ThO2 Research Articles

Fluorination of Rare Earth, Thorium and Uranium Oxides and Phosphates from Monazite: A Theoretical Approach

Monazite is a light rare earth phosphate which is difficult to process when using conventional chemical processes. It is considered to be one of the most important commercial sources of thorium and lanthanides. Using conventional chemicals to process monazite have caused severe environmental damage as has been demonstrated in the Baotou region, China. Monazite is found in combination with other minerals in nature such as bastnaesite and xenotime. The conventional techniques and chemicals used for the processing of monazite are expensive. In order to address this issue, South Africa would like to beneficiate monazite as part of its mineral beneficiation strategy. Doing so competitively would require a new cheaper and more environmentally friendlier process. A new method for the processing of monazite is currently being investigated. It is proposed to feed the monazite is fed into a plasma reactor to crack it. Potentially the cracking will allow the monazite to be more reactive and susceptible to react with less harsh chemicals. The plasma treated monazite is reacted with a fluorinating compound such as ammonium bifluoride. Ammonium bifluoride is used rather than fluorine or hydrogen fluoride as it is less dangerous to handle. The fluorinated rare earth mixture can now be separated using various methods.

The formation of submonolayer thorium coatings on a silicon oxide surface by electrochemical deposition

Data are reported on the mechanisms of the formation of submonolayer coatings based on thorium oxide on the surface of Si(111) single crystal with natural oxide as a result of electrochemical deposition. It is experimentally shown that the deposition of thorium atoms from an acetone solution of Th(NO3)4 onto the surface of natural silicon oxide leads to the formation of defects in the thin silicon oxide layer, which is accompanied by the deposition of thorium nanoclusters onto a pure silicon surface. The observed effects have been qualitatively explained assuming the breakdown of natural silicon oxide due to the presence of an electrical double layer in the near-surface region of the cathode.

Commercial thorium fuel manufacture and irradiation: Testing (Th, Pu)[formula omitted] and (Th, U)[formula omitted] in the “Seven-Thirty” program

Thorium based fuels are being tested in the Halden Research Reactor in Norway with the aim of producing the data necessary for licensing of these fuels in today’s light water reactors. The fuel types currently under irradiation are thorium oxide fuel with plutonium as the fissile component, and uranium fuel with thorium as an additive for enhancement of thermo-mechanical and neutronic fuel properties. Fuel temperatures, rod pressures and dimensional changes are monitored on-line for quantification of thermo-mechanical behavior and fission gas release. Preliminary irradiation results show benefits in terms of lower fuel temperatures, mainly caused by improved thermal conductivity of the thorium fuels. In parallel with the irradiation, a manufacturing procedure for thorium–plutonium mixed oxide fuel is developed with the aim to manufacture industrially relevant high-quality fuel pellets for the next phase of the irradiation campaign.

Advanced x-ray spectrometric techniques for characterization of nuclear materials: An overview of recent laboratory activities

Advancements in x-ray spectrometric techniques at different stages have made this technique suitable for characterization of nuclear materials with respect to trace/major element determinations and compositional uniformity studies. The two important features of total reflection x-ray fluorescence spectrometry: 1) requirement of very small amount of sample in ng level 2) multielement analytical capability, in addition to other features, make this technique very much suitable to nuclear materials characterization as most of the nuclear materials are radioactive and the radioactive waste generated and radiation hazards to the operator are minimum when such low amount of sample is used. Similarly advanced features of energy dispersive x-ray fluorescence e.g. better geometry for high flux, reduction in background due to application of radiation filters have made the measurements of samples sealed inside thin alkathene/PVC covers possible with good sensitivity. This approach avoids putting the instrument inside a glove box for measuring radioactive samples and makes the operation/maintenance of the instrument and analysis of the samples possible in easy and fast manner. This approach has been used for major element determinations in mixed uranium–plutonium samples. Similarly μ-XRF with brilliant and micro-focused excitation sources can be used for compositional uniformity study of reactor fuel pellets. A μ-XRF study using synchrotron light source has been made to assess the compositional uniformity of mixed uranium–thorium oxide pellets produced by different processes. This approach is simple as it does not involve any sample preparation and is non-destructive. A brief summary of such activities carried out in our laboratory in past as well as ongoing and planned for the future have been discussed in the present manuscript.

