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Cesium uranate [Cs2(Н2О)3][(UO2)6O3(OH)8]·2H2O was obtained by reacting hydrated uranium(VI) oxide UO3·2.25H2O with a cesium nitrate aqueous solution under hydrothermal conditions at 100°C within two weeks. Composition and structure of the resulting compound was determined by the chemical analysis, IR spectroscopy, X-ray diffraction, and differential thermal analysis. Its dehydration and thermal decomposition were studied.

Portlandite [Ca(OH)2] is a potentially dominant solid phase in the high pH fluids expected within the cementitious engineered barriers of Geological Disposal Facilities (GDF). This study combined X-ray Absorption Spectroscopy with computational modelling in order to provide atomic-scale data which improves our understanding of how a critically important radionuclide (U) will be adsorbed onto this phase under conditions relevant to a GDF environment. Such data are fundamental for predicting radionuclide mass transfer. Surface coordination chemistry and speciation of uranium with portlandite [Ca(OH)2] under alkaline groundwater conditions (ca. pH 12) were determined by both in situ and ex situ grazing incidence extended X-ray absorption fine structure analysis (EXAFS) and by computational modelling at the atomic level. Free energies of sorption of aqueous uranyl hydroxides, [UO2(OH)n]2–n (n = 0–5) with the (001), (100) and (203) or (101) surfaces of portlandite are predicted from the potential of mean force using classical molecular umbrella sampling simulation methods and the structural interactions are further explored using fully periodic density functional theory computations. Although uranyl is predicted to only weakly adsorb to the (001) and (100) clean surfaces, there should be significantly stronger interactions with the (203/101) surface or at hydroxyl vacancies, both prevalent under groundwater conditions. The uranyl surface complex is typically found to include four equatorially coordinated hydroxyl ligands, forming an inner-sphere sorbate by direct interaction of a uranyl oxygen with surface calcium ions in both the (001) and (203/101) cases. In contrast, on the (100) surface, uranyl is sorbed with its axis more parallel to the surface plane. The EXAFS data are largely consistent with a surface structural layer or film similar to calcium uranate, but also show distinct uranyl characteristics, with the uranyl ion exhibiting the classic dioxygenyl oxygens at 1.8 Å and between four and five equatorial oxygen atoms at distances between 2.28 and 2.35 Å from the central U absorber. These experimental data are wholly consistent with the adsorbate configuration predicted by the computational models. These findings suggest that, under the strongly alkaline conditions of a cementitious backfill engineered barrier, there would be significant uptake of uranyl by portlandite to inhibit the mobility of U(VI) from the near field of a geological disposal facility.

The interaction of hydrated uranium(VI) oxide UO3·2.25H2O (schoepite) with an aqueous solution of rubidium hydroxide in an autoclave at 100°C has yielded rubidium uranate Rb2(UO2)6O3(OH)8·6H2O. Composition and structure of the obtained compound have been determined by chemical analysis, IR spectroscopy, X-ray diffraction, and differential thermal analysis. The processes of its dehydration and thermal decomposition have been studied.

A2B2O7 type pyrochlores have been recently proposed as a potential nuclear waste host due to their many interesting properties. To assess and understand the performance of these compounds as nuclear waste hosts, the speciation and structural investigations on actinide-doped RE2Hf2O7 are needed since both are imperative from their application perspective. In this work, we investigated the effect of uranium doping at different concentrations in the range of 0–10% on the structural and optical properties of RE2Hf2O7:U (RE = Y, Gd, Nd, and Lu) nanoparticles (NPs). The Y2Hf2O7 NPs exist in slightly disordered pyrochlore structure and the extent of disordering increases as a function of uranium doping while the structure reaches a cotunnite phase at 10.0% doping level. The Nd2Hf2O7 NPs also exist in a distorted pyrochlore structure and their distortion increases with increasing uranium doping inducing a phase transition into a disordered fluorite structure at 10.0% uranium doping. Both Gd2Hf2O7 and Lu2Hf2O7 NPs exist in a disordered fluorite structure and transforms into cotunnite structure at higher U concentrations (≥5.0%). Photoluminescence spectroscopy showed that uranium ions are stabilized in +6 oxidation state in all samples: in the form of uranate ion UO66− in the Y2Hf2O7, Nd2Hf2O7 and Lu2Hf2O7 NPs while in the form of uranyl ion UO22+ in the Gd2Hf2O7 NPs. Therefore, this work deepens the understanding of the behavior of uranium ions doped in different RE2Hf2O7 host matrices in the perspective of their application as nuclear waste hosts.

Single crystals of trichloridosulfonium(IV) hexachloridouranate(V) were obtained from the reaction of uranium(IV) chloride with an excess of disulfur dichloride and studied by single-crystal X-ray diffraction. In comparison with the structure model reported previously [Sawodny et al. (1983). Z. Anorg. Allg. Chem. 499, 81–88.], the lattice parameters and fractional atomic coordinates were determined to a much higher precision, leading overall to an improved structure model. The ionic compound contains trigonal–pyramidal (SCl3)+ cations and slightly distorted octahedral [UCl6]− anions. The structure was refined as an inversion twin with a twin ratio of 4.4:1.

