Uranium Incorporation Research Articles

Preparation of Optimized Uranium and Thorium Bearing Brabantite or Monazite/Brabantite Solid Solutions

The simultaneous incorporation of thorium and uranium(IV) in the rare‐earth orthophosphates from a mixture of powdered AnO2, (NH4)(H2PO4), and Ca(HPO4)·2H2O was successfully performed, leading to the formation of single‐phase Th‐brabantite (Ca0.5Th0.5PO4), U‐brabantite (Ca0.5U0.5PO4), and (Th,U)‐brabantites (Ca0.5Th0.5−yUyPO4). The repetition of several cycles composed of a grinding step (30 Hz, 15 min) and of a heat treatment (T=1200°C, 10 h, Ar atmosphere) allowed preparation of single‐phase and homogeneous compounds. The complete characterization of the samples by the means of X‐Ray powder diffraction, electron probe microanalyses, and μ‐Raman spectrometry did not reveal either the presence of minor phases or the partial oxidation of uranium(IV) into uranyl. This study also evidenced the formation of a complete Ca0.5Th0.5−yUyPO4 solid solution with the associated linear decrease of a, b, and c unit cell parameters and β increase versus the uranium mole loading. Even if the samples were found to be single phase, the significant improvement of the cation distribution in the materials was reached by the mean of the repetition of grinding/heating cycles. The same solid‐state process was applied with success to the preparation of homogeneous and single‐phase (Th,U)‐monazite/brabantite solid solutions with general formula Ln1−2xCaxThx−yUyPO4 (with 0.1≤x≤0.4 and y=x/5).

Experimental high pressure and high temperature study of the incorporation of uranium in Al-rich CaSiO 3 perovskite

The high ability of the Al-rich CaSiO 3 perovskite to contain large amounts of uranium (up to 4 at.% U) has been studied up to 54 GPa and 2400 K, using laser-heated diamond anvil cell (LH-DAC) and up to 18 GPa and 2200 K using a multi-anvil press (MAP). Both latter HP-HT techniques proved to be complementary and gave similar results, in spite of different heating modes (laser and furnace). Chemical reactions were characterized and described by electron probe microanalysis and analytical scanning electron microscopy while associated structural changes were precisely characterized by synchrotron angle dispersive X-ray diffraction and by X-ray micro-diffraction. The diffusion of uranium into the CaSiO 3 matrix was measured as a function of run duration and temperature. We obtain diffusion coefficients with the same order of magnitude (about 10 −16 m 2 s −1) than for those found in the literature. After this work, coupled cationic substitutions of Ca by U and Si by Al are proposed to generate new interesting crystallographic features for a CaSiO 3 perovskite: a higher compressibility, a tetragonal distortion along the c-axis with c/ a ratio >1, a different compression behaviour of c-axis relative to a-axis, and a perovskite structure quenchable to ambient P and T conditions. The tetragonal U-bearing aluminous CaSiO 3 perovskite is observed to remain stable at pressures up to 54 GPa, then in the ( P, T) range of the upper part of the lower mantle. The influence of the present results, in terms of both uranium and aluminium partitioning related to the coexisting mineral phases as the (Mg,Fe)SiO 3 perovskite, is discussed. Uranium provides approximately 25% of the total energy generated within the deep Earth through its radioactive decay. The location of this source within the deep mantle is fundamental to the understanding of the geodynamics and thermal behaviour of our planet. Since the tetragonal structure of the U-bearing Al-rich CaSiO 3 perovskite is expected to remain stable towards the base of the Earth's mantle, this latter phase is proposed to be the main storage mineral for heat producing actinides of the lower mantle.

