Radionuclide disposal using the pyrochlore supergroup of minerals as a host matrix-A review

Effect of PFDS on the immobilization of Cs+ by metakaolin-based geopolymers in complex environments

Metakaolin-based geopolymers are very promising materials for improving the safety of low and intermediate level radioactive waste disposal, with respect to ordinary Portland cement, due to their excellent immobilization performance for Cs+ and superior chemical stability. However, their application is limited by the fact that the leaching behavior of Cs+ is susceptible to the presence of other ions in the environment. Here, we propose a way to modify a geopolymer using perfluorodecyltriethoxysilane (PDFS), successfully reducing the leaching rate of Cs+ in the presence of multiple competitive cations due to blocking the diffusion of water. The leachability index of the modified samples in deionized water and highly concentrated saline water reached 11.0 and 8.0, respectively. The reaction mechanism between PDFS and geopolymers was systematically investigated by characterizing the microstructure and chemical bonding of the material. This work provides a facile and successful approach to improve the immobilization of Cs ions by geopolymers in real complex environments, and it could be extended to further improve the reliability of geopolymers used in a range of applications.

Synthesis and feasibility studies of doping U at Ti site of Y2Ti2O7 as a radioactive waste immobilization matrix

In pursuit of clean and green nuclear energy one of the major challenges is to effectively immobilize the nuclear waste. In this context A2B2O7 type pyrochlore owing to its structural flexibility, ability to accommodate ions at both A/B-sites and high radiation tolerance has demonstrated excellent capability to store highly radioactive actinide ions. To fill the major gap area of actinide doping at the B site we have taken up the challenge of doping uranium ions at the Ti site of Y2Ti2O7 type pyrochlore. An yttria titanate (Y2Ti2-xUxO7; x = 0.05, 0.075, 0.1, 0.2, and 0.3) based matrix with uranium doped at the Ti site was synthesized using a simple gel combustion route under an air atmosphere. Rietveld refined X-ray diffraction (XRD) demonstrated that Y2Ti2O7 can accommodate U up to 5 mol% in the Ti site without any phase separation, which was further confirmed using Raman spectroscopy. Y2Ti2O7 based matrices are found to be radiation stable up to 1000 kGy and at the same time they are moderately thermally stable and on a par with the values reported for pyrochlores. Uranium in Y2Ti2O7 stabilizes in +6 oxidation state in the form of uranyl ion distributed near and far off from titanium vacancies with distinct excited state lifetime. This work could provide a smart and strategic way for selecting a suitable advanced ceramic matrix for immobilization of high level waste with additional and important information on solubility limit, actinide speciation, radiation/thermal stability, actinide concentration, etc.

Crystal chemical design, synthesis and characterisation of U(IV)-dominant betafite phases for actinide immobilisation

Crystal chemical design principles were applied to synthesise novel U⁴⁺ dominant and titanium excess betafite phases Ca_1.15(5)U_0.56(4)Zr_0.17(2)Ti_2.19(2)O₇ and Ca_1.10(4)U_0.68(4)Zr_0.15(3)Ti_2.12(2)O₇, in high yield (85-95 wt%), and ceramic density reaching 99% of theoretical. Substitution of Ti on the A-site of the pyrochlore structure, in excess of full B-site occupancy, enabled the radius ratio (r_A/r_B = 1.69) to be tuned into the pyrochlore stability field, approximately 1.48 ≲ r_A/r_B ≲ 1.78, in contrast to the archetype composition CaUTi₂O₇ (r_A/r_B = 1.75). U L₃-edge XANES and U 4f_7/2 and U 4f_5/2 XPS data evidenced U⁴⁺ as the dominant speciation, consistent with the determined chemical compositions. The new betafite phases, and further analysis reported herein, point to a wider family of actinide betafite pyrochlores that could be stabilised by application of the underlying crystal chemical principle applied here.

Synthesis and Investigation of the Properties of Biphasic Hybrid Composites Based on Bentonite, Copper Hexacyanoferrate, Acrylamide and Acrylic Acid Hydrogel

This article presents a study of the synthesis and characterization of new biphasic hybrid composite materials consisting of intercalated complexes (ICC) of natural mineral bentonite with copper hexaferrocyanide (phase I), which are incorporated into the bulk of the polymer matrix (phase II). It has been established that the sequential modification of bentonite with copper hexaferrocyanide and introduction of acrylamide and acrylic acid cross-linked copolymers into its volume by means of in situ polymerization promote the formation of a heterogeneous porous structure in the resulting hybrid material. The sorption abilities of prepared hybrid composite toward radionuclides of liquid radioactive waste (LRW) have been studied, and the mechanism for binding radionuclide metal ions with the components of the hybrid composition have been described.