The synthesis and crystal structure determination of (Et3NH)2[UBr6]·CH2Cl2 is reported., Tri­ethyl­ammonium hexa­bromido­uranate(IV) di­chloro­methane monosolvate, [(C2H5)3NH]2[UBr6]·CH2Cl2, was obtained in the form of dark-brown crystals from the reaction of uranium penta­bromide with NEt3 and ethyl­ene glycol in di­chloro­methane at low temperature. During the progress of the reaction, the reduction of uranium(V) to uranium(IV) was observed, whose associated oxidation product could not be identified. The uranium atom of the [UBr6]2– anion is coordinated by six bromido ligands in the shape of an octa­hedron. Between cations, anion and solvent mol­ecules of crystallization, numerous C—H⋯Hal hydrogen-bond-like inter­actions are present, leading to a three-dimensional network structure.

Sm6UO12 has been synthesized by urea-combustion method and characterized using synchrotron x-ray source. The compound is found to be in single phase crystallizing in rhombohedral lattice with lattice parameters a = 10.153 A and c = 9.625 A. High-pressure experiment at ambient temperature reveals the lattice to be stable up to 27.0 GPa. The bulk modulus of the compound is found to be 157.0 GPa. At higher pressures, reflections broaden out as a result of disorder in the system, leading to amorphization at further higher pressures. High pressure and high temperature (HP-HT) x-ray diffraction studies have been carried out up to 6.0 GPa and 673 K. A softening is seen in the material at HP-HT resulting in lower bulk modulus at high temperature. Volume thermal expansion coefficients are found to be 22 × 10−6 K−1 and 17.7 × 10−6 K−1 in the temperature range 298 K–473 K and 298 K-673 K, respectively.

Uranium doped yttrium aluminum garnet (YAG) transparent ceramic with Ca2+ as charge compensator and sintering aid was firstly fabricated by vacuum sintering technique using the co-precipitation synthesis of raw powders. The structural, optical spectral and luminescence properties of the U:YAG ceramic have been investigated in detail. The experimental results show that uranium doped in YAG matrix has not changed the crystal structure. Vacuum sintering 15 h in 1800 °C, the sample has a pore-free structure with an average grain size of about 3.5 μm, and in-line transmittance reaches up to 78% in the visible wavelength region. The excitation spectrum showed a sharp band around 458 nm, and the emission spectrum revealed the intense green emission located at 520 nm. Further, with respect to the shape of emission band and luminescence color, uranium was inferred to exist as uranate ion (UO66−) in the YAG host where it replaces the ‘Y’ ions at dodecahedral site with surrounding defect centers created for charge compensation.

Alkaline-earth monouranates with the formula MUO4 (M = Ca, Sr, Ba) were synthesized using a novel method which consists of a combination of solid state reaction between U3O8 and alkaline-earth oxide or carbonate in air, and chlorination reaction of the product of the solid state reaction using Cl2(g) as chlorinating agent. The chlorination temperature was optimized to obtain the desired uranate. The products obtained after the chlorination reactions were washed with deionized water and thermally treated to crystallize the uranates. MgUO4 could not be synthesized and presents a different behavior due to the similarity in the starting temperatures of the magnesium and uranium oxide chlorination reactions. The monouranates were characterized by PXRD, SEM and chemical analysis to determinate their structures. Fil: Pomiro, Federico J. Comisión Nacional de Energía Atómica. Gerencia Complejo Tecnológico Pilcaniyeu; Argentina Fil: Guibaldo, Cristina Noemi. Comisión Nacional de Energía Atómica. Gerencia Complejo Tecnológico Pilcaniyeu; Argentina Fil: Quinteros, Raúl D.. Comisión Nacional de Energía Atómica. Gerencia Complejo Tecnológico Pilcaniyeu; Argentina Fil: Bohe, Ana Ester. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia Complejo Tecnológico Pilcaniyeu; Argentina. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche; Argentina Fil: de Micco, Georgina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia Complejo Tecnológico Pilcaniyeu; Argentina

An effort was taken to carry our speciation study of uranium ion in technologically important cerate host Sr2CeO4 using time resolved photoluminescence spectroscopy. Such studies are not relevant only to nuclear industry but can give rich insight into fundamentals of 5f electron chemistry in solid state systems. In this work both undoped and varied amount of uranium doped Sr2CeO4 compound is synthesized using complex polymerization method and is characterized systematically using X-ray diffraction (XRD), Raman spectroscopy, impedance spectroscopy and scanning electron microscopy (SEM). Both XRD and Raman spectroscopy confirmed the formation of pure Sr2CeO4 which has tendency to decompose peritectically to SrCeO3 and SrO at higher temperature. Uranium doping is confirmed by XRD. Uranium exhibits a rich chemistry owing to its variable oxidation state from +3 to +6. Each of them exhibits distinct luminescence properties either due to f-f transitions or ligand to metal charge transfer (LMCT). We have taken Sr2CeO4 as a model host lattice to understand the photophysical characteristics of uranium ion in it. Emission spectroscopy revealed the stabilization of uranium as U (VI) in the form of UO66- (octahedral uranate) in Sr2CeO4. Emission kinetics study reflects that uranate ions are not homogeneously distributed in Sr2CeO4 and it has two different environments due to its stabilization at both Sr2+ as well as Ce4+ site. The lifetime population analysis interestingly pinpointed that majority of uranate ion resided at Ce4+ site. The critical energy-transfer distance between the uranate ion was determined based on which the concentration quenching mechanism was attributed to electric multipolar interaction. These studies are very important in designing Sr2CeO4 based optoelectronic material as well exploring it for actinides studies.