Evaluation of uranium incorporation from contaminated areas using teeth as bioindicators--a case study

The Southwest region of the Bahia state in Brazil hosts the largest uranium reserve of the country (100 kton in uranium, only), plus the cities of Caetité, Lagoa Real and Igaporã. In this work, aim was at the investigation of uranium burdens on residents of these cities by using teeth as bioindicators, as a contribution for possible radiation protection measures. Thus, a total of 41 human teeth were collected, plus 50 from an allegedly uranium free area (the control region). Concentrations of uranium in teeth from residents of 5- to 87-y old were determined by means of a high-resolution inductively coupled plasma mass spectrometer (ICP-MS). The highest uranium concentration in teeth was measured from samples belonging to residents of Caetité (median equal to 16 ppb). Assuming that the uranium concentrations in teeth and bones are similar within 10-20% (for children and young adults), it concluded that uranium body levels in residents of Caetité are at least one order of magnitude higher than the worldwide average. This finding led to conclude that daily ingestion of uranium, from food and water, is equally high.

Inositol hexaphosphate: a potential chelating agent for uranium

Chelation therapy is an optimal method to reduce the radionuclide-related risks. In the case of uranium incorporation, the treatment of choice is so far i.v infusion of a 1.4% sodium bicarbonate solution, but the efficacy has been proved to be not very high. In this study, we examine the efficacy of some substances: bicarbonate, citrate, diethylenetriamine pentaacetic acid (DTPA), ethidronate (EHBP) and inositol hexaphosphate (phytic acid) to chelate uranium using a test developed by Braun et al. Different concentrations of phytic acid, an abundant component of plant seeds that is widely distributed in animal cells and tissues in substantial levels, were tested and compared to the same concentrations of sodium citrate, bicarbonate, EHBP and DTPA. The results showed a strong affinity of inositol hexaphosphate for uranium, suggesting that it could be an effective chelating agent for uranium in vivo.

Synthesis and characterization of uranium-bearing britholites

Unlike thorium which incorporation is easy and quantitative in the britholite structure, that of tetravalent uranium appears rather difficult even after heating at high temperature. Indeed, only about 3.5 wt% of uranium (instead of 10.3 wt% expected) are incorporated in the britholite structure when using a manual grinding of the initial mixture while it is increased to 5.9 wt% when performing mechanical grinding/heating cycles. The optimized grinding and heating conditions can be fixed to 15 min at 30 Hz and to 1400 °C for 6 h, respectively. All the samples prepared at 1400 °C are found to be composed by (Nd, U)-britholite and calcium uranate CaU 2O 5+ y which formation results from that of CaUO 4 above 800 °C consequently to the direct reaction between UO 2 and CaO. The incorporation of tetravalent uranium in (Nd, U)-britholites begins simultaneously to the transformation of CaUO 4 into CaU 2O 5+ y above 1100 °C. Due to these redox reactions, the incorporation of uranium remains partial even though it is significantly increased at 1400 °C and mainly proceeds through diffusion phenomena. Two main methods can be used to improve significantly the incorporation of uranium in (Nd, U)-britholites. The first one deals with the compaction of the powdered initial mixture prior to perform the heating treatment at 1400 °C. The second one is based on the simultaneous incorporation of tetravalent thorium and uranium, leading to the formation of (Nd, Th, U)-britholites. Like for uranium, such methods could be of significant interest in the field of the incorporation of other tetravalent actinides, e.g. neptunium or plutonium, which could be also stabilized in several oxidation states.

Structural aspects of barium borosilicate glasses containing thorium and uranium oxides

Barium borosilicate glasses incorporated with 15.86 wt% ThO 2 and containing different amounts of uranium oxide were prepared by conventional melt quench method. Based on 29Si and 11B magic angle spinning nuclear magnetic resonance (MAS NMR) studies, it has been confirmed that uranium oxide incorporation is associated with distortion of borosilicate network as revealed by the increase in the relative concentration of Q 2 structural units of silicon as well as the increase in the quadrupolar coupling constant ( C q) of BO 3 structural units. The increased number of non-bridging oxygen atoms brought about by the increase in Q 2 structural units of silicon facilitates the incorporation of both uranium and thorium ions in the sites created by non-bridging oxygen atoms (network modifying positions) in the glass. Uranium oxide incorporation above 7.5 wt% resulted in the phase separation of ThO 2 as revealed by the X-ray diffraction studies. The present study focuses on the structural changes with the borosilicate network of barium borosilicate glasses brought about by the introduction of thorium and uranium ions.

