Phase study in Sr-Th-P-O system: Structural and thermal investigations of quaternary compounds

The sub-solidus phase relations in Sr-Th-P-O quaternary system were determined at 1223 K in air. To confirm the formation and stability of reported phases, ternary and quaternary compounds in Sr-Th-O, Sr-P-O, Th-P-O and Sr-Th-P-O systems were synthesized by solid state reactions of SrCO₃, ThO₂ and NH₄H₂PO₄ in desired molar proportions at 1223 K. A pseudo-ternary phase diagram of SrO-ThO₂-P₂O₅ system was drawn on the basis of the phase analysis of various phase mixtures and phase fields were established by powder X-ray diffraction. In the phase diagram, three quaternary compounds SrTh(PO₄)₂, SrTh₄(PO₄)₆ and Sr₇Th(PO₄)₆ were identified. When heated in air at 1673 K, these compounds decompose to ThO₂. Structures of SrTh(PO₄)₂, SrTh₄(PO₄)₆ and Sr₇Th(PO₄)₆ were derived from X-ray powder data using the Rietveld refinement method. Thermal expansion behaviors of SrTh(PO₄)₂, SrTh₄(PO₄)₆ and Sr₇Th(PO₄)₆ were investigated using high-temperature X-ray diffraction in the temperature range of 298-1273 K.     

Phase relations in the quaternary Fe-Pu-U-Zr system

The phase diagram in the quaternary Fe-Pu-U-Zr system was established at 923 K in the uranium-rich region to understand better the compatibility between the metal fuel and stainless steel cladding in a fast reactor. The experimental phase relation data obtained in this study was applied to the thermodynamic methodology for construction of the phase diagram. The calculated phase diagram was consistent and well within the experimental data. The applicability of the thermodynamic model to other temperatures was confirmed by comparing the present results of differential thermal analysis with the calculated phase diagrams. These consistencies mean that both the thermodynamic model and the assessed parameters in the binary and ternary subsystems developed so far are reasonable. The calculated phase diagram established in this study was also in good agreement with the analysis of the diffusion zone in tests on Pu-U-Zr/Fe couples. This suggests that the diffusion zone formed at the fuel-cladding interface in the reactor system can be assessed using the phase diagram in the quaternary Fe-Pu-U-Zr system.     

Phase relations in the U-Mo-Al ternary system

The phase relations in the U-Mo-Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo_2−xAl_20+x and U₆Mo_4+xAl_43−x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl₂ (cubic MgCu₂-type) giving a quite large extension (UAl_2−xMo_x, 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl_2−xMo_x with the hexagonal MgZn₂-type. There is no detectable solubility of Mo in UAl₄, and it is of the order of 1 at.% in UAl₃. The interaction layers between the γU-Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.     

Phase diagram of the zirconium-rhenium system

The phase diagram of the zirconium-rhenium system was constructed by x-ray and microscopic methods and by measurements of melting points, hardness and microhardness of the phases. The region of solid solutions of rhenium in α-zirconium at 800 ° C is ～0.5 weight %, and at the temperature of the eutectoid transition it increases to 2–3 weight %. At 1600 ° C, 14.68 weight % of rhenium dissolves in β-zirconium and at the eutectoid transition point at 550–600 ° C, 8 weight % of rhenium. The β-phase is stabilized in alloys containing more than 4 weight % of rhenium. An eutectic is formed at 1600 ° C and 25 weight % of rhenium. The presence of a metastable ω-phase in melts rich in zirconium was established. The solubility of zirconium in rhenium at 2500 ° C is less than 2 weight %. Three chemical compounds are formed in the system by peritectic reactions:

1. 1)
   At 2500 ° C, Zr₅Re₂₄ of the α-Mn type with a body-centered cubic lattice (a = 9.6–9.7 kX) and a microhardness of 1000 kg/ mm²;     
   
   

Phase diagram of the Nb-T system

The phase diagram of the niobium-tritium system was determined for the first time. Using X-ray diffraction and DTA techniques a shift of the monotectoid horizontal and of the boundary between the α' + β and α' phases to higher temperatures is observed for increasing hydrogen isotope mass. The monotectoid temperature is $\text{(103.9 + 1.0)°C}$ for T in Nb in comparison to $\text{(87.5 ± 1.0)°C}$ for H. In addition, a small decrease of the terminal solubility of the α phase at temperatures below the monotectoid horizontal was found with increasing isotope mass.     