Measurement of the J = 1–0 pure rotational transition in excited vibrational states of [formula omitted] thorium (II) oxide, ThO

Thorium metal has been ablated by a Nd:YAG laser in the presence of oxygen and argon. The chemical products of this reaction have been entrained in supersonic pulses of argon gas and examined using Fourier transform microwave spectroscopy. One reaction product is thorium (II) oxide and for this species the J=1–0 pure rotational transition has been recorded in newly observed vibrational states ν=8 through ν=15. Combining this new data, with that of a previous study [1], has allowed for the rotational constants to be re-determined with higher precision. Analysis of the relative intensities of the observed transitions suggests a vibrational temperature in our experiment of approximately 2500±300K.

Light hydrocarbons production over bimetallic calcium–actinide oxide catalysts using N2O as oxidant

New bimetallic calcium–actinide (Th and U) oxides were synthesized and tested as catalysts for the oxidative coupling of methane (OCM) using nitrous oxide as oxidant. The reaction main products were primarily C2 hydrocarbons (C2H4 and C2H6), particularly over the bimetallic calcium–thorium oxide catalyst (CH4 conversion≈28%; C2 selectivity≈53%; C2 yield≈15% at 800°C). The selectivity to hydrocarbons is promoted when thorium replaces uranium on the bimetallic calcium–actinide oxides (at CH4 isoconversion=15±3%; C2 selectivity≈32% and 64% over the bimetallic Ca–U–O and Ca–Th–O catalysts at 750°C, respectively), which can be linked to the actinide oxide phase properties. The bimetallic calcium–actinide oxides catalytic performance correlates with their acid–base and conductivity properties (selectivity to hydrocarbons increases with the catalysts basicity and it is higher over p-type conductors, e.g. ThO2). The influence of the reaction temperature, gas phase CH4/N2O molar ratio and type of oxidant agent were also studied. In particular, the studies performed either under O2 or N2O confirm that the use of nitrous oxide improves the selectivity to hydrocarbons. To the best of our knowledge, this is the first time that an OCM study using N2O as oxidant agent is reported over bimetallic Ca–An (An=Th and U) oxides aiming the production of C2 hydrocarbons.

Stabilisation/solidification of radionuclide polluted soils — Part I: Assessment of setting time, mechanical resistance, γ-radiation shielding and leachate γ-radiation

Stabilisation/solidification treatment was performed in order to assess the possibility of remedying radionuclide polluted soils. A sandy soil, spiked by Thorium oxide (ThO2) at levels of 2.2 and 5.0%, was stabilised/solidified (S/S) using different binder mixtures of Portland cement (32.5 Rck and 42.5 Rck) and barite aggregates at different soil-binder ratios and tested for its setting time, unconfined compressive strength (UCS), γ radiation shielding and contaminant leaching.Results obtained from setting time tests showed that different treatments do not influence the performance of the treatment, whereas the level and the type of cement used significantly influenced the mechanical resistance of the S/S materials. The presence of the barite aggregates mixed with cement also gives a significant containment of the γ radiation and an excellent reduction of contaminant leaching but it slightly reduces the performance in terms of mechanical resistance, possibly representing an optimal choice to S/S treat low level radionuclide polluted soil. The obtained preliminary results are of scientific and practical interest and may be used in further research, whereas complete results will represent a suitable tool to optimize the treatment operating conditions and to guide the design and the scale-up of S/S treatment systems in full scale remediation activities of radionuclide polluted soils.

A tool to draw chemical equilibrium diagrams using SIT: Applications to geochemical systems and radionuclide solubility

ABSTRACTA set of computer programs has been developed to draw chemical-equilibrium diagrams. This new software is the Java-language equivalent to the Medusa/Hydra software (developed some time ago in Visual basic at the Royal Institute of Technology, Stockholm, Sweden). The main program, now named “Spana” calls Java programs based on the HaltaFall algorithm. The equilibrium constants that are needed for the calculations may be retrieved from a database included in the software package (“Database” program). This new software is intended for undergraduate students as well as researchers and professionals.The “Spana” code can be easily applied to perform radionuclide speciation and solubility calculations of minerals, including solubility calculations relevant for the performance assessment of a nuclear waste repository. In order to handle ionic strength corrections in such calculations several approaches can be applied. The “Spana” code is able to perform calculations based on three models: the Davies equation; an approximation to the model by Helgeson et al. (HKF); and the Specific Ion-Interaction Theory (SIT). Default SIT-coefficients may be used, which widens the applicability of SIT significantly.A comparison is made here among the different ionic strength approaches used by “Spana” (Davies, HKF, SIT) when modelling the chemistry of radionuclides and minerals of interest under the conditions of a geological repository for nuclear waste. For this purpose, amorphous hydrous Thorium(IV) oxide (ThO2(am)), Gypsum (CaSO4·2H2O) and Portlandite (Ca(OH)2) solubility at high ionic strengths have been modelled and compared to experimental data from the literature. Results show a good fitting between the calculated values and the experimental data especially for the SIT approach in a wide range of ionic strengths (0-4 M).