On the interest and the limits of using combined ESR/U-series model in the case of very late uranium uptake

Before ESR/U-series models were applied to ungulate teeth, it is generally assumed that calculated ages are close to the “real” ages when they range between the values obtained with the EU and LU models. When the samples show signs of uranium-leaching, a realistic mathematical modelling of the age is not possible. On the other hand, some samples have undergone a very late U-uptake “affecting” directly the U-series ages and rendering the calculated ages younger. We present results obtained on two open-air sites, which show the limits of such combined models in the case of late uranium incorporation. For the Middle Pleistocene site of Isernia la Pineta (Italy), the ESR dates are expected to fall within the limits imposed by the Ar–Ar results. The other site, the Lower Palaeolithic deposits of Ambrona (Spain), yielded two sets of samples: some at the top of the sequence with uranium uptake ranging between EU and LU limits and thus giving a good idea of the “real” age of the site; others at the base displaying a very late U-uptake, which render an age determination difficult.

Immobilization of tetravalent actinides in phosphate ceramics

Three phosphate-based ceramics were studied for the immobilization of tri- and tetravalent actinides: britholite Ca9Nd1-xAnxIV(PO4)5-x(SiO4)1+xF2, monazite/brabantite Ln1-2xIIICaxAnxIVPO4 and thorium phosphate diphosphate β-Th4-xAnxIV(PO4)4P2O7 (β-TPD). For each material, the incorporation of thorium and uranium (IV) was examined through dry chemistry routes, using mechanical grinding of the initial mixtures then heating at high temperature (1373–1673K). The quantitative incorporation of thorium in the britholite structure was obtained up to 20wt% through the coupled substitution (Nd3+,PO43-)⇔(Th4+,SiO44-). On the contrary, the incorporation of uranium was limited to 5–8wt% and always led to a two-phase system composed by U-britholite and CaU2O5+y. The incorporation of Th and U(IV) was also examined in both matrices, β-TPD and monazite/brabantite solid solutions. Homogeneous and single phase samples of β-TUPD and (Th,U)-monazite/brabantite solid solutions were obtained using successive cycles of mechanical grinding/calcination. The three matrices were prepared in the pellet form then leached in 10−1M or 10−4M HNO3 at 363K. The very low normalized dissolution rates confirmed the good resistance of the materials to aqueous alteration. Moreover, in over-saturation conditions, the formation of neoformed phases onto the surface of the pellets was evidenced for several sintered samples.

Evidence of a Stable Uranyl Site in Ancient Organic-Rich Calcite

The mechanism of uranium (U) incorporation into calcite (calcium carbonate) is of fundamental importance to the fate and transport of U at the surface and in the shallow subsurface and has implications for (a) the accuracy of U-Pb and U-series isotope ratio methods used to determine the ages of ancient deposits and (b) potential remediation strategies based on sequestration of U in the subsurface. Extended X-ray absorption fine structure (EXAFS) spectroscopy is uniquely suited to the study of U-calcite systems. The sensitivity of the EXAFS spectrum to the local atomic Ca coordination about U(VI) in the calcite structure results in an increase in the number and amplitude of Ca signals as the U(VI) becomes more ordered within the crystal structure. Our X-ray microprobe (10-microm) measurements of an ancient 298 million-year-old organic-rich calcite (calcrete) clearly revealed three coordination shells of Ca atoms, defining a well-ordered calcite structure about uranyl to a distance of approximately 6.5 angstroms. These results indicate that uranyl is incorporated at the Ca2+ site in calcite and that the uranyl environment may evolve over long time scales, becoming more calcite-like and more stable for long-term sequestration of uranium. These results therefore validate U-related dating methods and show that calcite can be effective at sequestering U in vadose zone sediments.

