Search Results (27)

Tetrakis (Hydroxymethyl) Phosphonium chloride (THPC) in aqueous solution reacts with amines to form aminomethylenephosphines. The reaction was studied with piperidine, and THPC was used with PEI. The reaction with PEI leads to new polymers with phosphine groups (PEI-P) and phosphine oxide (PEI-PO) after oxidation by hydrogen peroxide. These polymers coordinate cations of transition metals, lanthanides and actinides. Full article

Heterobimetallic uranium(V) alkoxides incorporating monovalent alkali metal counterions display remarkable structural versatility, dictated by the steric demands of the alkoxide ligands and the ionic radius of the alkali metal. Compounds of the general formula [UM(O^tBu)₆] (UM-O^tBu-type: M = Na, K, Rb, Cs) were obtained by: (i) reacting [U(O^tBu)₅(py)] with equimolar amounts of alkali metal silylamides in tert-butyl alcohol, and (ii) oxidative transformation of [UM₂(O^tBu)₆] (M = Na, K, Rb, Cs) upon reaction with iodine. Trans-alcoholysis of uranium heterobimetallic tert-butoxides with sterically less demanding iso-propyl alcohol yields oligomeric or polymeric iso-propoxide derivatives of the general formula [UM(OⁱPr)₆]_n, where the nuclearity depends on the alkali metal (n = 2 for M = Li; n = ∞ for M = Na, K, Rb). The capacity of alkali metal ions to adopt flexible coordination geometries results in different structural types ranging from finite clusters to infinite chains, with [ULi(OⁱPr)₆]₂ (ULi-OⁱPr-1) found to be dimeric, whereas [UM(OⁱPr)₆]_∞ (UM-OⁱPr-2-type, M = Na, K) and [URb(OⁱPr)₆]_∞ (URb-OⁱPr-3) exhibit a polymeric architecture. These findings provide fresh insights into the structure-directing influence of alkali metals on actinide coordination chemistry and broaden the chemistry of actinide alkoxides. All compounds were unambiguously characterized in both solution and solid-state through NMR and IR spectroscopic studies, as well as single crystal X-ray diffraction analysis. Full article

Several recent studies have evaluated technologies of spent nuclear fuel processing specifically for solidifying transuranic (TRU) waste as a by-product of fission. Of the TRU group, plutonium and the minor actinides will be responsible for the bulk of the radiotoxicity and heat generation of spent nuclear fuel in the long term (300 to 20,000 years). In this study, we investigated magnesium potassium phosphate (MKP)-based compounds as host waste forms for the encapsulation of inactive trivalent Nd and Sm as analogues of the minor trivalent actinides, Am and Cm. Waste forms were fabricated under ambient atmospheric conditions by adding 5 wt.% of substances containing Nd or Sm via the following two routes: powder oxides and aqueous solutions of nitrate salts. Waste form performance was established using strength and aqueous medium leaching tests of MKP-based specimens. The MKP materials were analyzed by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. The waste forms exhibited a compressive strength of ≥30 MPa and were durable in an aqueous environment. The leachability indices for Nd and Sm, as per the ANS 16.1 procedure, were 19.55–19.78 and 19.74–19.89, respectively, which satisfy the acceptable criteria (>6). The results of the present room temperature leaching study suggest that MKPs can be effectively used as a host material to immobilize actinides (Am and Cm) contained in TRU waste. Full article

We present a first-principles density functional theory with the Coulomb interaction U (DFT + U) investigation of the bulk properties, including structural, energetic, electronic, and mechanical properties for uranium–americium mixed oxides (U, Am)O₂. The various Am aggregation contents were investigated to better understand the impact of Am on the nuclear fuel UO₂. The supercell defect models at different scales were used to describe the solid solution (U, Am)O₂. The obtained results show that different contents of Am aggregation have a significant impact on the volume and energy of the formation of mixed oxide systems. The results of the electronic structure calculations exhibit no bandgap owing to the mixing of UO₂ and AmO₂. The mixing enthalpy of the Am aggregation systems is used to describe the phase stability of the solid solution. In particular, the mixing enthalpy of (U, Am)O₂ is significantly reduced as the Am content increases. The elastic properties of the (U, Am)O₂ mixed oxides have also been compared as a function of the Am content. Moreover, the impacts of the whole Pu aggregation content range on the bulk properties for the (U, Pu)O₂ mixed oxides are also discussed. Full article

The synthesis, isolation, and characterisation of well-defined low-valent actinide complexes are reviewed with a main focus on compounds featuring uranium and thorium metal centres in formal oxidation states ≤ +3. The importance of the ligand environment in enabling access to these highly reactive species, as well as its influence on ground state electronic configurations and their reactivity, are emphasised. Furthermore, we highlight cyclic voltammetry (C.V.) studies as a more widely used method that can guide the synthesis of these highly reducing species. Full article