Immobilization of NPP evaporator bottom high salt-bearing liquid radioactive waste into struvite-based phosphate matrices

The high salt-bearing liquid radioactive waste (evaporator bottoms, EB) makes up the most voluminous NPP waste and needs solidification. In the paper presented, we introduce a novel formation process study of the struvite-based phosphate matrices ((K, NH₄)MgPO₄·6H₂O) and the developed phosphate matrix compositions for the solidification of high salt-bearing solutions. The solutions simulate the EB of nuclear power plants with pressurized water reactors (NPP PWR). The effect of the EB's composition and salt content on the matrices' mechanical strength was investigated. The cesium-selective nickel-potassium ferrocyanide sorbent or 10-20% of MgO over the reaction stoichiometry, introduced at the matrix synthesis stage, allowed the production of matrices with the average ¹³⁷Сs leach rate of less than 10^-3 g cm^-2 day^-1 and the mechanical strength over 5 MPa. The matrices obtained completely satisfied the cemented radioactive waste requirements and contained up to 17-17.5 wt% of salts, which was 1.7-2.5 times higher compared to the Portland cement-based matrices.

High-entropy A2B2O7-type oxide ceramics: A potential immobilising matrix for high-level radioactive waste

The sustainable development of civil nuclear energy requires the fabrication of the durable nuclear wasteforms, in particular for high-level radioactive waste, which involves the design of the composition and microstructure. Herein, we demonstrated that high-entropy ceramics (Eu_1-xGd_x)₂(Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ce_0.2)₂O₇ are the potential candidate as immobilizing hosts for high-level radioactive waste. The static aqueous leaching test indicates that the normalized leaching rates for the simulated radionuclides Ce (LR_Ce) and Gd (LR_Gd) in as-prepared high-entropy ceramics are approximately 10^-6~10^-8 g·m^-2·d^-1 after 42 days testing, much lower than those reported values in doped-Gd₂Zr₂O₇ (10^-6~10^-3 g·m^-2·d^-1). The excellent chemical durability is mainly due to the synergistic effects of the compositional complexity and severe lattice distortion. Compared to their ternary oxides, the low oxygen vacancy concentration slows down the migration and diffusion of cations. Moreover, the lattice distortion increases the lattice potential energy, also inhibiting the migration of cations. This study provides a strategy for the development and application of high-entropy ceramics as the wasteforms.

Localised solution environments drive radionuclide fractionation in uraninite

We explore the role of various solution environments - chloride brines, acid mine drainage (sulfate) and groundwater (carbonate), as well as pore pressure in producing secular disequilibrium among the various radionuclides (RN) in the U-decay series upon leaching of uraninite - the most abundant U-ore and a widespread accessory mineral in U-rich rocks. We observed that the end products of the U-decay chain, ²⁰⁶Pb and ²⁰⁷Pb, exist primarily at the surface/edges of grains or within large pores in the uraninite. In contrast, the intermediate daughters ²²⁶Ra, ²¹⁰Pb, ²¹⁰Po, and ^234/230Th, exist primarily within the bulk of uraninite, requiring breakdown by leaching for subsequent mobility to occur. Overall, pore pressure had little effect on RN mobility, with solution environment being the primary factor in creating significant mobility and disequilibrium among the RN, as it drives the initial breakdown of uraninite and influences the subsequent differential solubility of individual RNs. This was particularly the case for carbonate-bearing fluids, leading to significant fractionation of the various daughter RN arising from variable complexation and sorption phenomena. Understanding the geochemical behaviour of the RN in the U-decay series is important for predicting and managing the risks associated with RN in both environmental (acid-mine drainage) and engineered (metallurgical extraction) processes. Effective modelling of long-term RN behaviour should incorporate this strong relative fractionation caused by contrasting geochemical behaviour of individual RN during and after their release into the water from uraninite and subsequent interaction with the surrounding aquifer host rocks.