Electrochemical properties of LiMn 2O 4 cathode material doped with an actinide

Metal substation as an efficient approach for improvement of battery performance of LiMn 2O 4 was performed by an actinide dopant. Uranium as the last natural element and most common actinide was employed for this purpose. Cyclic voltammetric studies revealed that incorporation of uranium into LiMn 2O 4 spinel significantly improves electrochemical performance. It also strengthens the spinel stability to exhibit better cycleability. Surprisingly, the capacity increases upon cycling of LiU 0.01Mn 1.99O 4 cathode. This inverse behavior is attributed to uniform distribution of dopant during insertion/extraction process. In other words, this is an electrochemical refinement of the nanostructure which is not detectable in microscale morphology, as rearrangement of dopant in nanoscale occurs and this is an unexceptional nanostructural ordering. In addition, uranium doping strengthens the Li diffusion, particularly at redox potentials.

Small angle X-ray scattering study on the pore characteristics of U-substituted MCM-48 metallosilicates

The technique of small angle X-ray scattering (SAXS) has been employed to monitor the influence of uranium substitution on the pore characteristics of MCM-48 silica mesoporous material. The incorporation of uranium, as uranyl ion, at the framework silica sites resulted in the progressive increase in average pore size, the extent of which depended upon uranium loading. These results are in agreement with powder XRD results, showing a progressive increase in unit cell constant ( a 0) with increasing U-loading. The changes in the pore structure and the unit cell parameter are attributed to the perturbations arising due to bonding of (O U O) 2+ units to Si–O − units, thus forming a local uranate type structure. However, the increase in uranium content beyond 3 wt.% resulted in partial precipitation of uranium oxide crystallites, thus causing blocking/narrowing of some of the pores in the host material. SAXS results reveal that, in spite of these structural changes, the fractal nature of the host material remained by-and-large unchanged. These findings are in harmony with the results of the parallel studies conducted using N 2-adsorption and TEM studies.

Uranium incorporation biokinetics in poultry bones as function of phytase doses

Neutron activation analysis has been used to study uranium incorporation in poultry bones as function of chow doped with: (a) uranium (20 ppm); (b) U-doped food (20 ppm) plus phytase (120 ppm) and (c) U-doped food (20 ppm) plus phytase (180 ppm). To investigate this situation experiments involving several groups of Cobb broilers was performed. Two animals per group were sacrificed weekly up to their adultness and uranium concentration in the tibia was measured. It was observed that the concentration of uranium (µg U/g bone) is decreasing all along the animal life spanning period of 14-42 days. This behavior suggests that the skeleton mass is growing faster than the corresponding accumulation of uranium. The administration of phytase seems not to alter this scenario.

Uranium incorporation biokinetics in poultry bones as function of phytase doses

Uranium incorporation biokinetics in poultry bones as function of phytase doses

Evaluation of Uranium Coprecipitation with Sodium Aluminosilicate Phases

Batch laboratory experiments performed to evaluate uranium incorporation into aluminosilicate structures during synthesis are described. This research was conducted in response to plant problems related to the accumulation of uranium with aluminosilicates in low-level radioactive waste evaporators. Conditions that favor precipitation of aluminosilicates also foster uranium solid precipitation, so it is difficult to attribute problems with uranium accumulation to, for example, only the formation of the aluminosilicates. Infrared spectra show that sodium uranates, uranium silicates, and other uranium solids are formed during the synthesis of sodium aluminosilicates structures in the presence of uranium. Both amorphous and sodalite aluminosilicate phases, unlike the zeolite A phase, show appreciable affinity for uranium incorporation during their formation in the presence of uranium.

