19]).
Figure 1. Contribution of decay of radionuclide groups to the heat released by spent fuel from a PWR reactor (burnup 60 GW × d/t) over 1 year to 109 years (see open access Refs. [18,19]).
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Figure 2. Time-dependence of heat release (a) and temperature (b) of a glass block with a diameter of 0.2 m with 30 wt.% REE-MA fraction (1—center; 2—surface). Spent fuel was stored for 1 year.
Figure 2. Time-dependence of heat release (a) and temperature (b) of a glass block with a diameter of 0.2 m with 30 wt.% REE-MA fraction (1—center; 2—surface). Spent fuel was stored for 1 year.
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Figure 3. Heat release (green curve) and temperature of the glass matrix with 30 wt.% REE on the axis (1) and surface (2) of a block with a diameter of 0.2 m in the time interval from 0.001 year to 100 years (a) and from 1 year to 100 years (b). SNF was stored for 1 year before reprocessing.
Figure 3. Heat release (green curve) and temperature of the glass matrix with 30 wt.% REE on the axis (1) and surface (2) of a block with a diameter of 0.2 m in the time interval from 0.001 year to 100 years (a) and from 1 year to 100 years (b). SNF was stored for 1 year before reprocessing.
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Figure 4. Change in glass viscosity with temperature (Pa × s); Tg—glass transition temperature. The insets show two types of the nucleation (I) and crystal growth (U) rates in arbitrary units [48,49].
Figure 4. Change in glass viscosity with temperature (Pa × s); Tg—glass transition temperature. The insets show two types of the nucleation (I) and crystal growth (U) rates in arbitrary units [48,49].
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Figure 5. SEM images of the glass–ceramics (a) with britholite (1) in residual glass (2), and (b) a sample after crystallization of the vitreous phase with britholite (light) in britholite–CaSiO3 aggregates.
Figure 5. SEM images of the glass–ceramics (a) with britholite (1) in residual glass (2), and (b) a sample after crystallization of the vitreous phase with britholite (light) in britholite–CaSiO3 aggregates.
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Figure 6. AF-1 glass after heating for 4 h at 500 °C. Images in an optical microscope, with crossed nicols, at a magnification of 60 (a) and 200 (b) (glass—dark; spherulites of Na-Al-P phases—light; monazite in the center—yellow). SEM images (c,d) of phosphate spherulites (dark gray) around monazite grains (white) in residual glass (light gray). Black—pores.
Figure 6. AF-1 glass after heating for 4 h at 500 °C. Images in an optical microscope, with crossed nicols, at a magnification of 60 (a) and 200 (b) (glass—dark; spherulites of Na-Al-P phases—light; monazite in the center—yellow). SEM images (c,d) of phosphate spherulites (dark gray) around monazite grains (white) in residual glass (light gray). Black—pores.
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Table 1. Isotopes of REEs and MAs in SNF, with a burnup 33 GW × d/t, after 3 years of storage [16,17].
Table 1. Isotopes of REEs and MAs in SNF, with a burnup 33 GW × d/t, after 3 years of storage [16,17].
ElementTotal Content, g/t SNFRadionuclideContent, g/tHalf-Life, T1/2
La1205--Stable
Ce2352144Ce23284 days
Pr1109--Stable
Nd 14000--Considered Stable
Pm86147Pm862.6 years
Sm 1777151Sm1693 years
Eu133 g/t,
including:		154Eu208.6 years
155Eu124.8 years
Gd76--Stable
Am369 g/t,
including:		241Am290433 years
243Am797370 years
Cm20 g/t,
including:		243Cm0.229 years
244Cm18.318 years
245Cm1.08500 years
246Cm0.14760 years
1 REEs with a very long half-life: 144Nd (2.38 × 1015 years), 150Nd (7 × 1018 years), 147Sm (1.06 × 1011 years).
Table 2. The contribution of radionuclides to the heat released by spent fuel from a PWR reactor with a burn-up of 40 (a) and 60 (b) GW × d/t, in the range from 1 year to 500 years [18,19].
Table 2. The contribution of radionuclides to the heat released by spent fuel from a PWR reactor with a burn-up of 40 (a) and 60 (b) GW × d/t, in the range from 1 year to 500 years [18,19].