Computational study of atomic mobility for the bcc phase of the U-Pu-Zr ternary system

Experimental diffusion data in literature has been evaluated to assess the atomic mobility for the bcc phase in the U-Pu-Zr system by means of the DICTRA-type (Diffusion Controlled TRAnsformation) phenomenological treatment. The developed mobility database has been validated by comprehensive comparisons made between the experimental and calculated diffusion coefficients, as well as other interesting details resulting from interdiffusion, e.g. the concentration profile and the diffusion path of diffusion couples.     

Thermodynamic and phase behaviour in the U-Rh-C system

A galvanic cell using a single crystal of CaF₂ as electrolyte has been used to measure the free energy of formation of the compounds occurring in the U-Rh-C system. Arc-melted samples were subjected to varying annealing conditions, then metallographic and electron probe micro-analysis were used for phase identification. In binary U-Rh system five compounds were found, URh₃, U₃Rh₅, U₃Rh₄, URh and U₂Rh. The free energy of formation values obtained for these compounds were:$\text{ΔG}{}_{\mathrm{<mtext>f</mtext>}}{}^0\text{(URh}{}_3\text{) = ?282.44 + 0.108TkJ/mol}$, $\text{ΔG}{}_{\mathrm{<mtext>f</mtext>}}{}^0\text{(}\text{U}{}_3\text{Rh}{}_5\text{) = ?623.02 + 0.252T}\text{kJ/mol}$, $\text{ΔG}{}_{\mathrm{<mtext>f</mtext>}}{}^0\text{(}\text{U}{}_3\text{Rh}{}_4\text{) = ?547.69 + 0.226T}\text{kJ/mol}$, $\text{ΔG}{}_{\mathrm{<mtext>f</mtext>}}{}^0\text{(}\text{URh}\text{) = ?152.81 + 0.0672T}\text{kJ/mol}$, $\text{ΔG}{}_{\mathrm{<mtext>f</mtext>}}{}^0\text{(}\text{U}{}_2\text{RhC}{}_2\text{) = ?303.38 + 0.0329T}\text{kJ/mol}$, all for the range 1000–1200 K and $\text{ΔG}{}_{\mathrm{<mtext>f</mtext>}}{}^0\text{(}\text{U}{}_2\text{Rh}\text{) = 182.08 + 0.085T}\text{kJ/mol}$ for 893–978 K. However, for U₂Rh the upper temperature range is not necessarily the transformation temperature of this compound.     

Thermodynamic analysis of phase equilibria in the iron-uranium-zirconium system

Thermodynamic analysis of phase equilibria in metallic nuclear reactor systems has progressed to the study of the iron-uranium-zirconium ternary system. This work is based on our previous assessments and optimizations of the constituent binary subsystems iron-uranium, iron-zirconium, and uranium-zirconium.     

Stability of the body-centered cubic gamma phase in the uranium-zirconium-niobium system

A thermodynamic representation of the stability of the binary gamma phases has been developed for the U-Zr, U-Nb and Zr-Nb binary systems. This has permitted the calculation of the miscibility gaps in the ternary system. The three binary gaps merge to form a large three-phase region in the middle of the composition trangle. Because of the large coherency energies, the spinodal decomposition in the U-Nb and Zr-Nb gaps will be drastically modified, but a large region remains in the ternary system where the decomposition products have small coherency energies such that spinodal decomposition is probable. From these results, we have identified the four decomposition processes in Mulberry (U-16.6 at% Nb-5.6 at% Zr) as being discontinuous and spinodal decomposition within the miscibility gap, the diffusionless formation of α and the formation of (α + γ) from the supersaturated gamma phase.     

Drop formation at nozzles submerged in quiescent continuous phase:an experimental study with TBP-dodecane and nitric acid system

Solvent extraction is an important process in the nuclear fuel cycle. Tributyl phosphate(TBP) diluted with dodecane is commonly used as a solvent for extracting heavy metals from nitric acid medium. Studies on hydrodynamics of a single drop, which is the smallest mass transfer entity, are required for better understanding of the complex mass transfer and phase separation phenomena that occur in extraction equipment. In this study, drop formation at nozzles is studied using 30% TBP-dodecane as the dispersed phase and dilute nitric acid as the quiescent continuous phase. Experiments are carried out to determine the drop diameter, jetting velocity, drop detachment height and drop detachment time for various dispersed phase velocities, nozzle diameters(1.91, 3.04, and 4.88 mm), and nitric acid concentrations(0.01, 1, 3 N). Drop formation is captured using high-speed imaging, which enables quantification of drop size, onset of jetting, drop detachment height, and drop detachment time. Experimental data are used to propose correlations for predicting drop diameter and minimum jetting velocity. The correlations are found to be very accurate with average absolute relative errors being 5.23 and 2.97%, respectively.     