Calculation of the neutronic behavior of minor actinides burning in a thermal research reactor using the MCNPX 2.6 code

Abstract Due to the reduction of accessible uranium resources as well as waste proliferation issues, researchers are looking for more suitable approaches, such as replacement of uranium as breeding fuels. Among the practical fuel matrixes, the thorium fuel matrix is favored for its naturally abundant and minor actinide proliferation resistance. Monte Carlo computational methods are widely used to successfully simulate neutronic behavior of nuclear reactors. Calculation of some neutronic and dynamic parameters of a 37-assembly simulated research reactor consisting of thorium oxide fuel and 1 minor actinide pin have been carried out in the present work using the MCNPX 2.6 code.

Catalytic ketonization over metal oxide catalysts. XIII. Comparative measurements of activity of oxides of 32 chemical elements in ketonization of propanoic acid

The ketonization of propanoic acid in the presence of many metal oxides deposited on the surface of inorganic supports leads to the formation of 3-pentanone as the main product. This paper is the first to investigate simple oxides of Ag, Bi, Cd, Cu, In, Pb, Re, Cr, Mg, Zn, Ca, Ga, Sr, Ba, Al, Eu, Gd, V, Co, Fe, La, Mn, Zr, Ce, Th and U in the ketonization of propanoic acid under the same conditions. Such experiments allowed us to compare the activities of these oxides. Our results clearly show that the studied oxides can be divided into three groups: slightly active, fairly active and highly active. The last group consists of catalysts containing oxides of: manganese, zirconium, cerium, thorium, and uranium. It has also been checked whether any binary catalytic system containing two of these very active oxides shows a higher activity than the single oxide systems. However, no synergistic effect was noted between these oxides. Moreover, the influence of the addition of an alkali metal cation (K+) to the two most active catalysts on their activities has been studied. The results of these experiments indicate that the addition of potassium cations to uranium oxide significantly decreases its activity in this reaction due to the formation of potassium uranates, whereas thorium oxide is much more resistant to alkali metal ion poisoning. Another reason why thorium catalysts are better for practical implementation is their significantly lower radioactivity.

Thorium fuel production and results from beginning of life irradiation

An evolutionary, rather than a revolutionary approach to thorium fueled reactors is proposed with an introduction of thorium as the fertile component in mixed oxide fuel for conventional light water reactors. The utility of thorium as a component in today's light water reactors offers improved accident tolerance due to the superior material properties of thorium fuels over conventional uranium fuels. A few notable advantages include better thermal conductivity, higher melting point, higher oxide stability and superior spent fuel characteristics. Consequently, Thor Energy along with a consortium of industrial partners has established a fuel production and irradiation program aimed to license thorium fuels for use in today's light water reactors. Due to the morphology and physical properties of calcined thorium oxide powder, pellet fabrication used for standard uranium oxide fuels must be altered slightly for thorium bearing fuels to yield a product with acceptable theoretical densities, microstructure, and material integrity. At beginning of irradiation life fuel temperature data demonstrates improved fuel characteristics over standard uranium oxide fuel. Fuel centerline operating temperatures are 30–40 K less with a thorium mixed oxide fuel as compared to standard uranium fuel.

Warm white light emitting ThO2:Sm3+ nanorods: Cationic surfactant assisted reverse micellar synthesis and Photoluminescence properties

Sm3+ activated thorium oxide nanorods were synthesized by cationic surfactant assisted reverse micellar route. Phase purity, morphological and luminescent properties were investigated by X-ray diffraction, high resolution transmission electron microscopy and photoluminescence spectroscopy. Upon UV light excitation (245nm), ThO2:Sm3+ exhibited host emission at 447nm, along with characteristic emission lines of Sm3+ at 569, 609, 662 and 716nm. Lifetime spectroscopy shows the presence of two types of Sm3+ (τ=1.1ms and 4.9ms) with different asymmetric ratios.