Role of cation size on swelling pressure and free energy of mica pores

The ion exchange capacity of clay plays an important role in many industrial applications ranging from radioactive waste disposal to cosmetics. However, swelling or shrinking of clay platelets due to water and ions adsorption in the interstitial zone is also a well-known phenomenon. For their applications, it is crucial to understand the stability of these layered materials, especially after exchange of interstitial ions with surrounding ions having different properties. Here, we probed the role of cation size on swelling pressure and free energy profile. We used molecular simulations to investigate the stability of mica pore, having K⁺, Rb⁺, and Cs⁺ ions. We performed a series of grand canonical Monte Carlo simulations at various pore widths. We probed water adsorption in mica pores from which disjoining pressure, grand potential (swelling free energy), and structural properties of confined water and ions were calculated. While the behavior of these three systems is similar qualitatively because of similar hydration properties of ions, significant differences are observed at the quantitative level due to changes in the hydration structure of cations. The global minimum in swelling free energy is found to be at the smaller pore widths (first minimum) for Rb- and K-mica and at bigger pore widths (second minimum) for Cs-mica pores. We find that ±0.1 Å change in the interstitial cation size leads to a -15 to 5% change in equilibrium loading of adsorbed water and -2 to 35% change in swelling. Our thermodynamic analysis reveals an intricate interplay between enthalpic and entropic contributions caused by the structural change of water in the pores due to the hydration of interstitial cations.

Understanding the mobility and retention of uranium and its daughter products

Knowledge of the behavior of technologically enhanced naturally occurring radioactive materials derived through the decay of U and its daughter products, and their subsequent fractionation, mobilization and retention, is essential to develop effective mitigation strategies and long-term radiological risk prediction. In the present study, multiple state-of-the-art, spatially resolved micro-analytical characterization techniques were combined to systematically track the liberation and migration of radionuclides (RN) from U-bearing phases in an Olympic Dam Cu flotation concentrate following sulfuric-acid-leach processing. The results highlighted the progressive dissolution of U-bearing minerals (mainly uraninite) leading to the release, disequilibrium and ultimately upgrade of daughter RN from the parent U. This occurred in conjunction with primary Cu-Fe-sulfide minerals undergoing coupled-dissolution reprecipitation to the porous secondary Cu-mineral, covellite. The budget of RN remaining in the leached concentrate was split between RN still hosted in the original U-bearing minerals, and RN that were mobilized and subsequently sorbed/precipitated onto porous covellite and auxiliary gangue mineral phases (e.g. barite). Further grinding of the flotation concentrate prior to sulfuric-acid-leach led to dissolution of U-bearing minerals previously encapsulated within Cu-Fe-sulfide minerals, resulting in increased release and disequilibrium of daughter RN, and causing further RN upgrade. The various processes that affect RN (mobility, sorption, precipitation) and sulfide minerals (coupled-dissolution reprecipitation and associated porosity generation) occur continuously within the hydrometallurgical circuit, and their interplay controls the rapid and highly localized enrichment of RN. The innovative combination of tools developed here reveal the heterogeneous distribution and fractionation of the RN in the ores following hydrometallurgical treatment at nm to cm-scales in exquisite detail. This approach provides an effective blueprint for understanding of the mobility and retention of U and its daughter products in complex anthropogenic and natural processes in the mining and energy industries.

Selective radionuclide co-sorption onto natural minerals in environmental and anthropogenic conditions

Anthropogenic activities can redistribute the constituents of naturally occurring radioactive materials (NORM), posing potential hazards to populations and ecosystems. In the present study, the co-sorption of several RN from the U decay chain- ²³⁸U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po, onto common minerals associated with mining activities (chalcopyrite, bornite, pyrite and barite) was investigated, in order to identify the various factors that control long-term NORM mobility and retentivity in environmental acid-mine drainage systems and hydrometallurgical processing. The results show selective RN co-sorption to the various natural minerals, suggesting that mineral-specific mechanisms govern the variability in NORM mobility and retentivity. Both ²²⁶Ra and ²¹⁰Po underwent significant sorption onto the natural minerals investigated in this study. The order of co-sorption in sulfate media for chalcopyrite and bornite was ²¹⁰Po>²²⁶Ra>²⁰⁶Pb>²¹⁰Pb>²³⁸U/^230Th. Conversely, both pyrite and barite showed increased affinity for ²²⁶Ra; the order of co-sorption in sulfate media was ²²⁶Ra>²¹⁰Po>²⁰⁶Pb/²¹⁰Pb>²³⁸U/²³⁰Th for pyrite and ²²⁶Ra>²⁰⁶Pb/²¹⁰Pb>²³⁰Th/²³⁸U/²¹⁰Po for barite. Similar orders of co-sorption were observed in the nitrate media: for chalcopyrite and bornite the order was ²¹⁰Po>²²⁶Ra/²⁰⁶Pb/²¹⁰Pb/²³⁸U/²³⁰Th compared to ²²⁶Ra>²¹⁰Po/²⁰⁶Pb/²¹⁰Pb/²³⁸U/^230Th for pyrite and barite. The behavior of ²¹⁰Po was found to the anomalous: in both sulfate and nitrate solutions, ²¹⁰Po had little affinity for barite compared to the sulfides. Thermodynamic modeling indicated the formation of a reduced PoS(s) phase at the surface of sulfide minerals, leading to the suggestion that ²¹⁰Po likely undergoes reductive precipitation on the surface of sulfide minerals. The high sorption of both ²⁰⁶Pb and ²¹⁰Pb observed in the sulfate systems were likely as a result of co-precipitation as insoluble anglesite compared to nitrate where they mainly remained in solution. Overall, barite showed the highest affinity for ²²⁶Ra, given its propensity to sorb ²²⁶Ra (similar ionic size). Both ²³⁸U and ²³⁰Th were highly mobile in acidic sulfate and nitrate solutions. The results highlighted here identify the various constraints on the natural variability and fractionation of NORM in the environment, as well as the mineral-specific mechanisms that control co-sorption of RN. This information provides a framework for predicting RN transport within soils and ground waters with variable geochemical conditions and in metallurgical extraction processes, in order to develop effective strategies towards NORM mitigation.