In order to evaluate the trap level spectroscopic properties of Uranium in ZnAl2O4 spinel host, undoped and Uranium doped ZnAl2O4 samples were synthesized. From photoluminescence (PL) data it was confirmed that uranium gets stabilized in the system as UO66− (octahedral uranate). Electron spin resonance (ESR) studies for the gamma irradiated sample suggested the formation of O2−, F+ and V centres. From the TSL (thermally stimulated luminescence) data, the trap parameters such as frequency factor and activation energy etc. were evaluated. From ESR-TSL correlation it was confirmed that the destruction of O2− ion coincides with TSL glow peak appeared at 332 K.

An ultrasound assisted technique has been developed for the precipitation of ammonium di uranate from uranyl nitrate solution with ammonia employing an ultrasonic horn of 35 KHz. The effect of ultrasonic power on reaction kinetics, physical and morphological characteristics of the ADU powder, crystal size, crystallinity, yield etc has been investigated under experimental conditions. The results obtained with ultrasonic precipitation have been compared with that of the conventional route of precipitation. The resultant powder was characterised by the analytical techniques such as particle size analysis, BET surface area analysis, XRD and SEM. Increase in power in the range of 20 W–200 W led to an increase in precipitation kinetics and thereby decrease in induction time of nucleation due to micromixing effect of ultrasound. Additionally, surface area and tap density of the ADU powder has also been found to be improved. XRD and SEM studies confirmed an improved crystallinity and homogeneity with flower petal like intrinsic particle morphology of the resultant ADU in sonochemical route compared to conventional route.

The design and development of efficient and stable nuclear waste hosts has drawn intensive interest for long-lived lanthanides and actinides. A detailed investigation of their structure and potential structural evolution are crucial. In this study, we have synthesized lanthanum hafnate La2Hf2O7 nanoparticles (NPs) doped with uranium at different concentrations (0–10%) and investigated their structural transition. We have discovered that in our La2Hf2O7:U NPs, the uranium dopants are stabilized at both U4+ and U6+ oxidation states in which the U6+ oxidation state exists in octahedral uranate UO66− form. We also confirmed that the U4+ ions substituted the Hf4+ ions with a lifetime of ∼1.0 μs and the UO66− ions resided at the La3+ sites with a lifetime of ∼9.0 μs. More interestingly, the proportion of the U4+ ions in the La2Hf2O7:U NPs was higher than that of the UO66− ions at low doping level, but at the doping level higher than 2.5%, the fraction of the UO66− ions was greater than that of the U4+ ions. Furthermore, we studied the structural phase transformation from order pyrochlore to cotunnite of these La2Hf2O7:U NPs with increasing uranium doping level, and found that ordered pyrochlore phase favors the U4+ ions whereas disordered cotunnite phase favors the UO66− ions. We further used in situ Raman spectroscopy to confirm the reversible cotunnite to pyrochlore phase transformation of the La2Hf2O7:10%U NPs at 900 °C. Therefore, this work demonstrated the successful development of uranium doped La2Hf2O7 NPs and thorough characterization of the fundamental spectra of uranium ions, doping induced phase transformation, and structure–optical property correlation.

The analysis and characterization of uranium dioxide (UO2) under controlled storage conditions can provide insight into potential alteration products and overall stability, which has relevance for nuclear fuel cycle processes, environmental management, and the stability of used nuclear fuel in a repository. In this work, UO2 was monitored as a function of time (6–24 months) and temperature at constant relative humidity (81% RH) in the presence of ammonium sulfate. The morphology of the aged material was described using scanning electron microscopy and a previously published lexicon, while phase changes were confirmed using X-ray diffraction. Additional characterization was carried out using energy dispersive X-ray spectroscopy, transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy to further identify aging effects. Over time, UO2 transitioned to a mixed phase assemblage. Among the alteration was ammonium uranate, a common fuel cycle compound, which was identified from its morphological and spectral characteristics. This work highlights the affinity and persistence of ammonium uranate compounds and emphasizes the need to continue investigating these alteration systems.

The paper discusses a method developed for the determination of uranium in uranyl nitrate raffinate (UNR) generated through sodium di-uranate (SDU) process using inductively coupled plasma atomic emission spectrometer (ICP-AES). High concentration of Na in UNR limits the direct determination of uranium in ICP-AES. Therefore, selective extraction of uranium from UNR using tri-n-octyl-phosphine oxide (TOPO) and stripping by ammonium carbonate was followed. Effect of sodium on estimation of uranium is discussed. The method has been validated by standard recovery of spiked real time UNR samples with known amounts of uranium CRM (Merck-HC42984673). The developed method compared well with EDXRFS and Redox titrimetric methods.

Chemical stabilization of tank residual waste is part of a Hanford Site tank closure strategy to reduce overall risk levels to human health and the environment. In this study, a set of column leaching experiments using tank C-104 residual waste were conducted to evaluate the leachability of uranium (U) and technetium (Tc) where grout and hydrated lime were applied as chemical stabilizing agents. The experiments were designed to simulate future scenarios where meteoric water infiltrates through the vadose zones into the interior of the tank filled with layers of grout or hydrated lime, and then contacts the residual waste. Effluent concentrations of U and Tc were monitored and compared among three different packing columns (waste only, waste + grout, and waste + grout + hydrated lime). Geochemical modeling of the effluent compositions was conducted to determine saturation indices of uranium solid phases that could control the solubility of uranium. The results indicate that addition of hydrated lime strongly stabilized the uranium through transforming uranium to a highly insoluble calcium uranate (CaUO4) or similar phase, whereas no significant stabilization effect of grout or hydrated lime was observed on Tc leachability. The result implies that hydrated lime could be a great candidate for stabilizing Hanford tank residual wastes where uranium is one of the main concerns.