Laser-induced breakdown spectroscopy measurements of uranium and thorium powders and uranium ore

Laser-induced breakdown spectroscopy (LIBS) was used to analyze depleted uranium and thorium oxide powders and uranium ore as a potential rapid in situ analysis technique in nuclear production facilities, environmental sampling, and in-field forensic applications. Material such as pressed pellets and metals, has been extensively studied using LIBS due to the high density of the material and more stable laser-induced plasma formation. Powders, on the other hand, are difficult to analyze using LIBS since ejection and removal of the powder occur in the laser interaction region. The capability of analyzing powders is important in allowing for rapid analysis of suspicious materials, environmental samples, or trace contamination on surfaces since it most closely represents field samples (soil, small particles, debris etc.). The rapid, in situ analysis of samples, including nuclear materials, also reduces costs in sample collection, transportation, sample preparation, and analysis time. Here we demonstrate the detection of actinides in oxide powders and within a uranium ore sample as both pressed pellets and powders on carbon adhesive discs for spectral comparison. The acquired LIBS spectra for both forms of the samples differ in overall intensity but yield a similar distribution of atomic emission spectral lines.

Synthesis and characterization of thorium, uranium and cerium oxide nanoparticles

Abstract We describe the synthesis of cerium, thorium and uranium oxide nanoparticles embedded in a mesoporous matrix as template in a kind of nanocasting technique. The solid matrix is used as a template to obtain and stabilize the actinide oxide nanoparticles. We apply high resolution transmission electron microscopy (HR-TEM) to show evidence of metal oxide incorporation into the matrix pores and analyze their structure. Measured interplanar distances and calculated lattice parameters for synthesized nanosized CeO2−x and ThO2 samples differ from their bulk crystalline counterparts. We obtain with our synthesis CeO2−x particles containing both Ce4+ and larger sized Ce3+. The lattice parameter for these ceria nanoparticles is found to be larger than the bulk value due to the presence of Ce3+ with its larger ionic radius. The presence of Ce3+ was established by means of high resolution X-ray emission spectroscopy (HRXES), applied to the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as one might generally expected for these nanoclusters. However, the lattice distance decrease for our particles is remarkable, up to 5%, indicating that contact with the surrounding silica matrix may exert a bond distance shortening effect such as through significant external pressure on the particle surface.

Controlled Synthesis of Thorium and Uranium Oxide Nanocrystals

Very little is known about the size and shape effects on the properties of actinide compounds. As a consequence, the controlled synthesis of well-defined actinide-based nanocrystals constitutes a fundamental step before studying their corresponding properties. In this paper, we report on the non-aqueous surfactant-assisted synthesis of thorium and uranium oxide nanocrystals. The final characteristics of thorium and uranium oxide nanocrystals can be easily tuned by controlling a few experimental parameters such as the nature of the actinide precursor and the composition of the organic system (e.g., the chemical nature of the surfactants and their relative concentrations). Additionally, the influence of these parameters on the outcome of the synthesis is highly dependent on the nature of the actinide element (thorium versus uranium). By using optimised experimental conditions, monodisperse isotropic uranium oxide nanocrystals with different sizes (4.5 and 10.7 nm) as well as branched nanocrystals (overall size ca. 5 nm), nanodots (ca. 4 nm) and nanorods (with ultra-small diameters of 1 nm) of thorium oxide were synthesised.

Experimental Studies on the Self-Shielding Effect in Fissile Fuel Breeding Measurement in Thorium Oxide Pellets Irradiated with 14 MeV Neutrons

The 14 MeV neutrons produced in the D-T fusion reactions have the potential of breeding Uranium-233 fissile fuel from fertile material Thorium-232. In order to estimate the amount of U-233 produced, experiments are carried out by irradiating thorium dioxide pellets with neutrons produced from a 14 MeV neutron generator. The objective of the present work is to measure the reaction rates of 232Th + 1n → 233Th → 233Pa → 233U in different pellet thicknesses to study the self-shielding effects and adopt a procedure for correction. An appropriate assembly consisting of high-density polyethylene is designed and fabricated to slow down the high-energy neutrons, in which Thorium pellets are irradiated. The amount of fissile fuel (233U) produced is estimated by measuring the 312 keV gammas emitted by Protactinium-233 (half-life of 27 days). A calibrated High Purity Germanium (HPGe) detector is used to measure the gamma ray spectrum. The amount of 233U produced by Th232 (n, γ) is calculated using MCNP code. The self-shielding effect is evaluated by calculating the reaction rates for different foil thickness. MCNP calculation results are compared with the experimental values and appropriate correction factors are estimated for self-shielding of neutrons and absorption of gamma rays.