A comparative bio-oxidative leaching study of synthetic U-bearing minerals: Implications for mobility and retention

In this study, the effects of bio-oxidative leaching on several synthetic uranium minerals - Uraninite [UO₂], Pitchblende [U₃O₈], Coffinite [USiO₄], Brannerite [UTi₂O₆] and Betafite [(U,Ca)₂(Ti,Nb,Ta)₂O₇]) compared to chemical leaching in the presence of pyrite was investigated. In all cases, bio-oxidative leaching was faster and increased overall %U extraction compared to chemical leaching. The results indicated that the bio-oxidative leachability of the uranium minerals was in the order: pitchblende≈ uraninite > coffinite>> brannerite > betafite. The leaching of pitchblende and uraninite was fast and complete; U extraction from coffinite was slower over 28 days' during the bioleaching. The use of thermophiles doubled the recovery of U from refractory brannerite. The results highlight the significant capability of bio-leaching in the recovery of U from brannerite; both mesophilic and thermophilic bacteria was found to enhance U recovery likely through enhanced breakdown of the titanate structure. Brannerite is often found in significant quantities within ore tailings due to its refractory nature, which can lead to subsequent release of U into the environment. Conversely, betafite is highly stable in the presence of mesophile and moderate thermophiles, which suggested that betafite materials can be a viable future host for long term storage for spent nuclear fuels.

Uranium-Incorporated Pyrochlore La2(UxMgxZr1-2x)2O7 Nuclear Waste Form: Structure and Phase Stability

As efficient and stable nuclear waste forms, single-phase uranium (U⁶⁺)-incorporated La₂Zr₂O₇ nanoparticles were designed and synthesized in an air atmosphere. To obtain a high U loading, divalent magnesium (Mg²⁺) was introduced to balance the extra charge from the substitution of tetravalent zirconium (Zr⁴⁺) by U⁶⁺ with a minimized impact to the lattice. There is a composition-driven phase transition from order pyrochlore to defect fluorite as the U concentration increases from 10 to 30 mol %, demonstrating both good solubility and stability of the La₂Zr₂O₇ host for U and potentially for other actinides. La₂(U_xMg_xZr_1-2x)₂O₇ (x = 0-0.3) nanoparticles showed good dispersity and crystallinity with an average particle size of ∼48 nm. Furthermore, X-ray photoelectron spectroscopy, Raman spectroscopy, and emission spectroscopy revealed that U was stabilized in the hexavalent state in the form of a UO₂²⁺ ion. Spectroscopic methods also demonstrated that our samples caused a scintillating response with an orange emission (597 nm) by 230 nm excitation. In addition, density functional theory simulations were employed to investigate the atomic structures and electronic properties of the U-incorporated pyrochlores. The calculated bond lengths, atomic charges, and charge density confirm the existence of UO₂²⁺ ions. Supported by both experimental and computational results, a novel geometrical structure was proposed to explain the Mg²⁺-U⁶⁺ substitution. This work demonstrated the successful development of U-incorporated La₂Zr₂O₇ nanoparticles and provided an efficient way to immobilize U in these ceramic waste matrixes.