Solvent extraction studies on the purification of uranium from zirconium rich sodium diuranate (SDU) feed was carried out using n-tri butyl phosphate (TBP) as extractant and n-decanol as phase modifier. The presence of Zr in SDU leached solution leads to the formation of third phase during liquid–liquid extraction of uranium which was successfully prevented by addition of n-decanol in 30% (v/v) TBP/n-dodecane mixture. A seven stage counter current extraction of SDU feed solution followed by five stage stripping were carried out using optimum concentration of phase modifier 15% n-decanol-30% TBP in n-dodecane as solvent. Based on the findings a process flow-sheet has been developed for the purification of SDU to nuclear grade ammonium diuranate.

Actinyl species, [AnO2]2+, are well-known derivatives of the f-block because of their natural occurrence and essential roles in the nuclear fuel cycle. Along with their nitrogen analogues, [An(NR)2]2+, actinyls are characterized by their two strong trans-An–element multiple bonds, a consequence of the inverse trans influence. We report that these robust bonds can be weakened significantly by increasing the number of multiple bonds to uranium, as demonstrated by a family of uranium(VI) dianions bearing four U–N multiple bonds, [M]2[U(NR)4] (M = Li, Na, K, Rb, Cs). Their geometry is dictated by cation coordination and sterics rather than by electronic factors. Multiple bond weakening by the addition of strong π donors has the potential for applications in the processing of high-valent actinyls, commonly found in environmental pollutants and spent nuclear fuels. The field of high-valent uranium chemistry has been dominated by the linear uranyl moiety [UO2]2+ and its imido analogues. A family of tetrakis(imido)uranate dianions has now been developed that displays four uranium–nitrogen multiple bonds. Their geometry is dictated by cation coordination and steric factors rather than electronic ones.

Alkaline-earth (calcium, strontium, and barium) borophosphates doped with uranium (U) are prepared through conventional solid-state reaction route. The form of stabilized uranium in these solid matrices was characterized and investigated using X-ray diffraction (XRD), photo luminescence (PL) and extended X-ray absorption fine structure (EXAFS). XRD measurements confirmed the single phase formation of uranium-doped alkaline-earth borophosphate samples. The PL characteristics of uranium in calcium and barium borophosphate are studied for the first time. Photoluminescence studies indicated presence of uranium as Uranyl in SrBPO5 and CaBPO5 matrices whereas in case of BaBPO5 the indication was for uranate species. The life time data corroborated the presence of a different uranium species in SrBPO5, CaBPO5, and BaBPO5. The site occupancy of uranium was further probed using EXAFS which confirmed that in case of BaBPO5, uranium enters the host as uranate whereas in case of SrBPO5 and CaBPO5, uranium enters as Uranyl.

A combination of simultaneous thermal analysis, evolved gas analysis and non-ambient XRD techniques was used to characterise and investigate the conversion reactions of ammonium uranates into uranium oxides. Two solid phases of the ternary system NH 3 − UO 3 − H 2 O were synthesised under specified conditions. Microspheres prepared by the sol-gel method via internal gelation were identified as 3 UO 3 ⋅ 2 NH 3 ⋅ 4 H 2 O , whereas the product of a typical ammonium diuranate precipitation reaction was associated to the composition 3 UO 3 ⋅ NH 3 ⋅ 5 H 2 O . The thermal decomposition profile of both compounds in air feature distinct reaction steps towards the conversion to U 3 O 8 , owing to the successive release of water and ammonia molecules. Both compounds are converted into α- U 3 O 8 above 550 °C, but the crystallographic transition occurs differently. In compound 3 UO 3 ⋅ NH 3 ⋅ 5 H 2 O (ADU) the transformation occurs via the crystalline β-UO3 phase, whereas in compound 3 UO 3 ⋅ 2 NH 3 ⋅ 4 H 2 O (microspheres) an amorphous UO 3 intermediate was observed. The new insights obtained on these uranate systems improve the information base for designing and synthesising minor actinide-containing target materials in future applications.

High-efficiency extraction of uranium (U) is a key to determining the feasibility of radioactive hazards reducing and nuclear resources supplementing. So in this study, the influences of additives on U extraction during combustion were shown via Tessier extraction procedures and leaching tests. By co-firing U-rich coal with alkali metal-bearing, alkaline earth-bearing, and transition metal-bearing additives, the proportions of active U are decreasing, increasing, and ambiguous, respectively, among which CaCO3 displays the most remarkable improvement on U extraction. Thus, combining aqueous experiments with some characterizations (e.g., X-ray diffraction, scanning electron microscopy-energy dispersive spectroscopy), the fates of U during combustion with/without CaCO3 were comparatively studied as the roles of times (0–120 min), temperatures (450–1050 °C), and CaCO3 dosages (0–30%). Changes mainly include three points as co-firing time rising: 1) the oxidation of organics is suppressive in the CO2 atmosphere (owing to CaCO3 thermolysis), so U ions strongly associating with organics cannot turn into UOX at times ≤12 min; 2) the favorable captures of acidic phases, including UOx, by CaO promote the generation of uranate and its redistribution in alkaline eutectics; 3) although the former changes stabilize the existence of active U, iron oxides compete for the capture of UOX, contributing to its encapsulation in acid-insoluble glass. After co-firing with 20% CaCO3 and acid extraction, despite ~23.7% of U remaining in residue, it is promising to extract U by the method here, given a considerable improvement (~22.0%) on U extraction only under 5% CaCO3.