SNF Element GroupsHeat Release, W/t SNF, after 1–500 Years of SNF Storage
110305070100300500
Cs/Sr/Ba/Rb2765 (a)/
4608 (b)		1054/
1576		566/824354/
516		222/
323		110/
160		1/10
Ag/Pd/Ru/Rh2752/344711/14000000
La/Ce/Pr/Nd/Pm/Sm/Eu3593/384364/10910/172/30000
Np/Pu/Am/Cm/Bk819/
1515		348/
785		332/
613		309/
516		287/
449		258/
381		159/
199		116/
139
Others515/52215/212/31/1<0.1<0.1<0.1<0.1
Total10,444/
13,936		1492/
2505		910/
1458		666/
1036		509/
773		368/
541		160/
201		116/
139
Table 3. Content (a) in g/t of the main REE and actinide elements in SNF of light water reactors and their heat release (b) in W/t based on the fuel burn-up and storage time [10].
Table 3. Content (a) in g/t of the main REE and actinide elements in SNF of light water reactors and their heat release (b) in W/t based on the fuel burn-up and storage time [10].
ElementAfter 5 Years of SNF StorageAfter 30 Years of SNF Storage
45 GW × d/t60 GW × d/t45 GW × d/t60 GW × d/t
(a)(b)(a)(b)(a)(b)(a)(b)
Gd1500 (Stable)3100 (Stable)1800 (Stable)3460 (Stable)
Eu1906026090170823012
Sm 110600 (Stable)13700 (Stable)11200 (Stable)14300 (Stable)
Pm63216221- 3---
Ce3210104230103210Stable4220Stable
Pr154011420101131540Stable2010Stable
Nd 25570Stable7310Stable5570Stable7310Stable
La1670Stable2190Stable1670Stable2190Stable
Σ REE13,45320517,74223413,460817,73612
U941,0000.06923,0000.06941,0000.06923,0000.06
Pu11,20016412,60028310,20013811,500236
Np5700.017800.025700.017800.02
Am510477405813801461780178
Cm3388113292143450112
Am + Cm, MA54313585335013941801830290
MA share 44%40%5%60%9%96%9%96%
1 There is a long-lived 147Sm (T1/2 = 1.06 × 1011 years) and a small amount of 151Sm (T1/2 = 90 years). 2 Long-lived 144Nd (T1/2 = 2.4 × 1015 years) and 150Nd (T1/2 = 7 × 1018 years) can be considered stable. 3 absent. 4 It is the share of actinides in the REE–MA fraction and their contribution to heat release.
Table 4. Changes in the Am and Cm (MA) nuclide composition in SNF (burnup 45 GW × d/t) and their relative content in the hypothetical REE–MA fraction over storage time [20].
Table 4. Changes in the Am and Cm (MA) nuclide composition in SNF (burnup 45 GW × d/t) and their relative content in the hypothetical REE–MA fraction over storage time [20].
Nuclide, g/t SNFAfter 1 YearAfter 5 YearsAfter 30 Years
241Am 11354071272
243Am105105105
Total Am2405121377
242Cm3.80.1<0.01
244Cm35.330.311.6
245Cm2.22.22.2
Total Cm, including 243Cm41.933.014.4
Cm/(Am + Cm), %1561
MA/(REE + MA), %249
1 Content of 241Am rises due to the decay of short-lived 241Pu (T½ = 14.4 years).
Table 5. Properties of Am and Cm in the SNF (burn-up 50 GW day/t) after 6 years of storage [21].
Table 5. Properties of Am and Cm in the SNF (burn-up 50 GW day/t) after 6 years of storage [21].
Radionuclide
(T1/2, Years)		Content,
wt.%		Daughter Nuclide,
(T1/2, Years)		Type and Probability
of Nuclides Decay		Heat Release,
W/kg
241Am (433)63.8237Np (2.14 × 106)α (≈1.0), SF 1 (3.77 × 10−12)114.7
243Am (7300)25.4239Pu (2.41 × 104)α (≈1.0), SF (3.7 × 10−11)6.4
243Cm (29)0.1239Pu (2.41 × 104)α (0.9976), β+ (0.0024)1860.7
244Cm (18)9.8240Pu (6537)α (≈1.0), SF (1.35 × 10−6)2841.8
245Cm (8500)0.8241Pu (14.4)α (1.0)5.8
246Cm (4760)0.1242Pu (3.76 × 105)α (≈1.0), SF (2.61 × 10−4)10.2
1 Spontaneous fission.
Table 6. Examples of the use of elements—simulating radionuclides in HLW matrices.