A novel sorbent based on Ti-Ca-Mg phosphates: synthesis, characterization, and sorption properties

This work focuses on the synthesis procedure of a new sorbent based on a TiCaMg phosphate. The synthesis strategy includes stepwise interaction between solid precursors and phosphorus-containing agents. The solid precursors were ammonium titanyl sulfate and calcined dolomite, which were used as titanium, calcium, and magnesium sources. The effect of the nature and concentration of phosphoric agent on the sorbent composition and properties has been investigated using elemental analysis, TG, XRD, IR spectroscopy, BET, and SEM techniques. The novel sorbent has been demonstrated to be a composite material consisting of the following components: TiO(OH)H₂PO₄·H₂O, Ti(HPO₄)₂·H₂O, CaHPO₄·2H₂O, MgНPO₄·3H₂O, and NH₄MgPO₄·6H₂O. The ratio between these phases in the composite depends on synthesis conditions. The optimal conditions, ensuring full conversion of Ti, Ca, and Mg containing in the initial precursors into the final product, have been found. The sorption properties of the obtained composite sorbent towards Co²⁺, Cs⁺, and Sr²⁺ cations and their radionuclide analogues have been studied. The obtained data has indicated that the purification effect was based on both precipitation and ion exchange mechanism. The combined action of the individual components of the composite sorbent ensures its high sorption capacity towards different cations in a wide pH range. The new sorbent shows high sorption ability towards radionuclides in multicomponent liquid radioactive waste (LRW) systems, and the distribution coefficient of the studied radionuclides was found to be 10⁵ mL g^-1. The presence of different types of functional groups in the composite sorbent allows realizing the one-step purification process of LRW that, in turn, simplifies the sorption system design.

Template-directed synthesis of Sm2Ti2O7 nanoparticles: a FRET-based fluorescent chemosensor for the fast and selective determination of picric acid

Fluorescence “turn off” detection of picric acid using a Sm₂Ti₂O₇ nanoprobe.

Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization

Crystalline ceramics are intensively investigated as effective materials in various nuclear energy applications, such as inert matrix and accident tolerant fuels and nuclear waste immobilization. This paper presents an analysis of the current status of work in this field of material sciences. We have considered inorganic materials characterized by different structures, including simple oxides with fluorite structure, complex oxides (pyrochlore, murataite, zirconolite, perovskite, hollandite, garnet, crichtonite, freudenbergite, and P-pollucite), simple silicates (zircon/thorite/coffinite, titanite (sphen), britholite), framework silicates (zeolite, pollucite, nepheline /leucite, sodalite, cancrinite, micas structures), phosphates (monazite, xenotime, apatite, kosnarite (NZP), langbeinite, thorium phosphate diphosphate, struvite, meta-ankoleite), and aluminates with a magnetoplumbite structure. These materials can contain in their composition various cations in different combinations and ratios: Li-Cs, Tl, Ag, Be-Ba, Pb, Mn, Co, Ni, Cu, Cd, B, Al, Fe, Ga, Sc, Cr, V, Sb, Nb, Ta, La, Ce, rare-earth elements (REEs), Si, Ti, Zr, Hf, Sn, Bi, Nb, Th, U, Np, Pu, Am and Cm. They can be prepared in the form of powders, including nano-powders, as well as in form of monolith (bulk) ceramics. To produce ceramics, cold pressing and sintering (frittage), hot pressing, hot isostatic pressing and spark plasma sintering (SPS) can be used. The SPS method is now considered as one of most promising in applications with actual radioactive substances, enabling a densification of up to 98-99.9% to be achieved in a few minutes. Characteristics of the structures obtained (e.g., syngony, unit cell parameters, drawings) are described based upon an analysis of 462 publications.

Technetium Encapsulation by A Nanoporous Complex Oxide 12CaO•7Al2O3 (C12A7)

Technetium (99Tc) is an important long-lived radionuclide released from various activities including nuclear waste processing, nuclear accidents and atmospheric nuclear weapon testing. The removal of 99Tc from the environment is a challenging task, and chemical capture by stable ceramic host systems is an efficient strategy to minimise the hazard. Here we use density functional theory with dispersion correction (DFT+D) to examine the capability of the porous inorganic framework material C12A7 that can be used as a filter material in different places such as industries and nuclear power stations to encapsulate Tc in the form of atoms and dimers. The present study shows that both the stoichiometric and electride forms of C12A7 strongly encapsulate a single Tc atom. The electride form exhibits a significant enhancement in the encapsulation. Although the second Tc encapsulation is also energetically favourable in both forms, the two Tc atoms prefer to aggregate, forming a dimer.