Interaction behavior of uranyl nitrate hexahydrate (UNH) and lanthanum nitrate hexahydrate (LaNH) have been investigated on the mixtures in different molar ratios of the two precursors and monitoring the reactions at elevated temperatures with thermoanalytical and X-ray diffraction measurement techniques. During the decomposition of equimolar mixture of LaNH and UNH, formation of lanthanum uranate (U 0.5 La 0.5 )O 2 , was seen by the temperature of 500 °C along with lanthanum oxide (La 2 O 3 ) and uranium trioxide (UO 3 ). By the temperature of 700 °C, the formation of uranium sesquioxide (U 3 O 8 ) was observed along with (U 0.5 La 0.5 )O 2 as end products in uranium rich mixtures. Lanthanum rich compositions decomposed by the temperature of 700 °C to give (U 0.5 La 0.5 )O 2 and La 2 O 3 as end products.

Large stores of unstable waste uranic materials such as fluorides or nitrates exist internationally due to legacy civil nuclear enrichment activities. Conversion of these uranic materials to layered metal uranates prior to disposal is possible via aqueous quench - precipitation type reactions. Previous studies1 have shown facile in-situ formation of geologically persistent and labile uranate colloids2 under simulated nuclear waste repository conditions, though the effects of local solution metal-uranium ratios on uranate stoichiometry have yet to be covered. This affects our understanding of how key radionuclides present in repository porewaters such as strontium or caesium may be sequestered in these uranate structures. In this work, we demonstrate a synthesis reaction for calcium monouranate particles via rapid anhydrous curing of a sol-gel. We present some results showing aqueous nucleation of uranate nanoparticles and their phase transformations during thermal curing as well as the effects of solution phase calcium loading on uranate phase purity in the cured particles.

Uranium is one of the typical naturally occurring radioactive materials in coal. The release and speciation transformation of uranium was investigated at various combustion temperatures, and the thermodynamic modelling was performed to complement the experimental work. The results showed that the uranium release ratio did not increase consistently with the combustion temperature increasing, where the highest release ratio occurred at 500 °C. At the temperature range of 500–900 °C, the uranium release ratio obviously decreased, which could be attributed to the formation of uranate with the interaction of alkaline/alkaline-earth metal compounds in coal. However, some of the thermal unstable uranate was decomposed and released at the temperature above 1000 °C, while part of them remains stable in the combustion product even when the sample was heated at 1200 °C. Further, the interaction mechanism of uranium and alkaline/alkaline-earth metals during coal combustion was proposed based on the experimental and modelling results. This study will provide valuable information for understanding the primary factors and processes that affect the release of uranium during coal combustion.

An individual crystalline compound Pb(UO2)2O2(OH)2·(H2O) was obtained by reaction of synthetic schoepite UO3·2.25H2O with an aqueous solution of lead(II) nitrate under hydrothermal conditions. The composition and structure of this compound were determined, and the processes of its dehydration and thermal decomposition were studied by chemical analysis, X-ray diffraction, IR spectroscopy, and thermography.

Uranium doped Nd2Zr2O7 is synthesized at 800 °C (NUZO-800) using gel-combustion method and characterized using X-ray diffraction, scanning electron microscopy (SEM), X-ray photoelectron microscopy (XPS) and time-resolved photoluminescence (TRPL). As prepared sample was annealed further at 900, 1000, 1100 and 1200 °C leading to increase in the size of the particle and the effect of same has been investigated on the photophysical properties of NUZO. Based on excitation and emission spectroscopy it was inferred that uranium stabilizes as +6 oxidation state in the form of UO 6 6 − . XPS measurement also establishes +6 oxidation state of uranium in Nd2Zr2O7. Both emission intensity and photoluminescence (PL) decay time increases with increase in temperature till 1000 °C beyond which lifetime saturates whereas emission intensity decreases. The decay curve shows two different chemical environments for uranate ion in Nd2Zr2O7 pyrochlore matrix.

Two novel Ca2(UO3)(TeO3)2 (1) and K2(UO2)2O2(TeO3) (2) uranyl tellurites were obtained from telluric acid, used as a starting reagent for both compounds. In 1, the tetraoxido core is coordinated by TeO3 groups and UO4 squares polymerize into [UO3] chains. The tetraoxido core coordination modes in compound 1 are unique. New layered ⋡2[(UO2)2(TeO3)O2]2– topology is observed for 2. Both of the compounds were studied by the means of high-temperature X-ray diffraction. The thermal decomposition of 1 and 2 is different and leads to formation of uranate compounds.

The speciation of a particular element in any given matrix is a prerequisite to understanding its solubility and leaching properties. In this context, speciation of uranium in lanthanum zirconate pyrochlore (La2 Zr2 O7 = LZO), prepared by a low-temperature combustion route, was carried out using a simple photoluminescence lifetime technique. The LZO matrix is considered to be a potential ceramic host for fixing nuclear and actinide waste products generated during the nuclear fuel cycle. Special emphasis has been given to understanding the dynamics of the uranium species in the host as a function of annealing temperature and concentration. It was found that, in the LZO host, uranium is stabilized as the commonly encountered uranyl species (UO22+ ) up to a heat treatment of 500 °C at the surface. Above 500 °C, the uranyl ion is diffused into the matrix as the more symmetric octahedral uranate species (UO66- ). The uranate ions thus formed replace the six-coordinated 'Zr' atoms at regular lattice positions. Further, it was observed that concentration quenching takes place beyond 5 mol% of uranium doping. The mechanism of the quenching was found to be a multipolar interaction. Copyright © 2016 John Wiley & Sons, Ltd.