Rare earth (RE) aluminosilicate glasses exhibit several favorable chemical, mechanical and thermal properties. As such, they are considered to be model systems for long-half-life actinides and are candidate containment materials for long-term immobilization of radioactive wastes. The aim of the present study was to investigate the effect of the substitution of sodium oxide on the glass transition temperature and structure of lanthanum aluminosilicate glasses. The primary objective was to elucidate the relationship between the substitution of Na₂O for La₂O₃ on the Tg reduction and structural characteristics of lanthanum aluminosilicate glass, including identifying changes in the main Qn species and local environments of Si and Al. The structure of SiO₂–Al₂O₃–La₂O₃–Na₂O glasses has not been studied previously, and, thus, this investigation is the first to assess the structural changes occurring when La₂O₃ is substituted by Na₂O. Three glasses were prepared with general composition (mol.%): 55SiO₂–25Al₂O₃–20M₂O_n (M = La or Na; n = 3 or 1). Glass G1 contains 20 mol.% La₂O₃; in G2, 15 mol.% of La₂O₃ was substituted by 15 mol.% Na₂O; and Glass G3 contains 20 mol.% Na₂O. The glasses were characterized by DSC to determine glass transition temperatures. As expected, as Na is substituted for La, T_g decreases substantially. Structural studies were carried out by FTIR spectroscopy, ²⁹Si, and ²⁷Al MAS NMR. As Na is substituted for La in these aluminosilicate glasses, the main goals that were achieved were the identification of Qⁿ species and also changes in the local environments of Si and Al: {Qⁿ_Si(mAl)} and {Qⁿ_Al(mSi)}. Full article

The hydrolysis mechanism of americium was calculated using density functional theory, and the detailed microscopic reaction mechanism was obtained. The results show that americium reacts with water along the octet state to produce oxides and H₂, and that this reaction is exothermic. The interaction between Am and O atoms gradually changes from initially electrostatic interaction to covalent interaction, and continues to strengthen. During the reaction process, Am atoms always lose electrons, the 5f orbital is obviously involved, and there is df orbital hybridization. This study provides the necessary theoretical data support for the theoretical and experimental study of the actinide system. Full article

The adsorption of actinide ions (Am(III) and U(VI)) from aqueous solutions using pristine and oxidized carbon fabrics was investigated by means of batch experiments at different pH values (pH 4, 7 and 9) and temperatures (25, 35 and 45 °C) under ambient atmospheric conditions. The experimental results indicated that both the pH and the fabric texture affected the adsorption rate and the relative removal efficiency, which was 70% and 100% for Am(III) and U(VI), respectively. The K_d (L/kg) values for U(VI) were generally found to be higher (2 < log₁₀(K_d)< 3) than the corresponding values for Am(III) adsorption (1.5 < log₁₀(K_d) < 2). The data obtained from the experiments regarding the temperature effect implied that the relative adsorption for both actinides increases with temperature and that adsorption is an endothermic and entropy-driven reaction. The application of the fabrics to remove the two actinides from contaminated seawater samples showed that both the relative removal efficiency and the K_d values decreased significantly due to the presence of competitive cations (e.g., Ca²⁺ and Fe³⁺) and complexing anions (CO₃²⁻) in the respective waters. Nevertheless, the removal efficiency was still remarkable (50% and 90% for Am(III) and U(VI), respectively), demonstrating that these materials could be attractive candidates for the treatment of radionuclide/actinide-contaminated waters. Full article

A novel solid-phase synthetic approach was developed to produce a mineral-like composite ceramic based on strontium titanate (SrTiO₃) and yttrium titanate (Y₂Ti₂O₇) matrices for immobilizing radionuclides such as ⁹⁰Sr and its daughter product ⁹⁰Y, as well as lanthanides and actinides, via reactive spark plasma sintering technology (SPS-RS). Using XRD, SEM, and EDS analyses, the sintering kinetics of the initial mixed oxide reactants of composition Y_xSr_1–1.5xTiO₃ (x = 0.2, 0.4, 0.6 and 1) and structure-phase changes in the ceramics under SPS-RS conditions were investigated as a function of Y³⁺ content. In addition, a detailed study of phase transformation kinetics over time as a function of the heating temperature of the initial components (SrCO₃, TiO₂, and Y₂O₃) was conducted via in situ synchrotron XRD heating experiments. The composite ceramic achieved relatively high physicomechanical properties, including relative density between 4.92–4.64 g/cm³, Vickers microhardness of 500–800 HV, and compressive strength ranging from 95.5–272.4 MPa. An evaluation of hydrolytic stability and leaching rates of Sr²⁺ and Y³⁺ from the matrices was performed, demonstrating rates did not exceed 10⁻⁵–10⁻⁶ g·cm⁻²·day⁻¹ in compliance with GOST R 50926-96 and ANSI/ANS 16.1 standards. The leaching mechanism of these components was studied, including the calculation of solution penetration depth in the ceramic bulk and ion diffusion coefficients in the solution. These findings show great promise for radioactive waste conditioning technologies and the manufacturing of radioisotope products. Full article

