Slow Neutron Spectra in the Liquid and Solid Methane

An experimental study has been made on the carbothermic reaction of ThN with graphite in the form of loose powder mixture in vacuo. The effects of reaction temperature and particle size were examined. The graphite and ThN powders were mixed in proportions representing molecular ratios of about unity. Three kinds of samples of different particle sizes were utilized.

Gravimetric measurements on the reaction were made sothermally in the temperature range of 1,420°~1,630°C. For these samples of different particle sizes, the reaction kinetics were represented in common by first-order plots for the early part of the reaction. The apparent activation energies were about 60kcal/mol for the three samples.

X-ray examination revealed the reaction products to be mainly ThC_xN_1-x′ with some ThC₂ and/or ThC_1-x′ N_1-x′ in the case of the fine and the medium particle size samples. As for the coarse powder, the products were identified to be ThC_x′N_1-x and ThC_x′N_1-x′ without any ThC₂. As the reaction proceeded, each carbonitride took up more carbon and at the same time the carbon-rich carbonitride increased in quantity and the other decreased.     

Heat Capacity of Thorium Nitrides from 450 to 850K

The throium nitrides (ThN and Th₃N₄) were prepared by solid-gas reaction with thorium hydride and nitrogen.

The heat capacity of these samples were measured from 450 to 850K by a high-temperature double- adiabatic calorimeter and were determined as functions of temperature as follows:

Th₃N₄: C _p=41.30–8.47×10^?3 T-6.37×10⁵ T ^?2

ThN: C _p=12.50–2.66×10^?3 T-2.44×10⁵ T ^?2     

Hydrolysis of Uranium Nitrides

Studies were made on the hydrolysis of uranium nitrides (UN and cubic U₂N₃) by superheated steam. The resulting gaseous products were mainly hydrogen, though with U₂N₃ the hydrogen generation was replaced by that of nitrogen at higher temperatures. The condensed water contained ammonia, but not hydrazine. Both UN and U₂N₃ powders were ultimately converted to UO₂, after hydrolysis at temperatures up to 600°C; intermediate products (formed below 400°C) were presumably UO₂(N) or UN_1,7, in the case of UN and UN₁₈ with U₂N₃.

The reaction mechanisms corresponding to the formation of such chemical species were estimated mainly from the behavior of nitrogen and hydrogen evolution and from the yield of ammonia. The effect brought by N₂ atmosphere on the ammonia formation and on the dissolution of nitrogen into solids were studied also by the hydrolysis under flowing N₂ gas. It was revealed that the yield of ammonia increased and the dissolution was inhibited by N₂ atmosphere.     

Reaction between Uranium Mononitride and Graphite Powders

The carbothermic reaction of UN with graphite in the form of loose powder mixture has been studied in vacuo with the aid of a high-temperature thermobalance in the temperature range of 1,270~1,675°C. The gravimetric measurements were conducted continuously to determine the effect of varying reaction temperature and differences in sample particle size.

The kinetics of the reaction was found to follow the diffusion—controlled kinetic equation for mixed powder, and activation energies of 76.1, 55.1 and 72.7kcal/mol were obtained for samples of (1)—400 mesh UN and—400 mesh natural graphite mixture, (2)—150 +270 mesh UN and—150 +270 mesh natural graphite mixture, and (3)—150 +270 mesh UN and—150 +270 mesh artificial graphite mixture, respectively. X-ray diffraction studies revealed the reaction products of the coarse sample to be composed of two different kinds of UC_xN_1-x solid solutions, one having a x value of 0~0.2, and the other 0.8~0.9. From the above observations, the diffusion of C and/or N atoms through the outer carbon rich UC_xN_1-x layer may be considered to be the rate determining step.     

First-principles study of structural, elastic and electronic properties of thorium dicarbide (ThC2) polymorphs

The comparative study of the structural, elastic, cohesive and electronic properties of three polymorphs (α-monoclinic, β-tetragonal and γ-cubic) of thorium dicarbide ThC₂ is performed within the density-functional theory. The optimized atomic coordinates, lattice parameters, theoretical density (ρ), bulk moduli (B), compressibility (β), as well as electronic densities of states, electronic heat capacity (γ) and molar Pauli paramagnetic susceptibility (χ) for all ThC₂ polymorphs are obtained and analyzed in comparison with available experimental data. The peculiarities of inter-atomic bonding for thorium dicarbide are discussed. Besides, we have evaluated the formation energies (E_f) of ThC₂ polymorphs for different possible preparation routes (namely for the reactions with the participation of simple substances (metallic Th and graphite) or thorium monocarbide ThC and graphite). The results show that the synthesis of the ThC₂ polymorphs from simple substances is more favorable - in comparison with the reactions with participation of Th monocarbide.     