Zinc aluminate (ZAO), a member of spinel class of inorganic compounds has been of much interest of late due to its wide range of use in catalysis, optical, electronic and ceramic industries. When doped with several lanthanides, this material has proved to be a potential host matrix for phosphors. As lanthanides suffer from poor (direct) excitation and emission cross sections, the use of a co-dopant ion can help to circumvent this and extract better emission from a lanthanide doped ZAO system. In this connection, energy transfer mechanism from uranium to europium in the ZAO host was investigated by photoluminescence spectroscopic technique. It was seen that uranium gets stabilized in the hexavalent state as UO6(6-) (octahedral uranate) where as the lanthanide ion, Eu is stabilized in its trivalent state in the ZAO host. In the co-doped system, an efficient energy transfer pathway from the uranate to europium ion was observed. Based upon emission and life time data a suitable mechanism was proposed for the energy transfer (quenching) process. It was proposed that after excitation by photons, the uranate ions transfer their energy to nearby (5)D1 level of Eu(3+) ions which non-radiatively de-excites to the corresponding lower levels of (5)D0. Further this (5)D0 level decays in a radiative mode to the (7)F manifold giving the characteristic emission profile of trivalent Eu. It was proposed that both static and dynamic types of energy transfer mechanism were responsible for this process.

Although pentavalent uranium can exist in aqueous solution, its presence in the solid state is uncommon. Metal monouranates, MgUO4, CrUO4 and FeUO4 were synthesized for detailed structural and energetic investigations. Structural characteristics of these uranates used powder X-ray diffraction, synchrotron X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and (57)Fe-Mössbauer spectroscopy. Enthalpies of formation were measured by high temperature oxide melt solution calorimetry. Density functional theory (DFT) calculations provided both structural and energetic information. The measured structural and thermodynamic properties show good consistency with those predicted from DFT. The presence of U(5+) has been solidly confirmed in CrUO4 and FeUO4, which are thermodynamically stable compounds, and the origin and stability of U(5+) in the system was elaborated by DFT. The structural and thermodynamic behaviour of U(5+) elucidated in this work is relevant to fundamental actinide redox chemistry and to applications in the nuclear industry and radioactive waste disposal.

Rubidium uranate Rb4(UO2)8O7(OH)6·H2O was prepared by the reaction of an aqueous solution of uranyl acetate with an aqueous solution of rubidium nitrate under hydrothermal conditions at 200°С. The compound was studied by chemical analysis, X-ray diffraction, IR spectroscopy, and differential thermal analysis. The dehydration and thermal decomposition of the compound were studied.