Hydrothermal Synthesis and Characterization of Uranium-Containing MCM-48 Samples

MCM-48 samples containing uranium (U−MCM-48) to the extent of 1 to 6 wt % were synthesized hydrothermally and characterized using powder XRD, 29Si MAS NMR, FTIR spectroscopy, DR UV−Vis spectroscopy, N2 sorption, and TEM techniques. These analyses confirmed the formation of an ordered cubic-phase MCM-48 mesomaterial where the uranium existed in the hexavalent state forming a local uranate type structure. A part of the uranyl groups were found to be incorporated in the framework positions, while the rest were anchored on to the wall of the host silica matrix. Also, the U−MCM-48 samples exhibited an enlargement of the cubic cell unit as well as an increase in the degree of cross-linking, as revealed by XRD and MAS NMR results. The samples exhibited N2 sorption isotherms of type IV and an increase was observed in the pore diameter as a result of uranium incorporation. TEM investigation revealed that although U−MCM-48 samples were of cubic symmetry similar to that of Si−MCM-48, some distortion in the pore structure was clearly noticeable. Furthermore, in case of the samples with U loading of ∼6 wt %, formation of nanocrystallites of α-U3O8 was also detected after calcination and such samples consisted of a multiple-size pore system.

Uranium enrichment shorewards in black shales: A case study from the Scandinavian Alum Shale

The Scandinavian Alum Shale Formation (Middle Cambrian to Lower Ordovician) contains high levels of organic carbon (up to 25 wt.%) and syngenetic enriched trace elements. A regional survey of the uranium and organic carbon content reveals that uranium is preferentially enriched in sections located palaeo-shorewards compared with sections located farther offshore. This type of enrichment mode is evident in the Upper Cambrian (above the A. pisiformis Zone) and Tremadoc interval. In the Middle Cambrian no significant regional variation in uranium level is present. In the most enriched Upper Cambrian biozone (Peltura scarabaeoides Zone) the average concentrations of uranium (100 to 300 ppm) are inversely correlated to zone thickness. The variable bed thicknesses are believed to represent primary differences in the rate of deposition and the relationship thus indicates a strong time-dependency in the enrichment processes governing uranium incorporation in sediments. The high uranium levels generally found shorewards are interpreted to reflect a more vigorous bottom water circulation that promoted higher rates of mass-transfer across the sediment/water interface relatively to the mud deposited farther offshore. Very high levels of uranium (1000 to 8000 ppm) concentrated in discrete beds (known as kolm) are interpreted to reflect resuspension of sediment in an anoxic water column that enhanced diffusive exchange between suspended particles and sea-water. A Late Silurian to Early Devonian thermal overprint towards the Caledonian Front affected the total organic carbon (TOC) content but appears to have no effect on the uranium distribution in the thermally mature sections.

STUDY OF WORKER'S EXPOSURE TO THORIUM, URANIUM AND NIOBIUM MINERAL DUST

The aim of this study is to evaluate worker’s exposure to particles, with special attention to thorium, uranium and niobiumbearing particles. A cascade impactor (CI) with six stages was used to collect particles with aerodynamic diameter in the rangeof 0.64 to 19.4 μm. The elemental mass concentrations impacted in each stage of the CI were determined using the PIXE(Particle Induced X-ray Emission) technique. The Mass Median Aerodynamic Diameter (MMAD) and elemental mass concentration in the fine fraction of aerosol (0.5 to 2.5 μm) were determined.Personal air samplers were used by workers to collect fine particles of aerosol. The concentrations of 232Th and 228Th in these air samplers were measured by the alpha spectrometry technique. Faeces and urine samples from the workersand their wives were analyzed to determine the thorium, uranium and niobium mass concentration (mass spectrometry) and 232Th, 228Th, 238U and 234U concentrations (α spectrometry) and then compared to inhabitants of regions with low metal concentrations. The analysis of thefaecesand urine samples from workers and their wives indicated that themain pathway of thorium, uranium and niobium incorporation was byingestion. The observed concentrations of niobium and uranium inurine samples from workers showed a systemic incorporation of niobium and uranium.