Vaporization of uranium-carbon-nitrogen system

The vaporization of species from the uranium-carbon-nitrogen system has been investigated by mass spectrometry with the Knudsen cell technique. After calibration with gold, the vapor pressures of U and N₂ have been obtained over UC_1?xN_x in the temperature range of 1900–2300K. For uranium mono-nitride, values of enthalpies Δ$\text{H}$°₂₉₈ for overall reaction (1)$\text{UN(s) = U(g) + 0.5N}{}_2\text{(g)}$ and the partial reactions (2)$\text{UN(s) = U(L) + 0.5N}{}_2\text{(g)}$ and $\text{U(L) = U(g)}$ have been obtained with both second- and third-law treatments. For UC_1?xN_x the experimental vapor pressures were used to calculate activity of UN in the solutions. For three compositions ($\text{x = 0.69}$, 0.48, 0.30) measured, activity coefficients were found to be slightly smaller than unity.     

Change of Composition of UC1-xNx in Nitrogen Gas

The UC-UN solid solution, UC_1-xN_x, exists stably in monophase within a certain range of nitrogen pressure at constant temperature. At the upper limit of this range, UC_1-xN_x, coexists with UC₂, or with carbon, and it precipitates metallic uranium at the lower limit which corresponds to the decomposition pressure of UC_1-xN_x.

In order to measure the decomposition temperature of UC_1-xN_x with given x under constant nitrogen pressure, it is necessary to heat UC_1-xN_x without changing its composition, at a constant pressure.

In the present work, preliminary considerations have been given to change in composition. Experiments were also performed in which the solid solution was heated at a rate of 200°C/min under different nitrogen pressures.

From the results, it is concluded that there exists a temperature range within which the value of x in UC_1-xN_x, is maintained constant when the nitrogen pressure is fixed.     

Structural investigation of thorium in molten lithium-calcium fluoride mixtures for salt treatment process in molten salt reactor

X-ray absorption fine structure (XAFS) measurements on thorium fluoride in molten lithium-calcium fluoride mixtures and molecular dynamics (MD) simulation of zirconium and yttrium fluoride in molten lithium-calcium fluoride mixtures have been carried out. In the molten state, coordination number of thorium (N_i) and inter ionic distances between thorium and fluorine in the first neighbor (r_i) are nearly constant in all mixtures. However the fluctuation factors (Debye-Waller factor (σ²) and C₃ cumulant) increase until xCaF₂ = 0.17 and decrease by addition of excess CaF₂. It means that the local structure around Th⁴⁺ is disordered until xCaF₂ = 0.17 and stabilized over xCaF₂ = 0.17. The variation of fluctuation factors is related to the number density of F⁻ in ThF₄ mixtures and the stability of local structure around Th⁴⁺ increases with decreasing the number density of F⁻ in ThF₄ mixtures. This tendency is common to those in the ZrF₄ and YF₃ mixtures. However, in the case of YF₃ mixtures, the local structure around Y³⁺ becomes disordered until xCaF₂ = 0.40 and it becomes stabilized by addition of excess CaF₂. The difference between ThF₄ mixtures and YF₃ mixtures is related to the difference of Coulumbic interaction between Th⁴⁺-F⁻ and Y³⁺-F⁻. Therefore, the variation of local structure around cation is related to not only number density of F⁻ in molten salts but also the Coulumbic interaction between cation and anion.     

A model for the carbon activity in nqnstoichiometric thorium carbide

The Rees formulation of nonstoichiometry is adopted together with a vacancy/single carbon atom/C₂ group model to calculate the hitherto largely unknown carbon activity in ThC_x for 0.0 ? x ? 2.0. The form of the carbon activity isotherms from ThC_0.9 to ThC_1.95 closely resembles the experimentally determined isotherms in the U/C system. Based on the occupation of two kinds of sites, the present formalism also generates an asymmetric shape of the miscibility gap between ThC and ThC₂ as found in both the real Th/C and U/C systems.     

Electronic structures, mechanical and thermodynamic properties of ThN from first-principles calculations

Lattice parameter, electronic structure, mechanical and thermodynamic properties of ThN are systematically studied using the projector-augmented-wave method and the generalized gradient approximation based on the density functional theory. The calculated electronic structure indicates the important contributions of Th 6d and 5f states to the Fermi-level electron occupation. Through Bader analysis it is found that the effective valencies in ThN can be represented as Th^+1.82 N^−1.82. Elastic constant calculations show that ThN is mechanically stable and elastically anisotropic. Furthermore, the melting curve of ThN is presented up to 120 GPa. Based on the phonon dispersion data, our calculated specific heat capacities including both lattice and conduction electron contributions agree well with experimental results in a wide range of temperature.     