The interaction of the actinides Pu(III), Am(III), Np(V), Np(VI), and U(VI) with hardened cement paste (HCP) prepared from ordinary Portland cement was investigated by batch experiments in a diluted caprock solution (I = 2.5 M) as a function of the solid-to-liquid (S/L) ratio (0.5–20.0 g L⁻¹) and pH (10–13). Independent of the oxidation state of the actinides, strong sorption was observed with R_d values between 10⁴ and 5 × 10⁵ L kg⁻¹. For the hexavalent actinides U(VI) and Np(VI), a decrease in sorption was observed with increasing pH, which could be due to the formation of the AnO₂(OH)₄²⁻ species. CE-ICP-MS measurements of the supernatant solution from the U(VI) batch sorption experiment at pH ≥ 10 indicate that UO₂(OH)₃⁻ and UO₂(OH)₄²⁻ dominate the speciation. Pu L_III-edge XANES and EXAFS measurements showed oxidation of Pu(III) to Pu(IV) when interacting with HCP. Calcium silicate hydrate (C-S-H) phases effectively immobilize Pu(IV) by incorporating it into the CaO layer. This was observed in a C-S-H sample with C/S = 1.65 and HCP at pH 12.7. Compared to data published in the literature on the retention of actinides on HCP at low ionic strength, the influence of high ionic strength (I = 2.5 M) on the sorption behavior was insignificant. Full article

Mixed oxides of uranium and plutonium (MOX) are currently considered as a reference fuel for the new generation of fast breeder reactors such as ASTRID. The key factor determining the performance and safety of a fuel such as MOX is its operational limits in the application environment which are closely related to the material’s structure and thermodynamic stability. They are in turn closely related to the ambient (zero pressure) melting point $\left(T_m\right)$; thus, $T_m$ is an important engineering parameter. Furthermore, PuO${}_2$ and UO${}_2$ are two endpoints of the phase diagram of MOX; therefore, their ambient $T_m$s are fundamental reference points. However, the current knowledge of the $T_m$ of MOX is limited and controversial as several studies available in the literature do not converge on the unique behavior of $T_m$ as a function of $x.$ Specifically, some studies produced $T_m$ as a monotonically decreasing function of x such that, with $T_m$ of UO${}_2$$(x=0)$ of 3150 K, $T_m$ of PuO${}_2$$(x=1)$ is ∼2650 K, while other studies resulted in $T_m$ having a local minimum at $0.5<x<1$ such that $T_m$ of PuO${}_2$ is ∼3000 K, so that the difference between the two values of $T_m$ is as high as 350 K. In this study, using the ab initio Z method implemented with the Vienna Ab Initio Simulation Package (VASP), we carry out a suite of quantum molecular dynamics simulations to obtain the ambient $T_m$ of MOX at several values of $x,$ $0<x<1,$ including the two end points $(x=0,$ $x=1).$ Our results agree with the behavior of $T_m$ of MOX as a function of x having a local minimum at $x=0.7$ and $T_m$ of PuO${}_2$ of 3050 K. Our study suggests potential ambient density–melting point systematics of MOX which may be useful in subsequent research on MOX such as its thermoelasticity modeling. Full article

Numerous studies expose the potential of brannerite to become a good matrix, concentrating fission products and actinides. Minerals can complement the data collected from the synthetic materials and offer an advantage of a long-time exposure to radiation. Natural metamict brannerite from Akchatau, Kazakhstan, and its annealed sample were studied by EPMA, Raman spectroscopy, TGA, DSC, XRD and HTXRD. The radioactivity of pristine and annealed samples of brannerite was measured. Brannerite from Akchatau is characterized by the absence of significant amounts of REE and yttrium. The studied brannerite regains its structure at a temperature ~650 °C, revealed by the HTXRD and DSC. HTXRD was also performed on the annealed recrystallized brannerite. The thermal expansion for brannerite has been determined for the first time. The brannerite structure expands anisotropically with temperature increase. All the thermal expansion coefficients are positive except for α_β. The decreasing beta parameter indicates a “shear structural deformation“. The angle between the 1st axis of the tensor and the crystallographic a axis decreases with the increase of the temperature. The structure expands mostly in the α₁₁ direction, approaching the bisector of the β angle. Brannerite has a low CTE at room temperature—α_v = 16 × 10⁻⁶ °C⁻¹, which increases up to 39.4 × 10⁻⁶ °C⁻¹ at 1100 °C. In general, the thermal stability of brannerite is comparable to that of the other perspective oxide radioactive waste-immobilizing matrices (e.g., Ln₂Zr₂O₇, CePO₄, CaTiO₃, CaZrTi₂O₇). The calculated thermal expansion of brannerite and the understanding of its underlying crystal chemical mechanisms may contribute to the behavior prediction of the material (both metamict and crystalline) at high temperatures. Full article