Uranium is the main element present in spent nuclear fuel and accordingly contributes with the largest mass inventory to the nuclear waste. In spite of uranium being a relatively minor contributor to the overall radiological dose of the waste, it is certainly required to have an accurate knowledge on the solution chemistry and solubility phenomena of this key element. Uranium is also a redox-sensitive actinide, and accordingly its chemical behavior is strongly dependent on the redox boundary conditions of the system. Disposal of spent fuel in deep geological formations such as crystalline/granite, clay and rock salt is the option favored by international consensus. Water intrusion is a possible scenario that needs to be accounted for in the context of the long-term Safety Assessment of these repositories. The composition of the pore water contacting the waste will largely vary depending upon host-rock, backfill and other technical barriers, as well as the waste itself. Although a vast number of studies have previously investigated the solution chemistry of uranium, a number of key uncertainties remain. These affect to redox behavior, solid phases controlling solubility and hydrolysis, especially in the alkaline to hyperalkaline pH conditions of relevance in the context of nuclear waste disposal. U(IV) and U(VI) are the most stable oxidation states of uranium controlling its solution chemistry and solubility within the stability field of water and in the absence of strong complexing ligands. The study of this redox couple in the alkaline reducing conditions relevant in certain concepts for waste disposal (e.g. cementitious) is challenged by the further stabilization of U(VI) in the hyperalkaline pH-region, and the high sensitivity of U(IV) towards oxidation in the presence of traces of oxygen. Accordingly, an adequate knowledge of uranium redox chemistry in the aqueous and solid phases under geochemical boundary conditions (pH, pe, ionic strength, etc.) relevant in the context of nuclear waste disposal is important for a correct assessment of the long-term safety. For this reason, the redox chemistry of uranium in the presence of various reducing chemical systems in dilute to concentrated NaCl solutions is investigated in acidic to hyperalkaline pH conditions, and the results summarized in Chapter 3 of this PhD thesis. The kinetics of the reduction of U(VI) to U(IV), as well as the effect of the type and concentration of the reducing system are investigated by systematic measurements of the pHm (with pHm = –log [H+] in molal units), pe and U concentrations until attaining equilibrium conditions. Complete reduction of U(VI) to U(IV) is observed in most of the cases within the boundary conditions (pe + pHm) ≤ 4, although reduction kinetics are strongly impacted by [U(VI)]0, pHm, type and concentration of the reducing system and NaCl concentration. In (oversaturated) alkaline NaCl systems, solubility data and XANES indicate that the reduction proceeds via fast precipitation of Na2U2O7xH2O(cr), which slowly transforms into a UO2(am, hyd) solid phase. In less favourable conditions, the completion of this process required ≈ 635 days. These results also preclude the predominance of the U(IV) anionic hydrolysis species U(OH)5– and U(OH)62– below pHm ≈ 14.5, previously reported in the literature. Experimental data obtained within this PhD thesis indicate that previous observations reported in the literature can be possibly explained by insufficient equilibration time. Furthermore, this study confirms the key role of U(IV) in controlling the solubility and solution chemistry of uranium in reducing, alkaline systems. Very reducing conditions are expected to develop after the closure of underground repositories for nuclear waste disposal due to the anoxic corrosion of steel and iron components. As demonstrated in Chapter 3, U(IV) is expected to control the solubility and aqueous speciation of uranium under these very reducing conditions over a broad range of pH and background electrolyte concentrations. In spite of this, key uncertainties still affect the solution chemistry of U(IV), in particular with regard to the properties of the oxo-hydroxide/s solid phases forming, the aqueous speciation in alkaline to hyperalkaline pH conditions, as well as the formation and stability of U(IV) “intrinsic colloids”. In this context, Chapter 4 of this PhD thesis focuses on the investigation of the solubility and hydrolysis of U(IV) in reducing, dilute to concentrated NaCl, MgCl2 and CaCl2 solutions. A very thorough solid phase characterization including XRD, SEM-EDS, quantitative chemical analysis, EXAFS and TG-DTA confirms that a (nano-)crystalline phase, UO2xH2O(ncr), is responsible for the control of the solubility of U(IV) in the investigated conditions. The systematic investigation of the solubility of this solid phase in dilute to concentrated, acidic to hyperalkaline pHm conditions allows deriving comprehensive chemical, thermodynamic and SIT activity models for the system U4+–Na+–Mg2+–Ca2+–H+–Cl––OH––H2O(l). The investigation of supernatant solutions in solubility experiments without the use of phase separation methods gives also insight on the colloidal fraction in “equilibrium” with UO2xH2O(cr). Although a systematically increased uranium concentration (ca. 2–3 log10-units) is observed with respect to 10 kD ultrafiltered samples, a clear trend to decreasing [U]aq with longer equilibration times is also indicated in solubility experiments within t ≤ 200 days. Hence, the contribution of U(IV) “intrinsic colloids” to the solubility is evident in all salt systems investigated in this work, but the long-term stability of such species remains unclear. Uranium is mostly found as U(VI) under mildly reducing to oxidizing conditions. In the close vicinity of spent nuclear fuel surfaces, radiolysis effects can also promote the formation of U(VI) even in the presence of H2(g). Under alkaline pH conditions and in the absence of complexing ligands (e.g. carbonate, phosphate, silicate), the solubility of U(VI) is expectedly controlled by M–U(VI)–OH uranate solid phases (with M = Na, K, Ca, among others). In contrast to Ca- and Na-uranates, very little is known on the solubility of K–U(VI)–OH phases in spite of the abundance of K+ in many types of groundwaters and, in particular, the key role of this alkali ion in cementitious systems. In this context, Chapter 5 of this PhD is dedicated to the study of U(VI) solubility in alkaline, dilute to concentrated KCl solutions. Comprehensive solubility experiments with systematic variation of pHm and ionic strength, in combination with an extensive solid phase characterization (XRD, SEM–EDS, quantitative chemical analysis, TG-DTA) resulted in thermodynamic and activity models for the system UO22+–K+–Na+–H+–Cl––OH––H2O(l). Sensitivity analysis conducted using this updated thermodynamic model confirms that K- and Na-uranates (K2U2O7x1.5H2O(cr) and Na2U2O7xH2O(cr), respectively) are responsible of controlling the solubility of U(VI) under boundary conditions defined by cementitious systems. The absence of these solid phases in the corresponding thermodynamic databases leads to a very large overestimation (2–6 log10-units), depending upon pHm and alkali concentration) of U concentration in the underlined conditions. This work provides improved fundamental understanding of uranium solution chemistry, including redox processes, solubility phenomena and hydrolysis of both +IV and +VI redox states. Thermodynamic constants derived in the standard state and (SIT) ion interaction coefficients obtained can be implemented in thermodynamic databases and used in geochemical calculations under a variety of boundary conditions. This covers dilute to concentrated salt systems, thus allowing thermodynamic calculations under conditions representative of the different host-rocks foreseen for repositories for nuclear waste disposal, from crystalline and clay to salt-rock.

A chlorination process is proposed for the uranium extraction and separation using Calcium[sbnd]Magnesium uranates such as starting reactants which were obtained by precipitation from uranyl nitrate solutions with calcium hydroxide. The study is based on thermodynamic and reaction analysis using chlorine gas as chlorination agent. The results showed that the chlorination reaction of Ca uranate is more feasible to occur than the Mg uranate. The products obtained after chlorination reactions were washed with deionized water to remove the chlorides produced and analyzed. The XRD patterns of the washed products indicated that the chlorination between 400 and 500 °C result in a single phase of calcium uranate (CaUO4) as reaction product. The formation of U3O8 and MgU3O10 was observed at temperatures between 600 °C and 700 °C for 8 hs. The optimal conditions to recover uranium were 3 l h−1 of chlorine and 10 hs of reaction at 700 °C being U3O8 the single uranium product obtained. Fil: Pomiro, Federico J.. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Gaviría, Juan Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Quinteros, Raúl D.. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Bohe, Ana Ester. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional del Comahue; Argentina

The luminescence of uranium in strontium borate (SrB4O7, SBO) matrix was investigated by time resolved photoluminescence, thermoluminescence (TSL) and electron spin resonance techniques (ESR). The samples were synthesized using solid state fusion reaction route and characterized by X-ray diffraction. Photoluminescence excitation and emission data suggested the stabilization of uranium as uranate ( UO 6 6 - ) in the matrix. Luminescence decay time data suggested the stabilization of uranium at two different sites in the matrix. By giving suitable delay times and choosing proper gate widths, the two emission spectra due to the two uranate species could be obtained. Thermoluminescence investigation on the gamma-rays irradiated sample showed a strong glow peak at ∼415 K and a weak glow peak at 505 K. The dose response behavior, the trap parameters along with the order of kinetics for the strong glow peak were determined. To pinpoint the exact chemical nature of the defect centers responsible for the observed glow peaks, electron spin resonance technique was employed. Based on the ESR-TSL correlation data and the observed photoluminescence results, a plausible mechanism for the origin of the luminescence in the system was proposed.