Synthesis and characterization of low-temperature precursors of thorium-uranium (IV) phosphate-diphosphate solid solutions

Several compositions of new precursor of thorium-uranium (IV) phosphate-diphosphate solid solutions (Th_4−xU_x(PO₄)₄P₂O₇, called β-TUPD) were synthesized in closed PTFE containers either in autoclave (160 °C) or on sand bath (90-160 °C). All the samples appeared to be single phase. From XRD data and TEM observations, the diffraction lines matched well with that of pure thorium phosphate-hydrogenphosphate hydrate (TPHPH), Th₂(PO₄)₂(HPO₄) · H₂O, which confirmed the preparation of a complete solid solution between pure thorium and uranium (IV) compounds. TGA/DTA experiments showed that samples of thorium-uranium (IV) phosphate-hydrogenphosphate hydrate (TUPHPH) prepared at 150-160 °C were monohydrated leading to the proposed formula Th_2−x/2U_x/2(PO₄)₂(HPO₄) · H₂O. The variation of the XRD diagrams versus the heating temperature showed that TUPHPH remained crystallized and single phase from room temperature to 200 °C. After heating between 200 °C and 800 °C, the presence of diphosphate groups in the solid was evidenced. In this range of temperature, the solid was transformed into the low-temperature monoclinic form of thorium-uranium (IV) phosphate-diphosphate (α-TUPD). This latter compound finally turned into well-crystallized, homogeneous and single-phase β-TUPD (orthorhombic form) above 930-950 °C for x values lower than 2.80. For higher x values, a mixture of β-TUPD, α-Th_1−zU_zP₂O₇ and U_2−wTh_wO(PO₄)₂ was obtained. By this new chemical route of preparation of β-TUPD solid solutions, the homogeneity of the samples is significantly improved, especially considering the distribution of thorium and uranium.     

Kinetic and thermodynamic studies of the dissolution of thoria-urania solid solutions

The dissolution of Th_1−xU_xO₂ was investigated through leaching experiments combined with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses. These experiments were performed in acidic and in oxidizing conditions (nitric solutions), for several compositions of solid solutions ranging from x = 0.24 to 0.81. Static sequential experiments in acidic media performed at room temperature confirmed that higher concentration of uranium in the solid solution leads to higher release of uranium in the leachate whatever the pH. The normalized dissolution rate in oxidizing media is increasing all the more the content of uranium is increases in the mixed oxide. While for Th enriched solids, kinetic parameters remain similar to that of ThO₂, in the case of uranium enriched solids, a drastic change is observed, and kinetic parameters are similar to that of UO₂ ones. For x > 0.50, the saturation is reached in the leachate after 100 days. XPS and EXAFS analysis on leached samples pointed out an oxidation of U(IV) at the surface for x < 0.5, and in the bulk for x > 0.5. Enrichment in Th is also observed at the surface of the solid, indicating the formation of a protective layer of hydrated thorium oxide, or hydroxide. Finally, the solubility product of secondary phase was determined. The values obtained are in good agreement with that of ThO₂, Th(OH)₄ and ThO₂, xH₂O reported in the literature.     

Oxygen potentials of (th, u)o2+x solid solutions

The oxygen potential and the nonstoichiometry ($\text{x}$) for (Th_1?yU_y)O_2+x were measured in situ at 1000° 1&;#x0303;200°C using a solid electrolyte oxygen sensor and a thermobalance, respectively. The results showed that the oxygen potentials of (Th_1?yU_y)O_2+x are not a function only of the U valence in the investigated range of $\text{0.05?y?0.20}$. The oxygen potentials at 1200°C decrease negatively with increasing thorium content at a given U valence. The calculated activity coefficients of urania in the solutions indicate an increasing positive deviation from ideality with increasing U valence.     

Oxygen potentials of UO2 + x and (Th1 − yUy)O2 + x

Oxygen potentials of UO_{2 + x} and (Th_{1 ? y}U_y)O_{2 + x} (y = 0.2 and 0.4) were measured by means of thermogravimetry in the range of 1282 ≦ T ≦ 1373 K and 10su?19 ≦ P(O₂) ≦ 10^?1 Pa. The oxygen potentials of (Th_{1 ? y}U_y)O_{2 + x} plotted against the mean uranium valence (V_u) were seen to be more negative than those of UO_{2 + x} in the region of V_u < 4.08 and more positive than those of UO_{2 + x} in the region of V_u > 4.08. The partial molar enthalpies and entropies of oxygen for UO_{2 + x} and (Th_0.6U_0.4)O_{2 + x} showed that each has a maximum against $\text{O}\text{U}\text{or}\text{O}\text{M}$ ratio at about 2.001, but no clear maximum was seen for (Th_0.8U_0.2)O_{2 + x}. From the oxygen partial pressure dependences of x in UO_{2 + x} and (Th_{1 ? y}U_y)O_{2 + x}, the defect structure was discussed with a complex defect model consisting of oxygen vacancies and two different types of interstitial oxygens.     