Isotopically heterogeneous initial Pb and continuous 222Rn loss in fossils: The U-Pb systematics of Brachiosaurus brancai

Diagenetic incorporation of uranium into bone material establishes a U-Pb system that provides a minimum age for the fossil. Loss of Rn and Ra from the 238U-decay series, recent mobility of U and Pb, and incorporation of radiogenic Pb from an old basement, which was available during diagenesis, disturbed the U-Pb system: All data fall in a 206Pb∗/238U-207Pb∗/235U (concordia) diagram to the right of the concordia. While samples that do not experience redistribution of U, Pb, 222Rn, and 226Ra should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic Pb, loss of 222Rn and 226Ra has led to displacement of the present data set to lower 206Pb/238U and, therefore, to younger intercept ages and to older 207Pb/206Pb ages. The 207Pb/204Pb-208Pb/204Pb systematics demonstrates that (1) three out of 19 samples experienced significant postdiagenetic fractionation of U and Pb and that (2) the diagenetic Pb addition was isotopically heterogeneous. The 235U/207Pb∗-206Pb∗/207Pb∗ and 206Pb∗/238U-207Pb∗/235U systematics combined demonstrate that (3) the samples experienced 5 to 18% 222Rn loss for a 1.9-Ga-old source of the diagenetically introduced Pb (and, correspondingly, more for a younger Pb source) and that (4) there is no unique solution for the age of the source of diagenetic Pb, since Pb from a younger source appears in the 235U/207Pb∗-206Pb∗/207Pb∗ diagram in the same way as higher 222Rn loss. No precise age can be determined from the data, although the most consistent solution falls in the range of 140 to 150 Ma. Addition of radiogenic Pb to the fossil bone is obvious only through its anomalous, highly radiogenic character and the isotopic heterogeneity in 208Pb/204Pb (no Th, i.e., no in situ 208Pb growth). The effect of unrecognized diagenetic additions of radiogenic Pb is an overestimation of the age, whereas unrecognized 222Rn loss results in a too-young discordant age. Since fossil material formed at times for which the discordia only shows a small curvature, anomalies originating from diagenetic Pb additions and 222Rn loss may not always be recognized.

What do we learn from peaks of uranium and of manganese in deep sea sediments?

Abstract Uranium and manganese are two redox-sensitive trace elements with complementary behavior. Under anoxic conditions, U is immobile, while reduced Mn is dissolved in pore water of sediments. Uranium diffuses from sea water into reducing sediments and is enriched there. The depth of the reducing layers and the sediment accumulation rate determine the enrichment of authigenic uranium. On the contrary, Mn is dissolved in the reducing sediment sections, diffuses in pore water and forms peaks of Mn-oxide as soon as oxygen is available. Thus, the profiles of both Mn and authigenic U record the depth of the redoxcline during periods of oxygen limitation in the sediments. Estimates of the depth of incorporation of uranium during the last glacial were derived for several cores at localities in the Atlantic. Numerical modeling of the redoxcline as a function of the oxygen content of the water column and of the flux of organic matter to the sediments, constrains these values during periods of extreme shallow redoxcline depths. Burn-down of organic matter and deepening of the redoxcline during the Holocene may have significantly altered the organic matter and uranium signals of glacial sediments. At present-day conditions, sediments accumulating at rates smaller than 2 cm ky −1 may have lost the U signals. Peaks of manganese are then the only reliable record for a less ventilated sediment column in the past.

Distribution of uranium and thorium in sediments and plants from a granitic fluvial area

A study of the presence of natural uranium and thorium isotopes in sediments and plants belonging to a granitic fluvial region of the Ortigas river (west of Spain) has been carried out. The existence of two uranium mines in the neighbourhood of the sampled sites and the granitic characteristics of the zone produce significant concentrations of natural radionuclides. Temporal and spatial variations of uranium and thorium concentrations and the activity ratios 234U/ 238U, 228Th/ 232Th and Th/U were studied to better understand the mobilization mechanisms such as leaching and transport at play in the studied system. These determinations were made using alpha-particle spectrometry with silicon detectors. The measurements were also compared with the results previously found for water of this fluvial area. Uranium in sediments showed variations due to changes in rainfall, but thorium content was nearly constant. Uranium and thorium concentrations in plants were lower after rainfall. Incorporation of uranium into the plants seemed to be mainly from water, whereas incorporation of thorium seemed to be from both sediments and water.