Reanalysis of the Gas-cooled fast reactor experiments at the zero power facility Proteus – Spectral indices

The gas-cooled fast reactor (GCFR) concept was investigated experimentally in the PROTEUS zero power facility at the Paul Scherrer Inst. during the 1970's. The experimental program was aimed at neutronics studies specific to the GCFR and at the validation of nuclear data in fast spectra. A significant part of the program used thorium oxide and thorium metal fuel either distributed quasi-homogeneously in the reference PuO{sub 2}/UO{sub 2} lattice or introduced in the form of radial and axial blanket zones. Experimental results obtained at the time are still of high relevance in view of the current consideration of the Gas-cooled Fast Reactor (GFR) as a Generation-IV nuclear system, as also of the renewed interest in the thorium cycle. In this context, some of the experiments have been modeled with modern Monte Carlo codes to better account for the complex PROTEUS whole-reactor geometry and to allow validating recent continuous neutron cross-section libraries. As a first step, the MCNPX model was used to test the JEFF-3.1, JEFF-3.1.1, ENDF/B-VII.0 and JENDL-3.3 libraries against spectral indices, notably involving fission and capture of {sup 232}Th and {sup 237}Np, measured in GFR-like lattices. (authors)

Combining in situ HT-ESEM observations and dilatometry: An original and fast way to the sintering map of ThO2

The sintering of ThO2 pellets prepared from the initial precipitation of thorium oxalate was investigated by the means of HT-ESEM observations and dilatometric measurements. On the one hand, the use of environmental microscope allowed the in situ observation of the pellet behaviour during heat treatments between 1250 °C and 1400 °C. Subsequent image analysis led to the determination of local (i.e. at the grain scale) and global (i.e. at the pellet scale) kinetic parameters. Particularly, the average grain size was plotted versus the holding time for the three considered temperatures. On the other hand, analogous experiments were performed by dilatometry and led to monitor both linear shrinkage and density of the samples. On this basis, the combination of the two sets of data allowed us to establish for the first time a sintering map for ThO2. This latter clearly evidenced two different zones driven by densification (d ≤ 92%) then by grain growth (d ≥ 92%) which can be used efficiently to monitor the final microstructure of sintered thorium oxide.

Charge Distribution and Local Structure of Americium-Bearing Thorium Oxide Solid Solutions

The electronical and structural properties of Th(0.80)Am(0.20)O(2-x) materials have been studied by the coupling of X-ray diffraction and X-ray absorption spectroscopy techniques. A substoichiometric fluorite Th(IV)(0.80)Am(III)(0.20)O(1.90) solid solution is found following sintering in moisturized Ar-H(2). In contrast, heating of this sample in air leads to a nondefective fluorite Th(IV)(0.80)Am(IV)(0.20)O(2.00) solid solution. The structures of these solid solution compounds were fully characterized by assessing the interatomic distances, the coordination numbers, and the structural disorder. The effect of the sintering atmosphere on these crystallographical parameters and on the cation valences has been determined and the capability of ThO(2) to accommodate tri- and tetravalent actinides in the fluorite structure assessed.

Methane to Methanol Conversion Induced by Thorium Oxide through the CH3Th(O)H Intermediate in Solid Argon

Reactions of ThO molecules and CH(4) have been investigated in solid argon near 4 K. The CH(3)Th(O)H molecule is produced when the sample is exposed to UV irradiation. Identification of this new intermediate is substantiated by observation of the Th═O and Th-H stretching vibrational modes with isotopic substitution via matrix infrared spectroscopy, and the assignments are supported by electronic structure frequency calculations. Methanol absorptions increase together with formation of the CH(3)Th(O)H molecule, suggesting a methane to methanol conversion induced by thorium oxide proceeding through the CH(3)Th(O)H intermediate. The formation of CH(3)Th(O)H from ThO + CH(4) is exothermic (ΔH(rxn) = -11 kcal/mol) with an energy barrier of 30 kcal/mol at the CCSD(T)//B3LYP level. Decomposition of this intermediate to form methanol involves spin crossing, and the overall reaction from the intermediate is endothermic by 127 kcal/mol. There is no activation energy for the reaction of thorium atoms with methanol to give CH(3)Th(O)H, as observed in separate experiments with Th and CH(3)OH.