Preparation and characterization of synthetic Th0.5U0.5SiO4 uranothorite

The preparation of a synthetic uranothorite with desired formula Th_0.5U_0.5SiO₄ was performed under hydrothermal conditions from a mixture of tetravalent thorium and uranium in hydrochloric solution with sodium metasilicate. The XRD rietveld analysis revealed that the system obtained was composed by two crystallized phases. The first one corresponded to uranium-depleted Th_0.57U_0.43SiO₄ with a = 7.0571(1) Å and c = 6.2998(1) Å that confirmed the formation of a solid solution between thorite and coffinite end-members. On the other hand, U-enriched Th_0.21U_0.79O₂ dioxide was also pointed out while the formation of amorphous SiO₂ was stated from elementary analyses and μ-Raman spectroscopy. This latter also led to confirm the formulation of the silicate by rejecting unambiguously the presence of structural hydroxyl groups or water molecules.     

Statistical thermodynamic study of the ternary compounds ThXyHx (X = C,N;y < 1;x < 2)

The stability and the nature of bonding in hypostoichiometric ThH₂, hcp ThC_0.46H_x and fcc ThN_yH_x (y = 0.486 and 0.692) are studied on the basis of statistical thermodynamics. The discussions are made in terms of the atomic partition function of H in these condensed phases and of the interaction energy terms. It is concluded that the nature of bonding in ThN_yH_x is approximately the same as that in ThH₂. On the other hand, the nature of bonding in ThC_yH_x is found to be significantly different from that in ThH₂.     

Swelling analysis of highly rated MX-type LMFBR fuels. I. Restructuring and porosity behaviour

Four MX-type fuels (U, Pu)C, (U, Pu)C_0.5N_0.5, (U, pU)c_0.2N_0.8, and (U, Pu)N have been irradiated in helium-bonded pins at heat ratings up to 1300 W/cm. The swelling behaviour has been analyzed on the basis of the complex restructuring occurring during the irradiation. The swelling mechanisms in the different restructured zones have been investigated with the aid of laboratory annealing experiments. Finally the properties of the different fuels have been discussed. Carbides provide acceptable swelling behaviour only under restricted temperature and temperature gradient conditions, whereas nitrides exhibit homogeneous performance in a wide temperature range. Carbonitrides with high nitrogen content provide an unfavourable in-pile restructuring, while carbonitrides with low nitrogen content display a carbide-type restructuring, but with more favourable features.     

CxH1-x薄膜制备

The C_xH_1-x film are prepared using low-pressure plasma CVD with trans-2-butene.The Fourier transform infrared spectroscopy (FT-IR) and xPS spectra of C_xH_1-x film prepared at various flow ratio between H₂ and trans-2-butene are studied.The growth mechanism of C_xH_1-x film is briefly discussed.The production rate of sp³ carbon and sp² carbon is related with the deposition condition.     

Oxygen non-stoichiometries in (Th0.7Ce0.3)O2−x

Oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x oxide solid solutions was investigated from the viewpoint of Ce reduction. The oxygen non-stoichiometry was experimentally determined by means of thermogravimetric analysis as a function of oxygen potential at 1173, 1273 and 1373 K. Features of the isotherms of oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x similar to those in oxygen non-stoichiometric actinide and lanthanide dioxides were observed. The oxygen non-stoichiometry in (Th_0.7Ce_0.3)O_2−x was compared with those of CeO_2−x and (U_0.7Ce_0.3)O_2−x. It was concluded that the Ce reduction has some relation to defect forms and their transformations in the solid solutions.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen— I synthesis of high purity UN

Formation of uranium mononitride by the reaction between UO₂ and C in an ammonia stream, a mixed 75%H₂-25%N₂ stream and a mixed 8%H₂-92%N₂ stream has been studied at 1400–1600°C. For the formation of high purity UN, there exists a minimum mixing ratio ($\text{C}\text{UO}{}_2$). It largely depends on the reaction atmosphere and temperature. The mixing ratio in the three atmospheres decreases in the following order: ammonia >75%H₂ + 25%N₂ > 8%H₂ + 92%N₂, and it decreases with increasing temperature. The total amount of impurities (carbon + oxygen) in the UN obtained is in the range of 500–1000 ppm. The time necessary for completion of the reaction in ammonia is shorter than in a mixed hydrogennitrogen.