A comprehensive study regarding the speciation of uranium in five different solids, namely, YBO3, Sr2P2O7, SrB4O7, SrBPO5 and SrZrO3 is presented using time resolved photoluminescence spectroscopy. The ‘Sr’ based hosts are considered as potential phosphor materials where as the borate based matrices are known to have near tissue equivalent absorption coefficients making them potential candidates for dosimetric applications. It was observed that, in case of the pyrophosphate, borophosphate and yttrium borate matrices, uranium gets stabilized as uranyl ( $$ {\text{UO}}_{2}^{2 + } $$ ), whereas, in case of the tetraborate and zirconate matrices, it was the uranate species, $$ {\text{UO}}_{6}^{6 - } $$ that gets stabilized preferentially.

SrZrO 3 :U was synthesized using gel-combustion method and characterized using powder X-ray diffraction. In view of the fact that zirconate based perovskite material can be a probable candidate for immobilization of nuclear waste, 1.0 mol% of uranium was doped in strontium zirconate and its valence state and coordination behavior was investigated using voltammetry and photoluminescence (PL) studies. Cyclic voltammetry, differential pulse voltammetry and UV–Visible spectroscopy have shown the existence of uranium in +6 oxidation state in strontium zirconate. Linearity of the current vs. scan rate plot showed that on doping, uranium is adsorbed in strontium zirconate matrix. The PL spectrum of the sample showed green emission band at 537 nm without any vibronic structure along with excitation peaks at 326, 362,392, 412, 424, 445 and 487 nm. This indicated the presence of uranium as uranate (UO 6 ) 6− in the strontium zirconate. These results were further corroborated with lifetime measurement.

Vorlanite $(CaU^{6+})O_{4}$ [$Fm\bar{3}m$, a = 5.3647(9) Å, V = 154.40(4) $A^{3}$, Z = 2] was found in larnite pyrometamorphic rocks of the Hatrurim formation at the Jabel Harmun locality, Judean Desert, Palestinian Autonomy. Vorlanite crystals from these larnite rocks are dark-gray with greenish hue in transmitted light. This color in transmitted light is in contrast to dark-red vorlanite [$Fm\bar{3}m$, a = 5.3813(2) Å, V = 155.834(10)$A^{3}$, Z = 2] from the type locality Upper Chegem caldera, Northern Caucasus. Heating above 750 $^{o}\textrm{C}$ of dark-gray vorlanite from the Jabel Harmun, as well as dark-red vorlanite from Caucasus, led to formation of yellow trigonal uranate $CaUO_{4}$. The unusual color of vorlanite from Jabel Harmun is assumed to be related to small impurities of tetravalent uranium.

Effect of ammonium nitrate on precipitation of Ammonium Di-Uranate (ADU) from nitric acid medium via gaseous ammonia route had been investigated. Studies on effect of ammonium nitrate on precipitation time, particle size, shape, surface morphology, flowability, oxygen/uranium (O/U) ratio and tap density of calcined ADU were carried out at various ammonium nitrate concentrations. It was observed that, the presence of excess ammonium nitrate influences the precipitation time, particle size distribution and surface morphology of the ADU. ADU and uranium oxide were characterized with Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). Presence of ammonium nitrate during precipitation leads to the formation of bigger, porous and uniform particles as compared to the ADU prepared without ammonium nitrate additions.

Current studies describe the application of simultaneous thermogravimetry–differential thermal analysis – evolved gas analysis techniques for the compositional characterization of magnesium diuranate (MDU) with respect to the impurities present in the matrix. The stoichiometric composition of MDU was identified as MgU 2 O 7 ⋅3H 2 O. Presence of carbonate and sulphate as impurities in the matrix was confirmed through the evolved gas analysis using Fourier Transformation Infrared Spectrometry detection. Carbon and magnesium hydroxide content present as impurities in magnesium diuranate have been determined quantitatively using TG and FTIR techniques and the results are in good agreement. Powder X-ray diffraction analysis of magnesium diuranate suggests the presence of magnesium hydroxide as impurity in the matrix. Also these studies confirm the formation of magnesium uranate, uranium sesquioxide and uranium dioxide above 1000 °C, due to the decomposition of magnesium diuranate.

Metal uranates(VI) are solubility limiting U(VI) phases under high pH conditions and may act as suitable long-term wasteforms. The precipitation and thermal phase development mechanisms of calcium uranate particles formed via aqueous hydroxylation reactions are studied in order to address the lack of aqueous synthesis methods currently available. Hydrous Ca-deficient uranate particles formed from aqueous solutions saturated in U(VI) oligomers were found to thermally decompose via several weight-loss steps between 100 and 800 °C. Crystalline calcium uranate (Ca2U3O11) is initially formed at 700 °C via dehydration and dehydroxylation-olation reactions under redox-neutral conditions. This initial phase decomposes to biphasic CaUO4-UO2 particles at 800 °C via a reductive pathway.