To separate the long-lived minor actinides (MA = Am, Cm) from high-level liquid waste (HLLW), we have been studying an advanced separation process via selective adsorption that uses minimal amounts of organic solvent and compact equipment. The process consists of two separation columns packed with a CMPO (octyl(phenyl)-N,N-diisobutylcarbamoyl-methyl phosphine oxide) adsorbent for elemental group separation and a soft-donor named the R-BTP (2,6-bis-(5,6-dialkyl-1,2,4-triazine-3-yl) pyridine) adsorbent for the isolation of MA from lanthanides (Ln). In this work, the effects of nitrate ion (NO₃⁻) on the adsorption behavior of Am(III) and a typical fission product Ln(III) onto the isoBu-BTP/SiO₂-P adsorbent were studied experimentally. Then, the desorption properties of the adsorbed element were examined using different eluting agents. A hot test for the separation of MA from the fission product Ln in a genuine MA containing effluent from the irradiated MOX-fuel treatment process was carried out using a nBu-BTP/SiO₂-P packed column. It was found that the separation factor between Am(III) and Ln(III)-FP is over 100 in the measured 0.5–4 M NO₃⁻. The adsorbed elements could be effectively eluted off using a complexing agent such as DTPA or pure water. Complete separation between MA and Ln was achieved in the column results, indicating that the proposed MA separation process is feasible in principle. Full article

Calcium silicate hydrate (C-S-H) is the main product of cement hydration and has also been shown to be the main sorbing phase of actinide ions interacting with cement. U(IV) has been chosen as an exemplary actinide ion to study actinide sorption at C-S-H as U is the main element in highly active radioactive waste and because reducing conditions are foreseen in a deep geological repository for such waste. U(IV) surface adsorption, absorption in the interlayer, and incorporation into the calcium oxide layer of C-S-H has been modeled quantum mechanically, applying a density functional approach. For each sorption mode various sites have been considered and a combined dynamic equilibration and optimization approach has been applied to generate a set of representative stable sorption complexes. At the surface and in the interlayer similar U(IV) hydroxo complexes stabilized by Ca²⁺ ions have been determined as sorbates. Surface adsorption tends to be preferred over absorption in the interlayer for the same type of sites. Incorporation of U(IV) in the CaO layer yields the most favorable sorption site. This result is supported by good qualitative agreement of structures with EXAFS results for other actinides in the oxidation state IV, leading to a new interpretation of the experimental results. Full article

Potassium chloride crystals are good conductors in the UV to mid-infrared broadband and can be widely used as materials for high-power CO₂ lasers. However, the most important use of potassium chloride crystals has been overlooked; their stable chemistry and wide electrochemical window allow them to form molten eutectic salts with other chlorides for the dry reprocessing of spent fuel. The aim of this work is to investigate the electrochemical properties of the actinide substitute cerium metal on liquid indium media in order to provide basic data for further realization of lanthanides (Ln)/actinides (An) separation. In this paper, we investigated the electrochemical behavior of 3LiCl-2KCl-CeCl₃ melt at liquid metal media indium (In) electrode using various transient electrochemical analysis methods, such as cyclic voltammetry, square wave voltammetry and open circuit potential technique. The quasi-equilibrium potentials of Ce³⁺ at different temperatures on the liquid metal In electrode were determined. On the redox process, the reaction kinetics of the oxidation–reduction process in the 3LiCl-2KCl-CeCl₃ system is studied. It was determined that the reduction of Ce³⁺ at the liquid metal In electrode is an irreversible single-step process with three electron transfers. The relationship between the diffusion coefficient and the reduction process of Ce³⁺ ions on liquid metal indium electrodes at different temperatures was determined. The expression for the diffusion coefficient and temperature of Ce³⁺ at the liquid metal indium electrode is: lnD = 1.43 − 7974.69/T. Full article