Table 6. Examples of the use of elements—simulating radionuclides in HLW matrices.
RadionuclideRadionuclide Simulants: From the More Similar to the Less Similar Elements
Short-LivedLong-LivedStable Isotope of ElementOther Simulants
Np- 1U, 232ThDoes not existCePr
238Pu-U, 232ThDoes not existCeNd
239Pu238PuU, 232ThDoes not existCeNd
Am, Cm244Cm-Does not existNdSm
137Cs134Cs-Natural isotopes
(133Cs, 127I, 59Co)
or their mixtures
(86–88Sr, 90–96Zr)		Ba 2-
129I----
60Co----
90Sr--Zr 2-
93Zr----
99Tc--Does not existReMo
1 No data. 2 Simulants of decay products 137Cs (137Cs ⟶ 137Ba) or 90Sr (90Sr ⟶ 90Y ⟶ 90Zr).
Table 7. Thermophysical properties of the actinide matrix, sorption buffer, and host rocks [31,45].
Table 7. Thermophysical properties of the actinide matrix, sorption buffer, and host rocks [31,45].
Engineering Barrier Material, Waste-Hosting RockDensity, kg/m3Specific Heat Capacity, J/(kg·K)Thermal Conductivity, W/(m·K)
Glass with 30% REE-MA fraction30009001.1
Bentonite buffer layers170010000.8
Granite, granite gneiss28509651.5
Table 8. Dependence of the volumetric heat-release density of glass with 30 wt.% REE on time.
Table 8. Dependence of the volumetric heat-release density of glass with 30 wt.% REE on time.
Storage Time, Years1510305070
Heat release, kW/m321612.33.80.60.10
Table 9. Composition (wt.%) of glass (1), monazite (2), and spherulites (3) in the AF-1 sample.
Table 9. Composition (wt.%) of glass (1), monazite (2), and spherulites (3) in the AF-1 sample.
PhaseNa2OAl2O3P2O5Me*O2-xSrOZrO2MoO3Cs2OBaO(Ln)**2O3
(1)24.017.247.80.80.82.91.40.90.82.4
(2)<d.l. 1<d.l.27.9<d.l.0.5<d.l.<d.l.0.60.372.1
(3)25.519.151.31.4<d.l<d.l.0.40.50.60.6
1 Me*—Cr, Mn, Fe, Ni; Ln**—La, Ce, Nd; d.l—detection limit (0.3–0.5 wt.%).
Table 10. Values of Tg, Tr, TL, KH = (Tr − Tg):(TL − Tr), ΔTrg = (Tr − Tg), α = Tr:Tg for the waste forms.
Table 10. Values of Tg, Tr, TL, KH = (Tr − Tg):(TL − Tr), ΔTrg = (Tr − Tg), α = Tr:Tg for the waste forms.
Samples0 HLW5 mol.% HLW10 mol.% HLW15 mol.% HLW
Tg ± 1 (K)778790773770
Tr ± 1 (K)861864855834
TL ± 1 (K)1245124612551264
ΔTrg (K)83748264
KH0.2160.1940.2050.149
α0.6920.6930.6810.660
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Yudintsev, S.V.; Ojovan, M.I.; Malkovsky, V.I. Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste. J. Compos. Sci. 2024, 8, 70. https://doi.org/10.3390/jcs8020070

AMA Style

Yudintsev SV, Ojovan MI, Malkovsky VI. Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste. Journal of Composites Science. 2024; 8(2):70. https://doi.org/10.3390/jcs8020070

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Yudintsev, Sergey V., Michael I. Ojovan, and Victor I. Malkovsky. 2024. "Thermal Effects and Glass Crystallization in Composite Matrices for Immobilization of the Rare-Earth Element–Minor Actinide Fraction of High-Level Radioactive Waste" Journal of Composites Science 8, no. 2: 70. https://doi.org/10.3390/jcs8020070

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