Phase diagram investigations of K-Sr-U-O system

In continuation of our previous work on phase diagram investigations of alkali metal-alkaline earth metal-uranium oxides, the sub-solidus phase relations in K-Sr-U-O quaternary system were determined at 850 °C in air. A pseudo-ternary phase diagram of UO₃-SrO-K₂O was drawn using reported ternary compounds in K₂O-UO₃ and SrO-UO₃ systems, a single quaternary compound, K₈Sr₂U₆O₂₄ and various phase mixtures. These compounds and mixtures were synthesized by solid route, by heating K₂CO₃, SrCO₃ and UO₃ in different molar proportions at 850 °C. The phase fields of K₂O-SrO-UO₃ pseudo-ternary phase diagram were established by X-ray powder diffraction analysis. Gibbs energies of formation of binary and ternary uranates were estimated. The Gibbs energies of formations of A₈Sr₂U₆O₂₄ (A = Na/K/Rb/Cs) type compounds for different alkali metals were compared to find a reason for the stability of the compound for sodium, potassium and rubidium and its instability for cesium.     

Phase diagram of the ZrO2–FeO system

The results on the ZrO₂–FeO system studies in a neutral atmosphere are presented. The refined eutectic point has been found to correspond to a ZrO₂ concentration of 10.3 ± 0.6 mol% at 1332 ± 5 °C. The ultimate solubility of iron oxide in zirconia has been determined in a broad temperature range, taking into account the ZrO₂ polymorphism. A phase diagram of the pseudobinary system in question has been constructed.     

The phase diagram for the system zirconium-beryllium

Using the methods of metallographic, thermal, and x-ray qualitative phase analyses and by measuring the hardness, studies have been made of the system zirconium-beryllium, and its phase diagram has been plotted. The presence of four intermediate phases has been shown in the system: ZrBe₂, ZrBe₆, ZrBe₉. and ZrBe₁₃. The first three compounds formed by peritectic reactions at temperatures of 1235, 1475, and 1555 °C, respectively; the last compound melts with an open maximum at 1645 °C. At 965 °C and 5 weight % beryllium a eutectoid forms between the ZrBe₂ and the zirconium. Additions of beryllium to the zirconium lead to a reduction in temperature of the -ß transformation and to the formation of a eutectoid at 800 °C. The solubility of beryllium in -zirconium is less than 0.1 weight % and in ß-zirconium — less than 0.3 weight %. The solubility of zirconium in beryllium does not exceed 0.3 weight %.     

Phase relations and thermodynamic properties of the uranium-nitrogen system

The present state of knowledge of the phase relations and thermodynamic properties of the uranium-nitrogen system is given. Emphasis is placed on the nonstoichiometric and thermodynamic properties of uranium sesquinitride. The present paper consists of two parts. The first part is on the phase relations; an equilibrium phase diagram is proposed, and the relationship between structure and lattice parameter for the α-U₂N₃-UN₂ system is shown. The second part deals with thermodynamic properties of UN and α-U₂N₃; the data on decomposition pressures, specific heats, and heats and free energies of formation are summarized and evaluated.     

Characterization of the new scintillator Cs_2LiYCl_6:Ce~(3+)

The first domestic inorganic scintillator, Cs_2LiYCl_6:Ce~(3+)(CLYC), was grown at Beijing Glass Research Institute using the vertical Bridgman method. In this work, we evaluated the performance of this new CLYC crystal in terms of its gamma-ray energy resolution and pulse shape discrimination(PSD) capability between neutrons and gamma rays. The decay times associated with different scintillation mechanisms were obtained by fitting decay functions to the neutron and gamma-ray waveform structures. We found an energy resolution of * 4.5% for 662-ke V gamma rays and efficient neutron/gamma PSD with a figure of merit of * 2.6. Under gamma-ray excitation, there is an ultrafast scintillation mechanism in CLYC with a decay time of approximately 2 ns, whereas there is no evidence of ultrafast decay under thermal neutron excitation. This work contributes to the promotion of domestic development of CLYC.