The solidification behaviour of the UO 2 –ThO 2 system in a laser heating study (original) (raw)
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Melting behavior of (Th,U)O2 and (Th,Pu)O2 mixed oxides
Journal of Nuclear Materials, 2016
The melting behaviors of pure ThO2, UO2 and PuO2 as well as (Th,U)O2 and (Th,Pu)O2 mixed oxides (MOX) have been studied using molecular dynamics (MD) simulations. The MD calculated melting temperatures (MT) of ThO2, UO2 and PuO2 using two-phase simulations, lie between 3650-3675 K, 3050-3075 K and 2800-2825 K, respectively, which match well with experiments. Variation of enthalpy increments and density with temperature, for solid and liquid phases of ThO2, PuO2 as well as the ThO2 rich part of (Th,U)O2 and (Th,Pu)O2 MOX are also reported. The MD calculated MT of (Th,U)O2 and (Th,Pu)O2 MOX show good agreement with the ideal solidus line in the high thoria section of the phase diagram, and evidence for a minima is identified around 5 atom% of ThO2 in the phase diagram of (Th,Pu)O2 MOX.
Journal of Nuclear Materials, 2008
The coated agglomerate pelletization (CAP) process is a novel and innovative technique for the fabrication of (Th, 233 U) mixed oxide fuel pellets. This technique is being investigated to fabricate the fuel for the forthcoming Indian Advanced Heavy Water Reactor (AHWR). In the CAP process, ThO 2 is converted to free flowing agglomerate by powder extrusion route. As only ThO 2 is handled to this stage, all the operations are carried out in a normal glove-box facility. Subsequent operations are carried out in a shielded glove-box or hot cell facility using manipulators. In this study, fabrication of ThO 2-4%UO 2 and ThO 2-20%UO 2 (all compositions are in weight percent) pellets was carried out by CAP process. For having these ThO 2-UO 2 pellets, ThO 2 granules and U 3 O 8 powders were used as the starting materials. The densification behaviour of the pellets was studied in reducing and oxidizing atmospheres using a high temperature dilatometer. The microstructure was examined by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA).
Unidirectional solidification of UO -RO type refractory oxides with emphasis in the system UO -MgO
1973
Page 20. Longitudinal Section of UO2-25 w/o MgO. Near the Edge of the Solidified Area MgO was Presented as Random Shaped Primary Phase Areas(Dark Field 60OX) 52 21. Longitudinal Section of UO2-20 w/o MgO. (Dark Field 60OX) 54 22. Longitudinal Section of UO2-18 w/o MgO.Cell Structure(Bark Field 60OX). 55 23. Transverse Section of UOg-I8 w/o MgO. The Circled Area Shows a Fault Line.(Light Field 60OX).. . 57 24. Transverse Section of UO2-I8 w/o MgO. In the Central Part, There are Two Regions of Lamellae, one is Nearly Fault Free and the Other one is Faulted. The Spacing of the Former one is Smaller Than the Latter. (Light Field 6OOX) 58 25. Transverse Section of UOp-18 w/o MgO. In the Central Area, There are Many Broken Lamellae and Rods Caused by Very High Interphase Boundary Energy. (Light Field 60OX) 60 26. Longitudinal Section of UO-18 w/o MgO. Lamellar Spacing 28 Microns, Growth Rate 5.0 cm/hr.(Light Field 60OX) 53 27. Longitudinal Section of UOo-I8 w/o MgO. Lamellar Spacing I8 Microns, Growth Rate 9*0 cm/hr. (Dark Field bOOX) 64 28. Transverse Section of UO2-18 w/o MgO. Rod Density 18600 Rods/cm2. Rod Diameter 1.7 Microns. Growth Rate 5.0 cm/hr. (Light Field 60OX) 65
Synthesis, characterization and thermal expansion measurements on uranium–cerium mixed oxides
Journal of Nuclear Materials, 2011
Highly homogeneous Th 1Àx Sm x O 2 ; 0 x 0.8 solid solutions were synthesized by a co-precipitation technique and the co-precipitated samples were sintered at 1473 K. Compositions of the solid solutions were characterized by standard wet-chemical analysis. X-ray diffraction measurements were performed on the sintered pellets for structural analysis, lattice parameter calculation and determination of solid solubility of SmO 1.5 in ThO 2 matrix. Bulk and theoretical densities of solid solutions were also determined. A fluorite structurewas observed for ThO 2 -SmO 1.5 solid solutions with 0-55.2 mol% SmO 1.5 . Thermal expansion coefficients were measured using high temperature X-ray diffraction technique. The mean linear thermal expansivity,ā m for ThO 2 -SmO 1.5 solid solutions containing 17.9, 41.7 and 52.0 mol% of SmO 1.5 were determined in the temperature range 298-2000 K for the first time. The mean linear thermal expansion coefficients for ThO 2 -SmO 1.5 solid solutions are 10.47, 11.16 and 11.45 Â 10 À6 K À1 , respectively.
Thermochemistry of UO2 – ThO2 and UO2 – ZrO2 fluorite solid solutions
The Journal of Chemical Thermodynamics, 2017
The enthalpies of formation of cubic urania – thoria (c-Th x U 1-x O 2+y) and urania – zirconia (c-Zr x U 1-x O 2 , x < 0.3) solid solutions at 25 °C from end-member binary oxides (c-UO 2 , and c-ThO 2 or m-ZrO 2) have been measured by high temperature oxide melt solution calorimetry. The enthalpies of mixing for both systems are zero within experimental error. The interaction parameters for binary solid solutions MO 2 – M 0 O 2 (M, M 0 = U, Th, Ce, Zr, and Hf), fitted by regular and subregular thermodynamic models using both calorimetric and computational data, increase linearly with the corresponding volume mismatch. Cubic UO 2 – ZrO 2 appears to be an exception to this correlation and shows a zero heat of mixing despite large size mismatch, suggestive of some short-range ordering and/or incipient phase separation to mitigate the strain. The incorporation of ZrO 2 into UO 2 stabilizes the system and makes it a potential candidate for immobilization and disposal of nuclear waste.
Review of Scientific Instruments, 2019
Thermodynamic properties of refractory materials, such as standard enthalpy of formation, heat content and enthalpy of reaction, can be measured by high temperature calorimetry. In such experiments, a small sample pellet is dropped from room temperature into a calorimeter operating at high temperature (often 700 °C) with or without a molten salt solvent present in an inert crucible in the calorimeter chamber. However, for hazardous (radioactive, toxic, etc.) and/or airsensitive (hygroscopic, sensitive to oxygen, pyrophoric etc.) samples, it is necessary to utilize a sealed device to encapsulate and isolate the samples, crucibles, and solvent under a controlled atmosphere in order to prevent the materials from reactions and/or protect the personnel from hazardous exposure during the calorimetric experiments. We have developed a sample seal-anddrop device (calorimetric dropper) that can be readily installed onto the dropping tube of a calorimeter such as the Setaram AlexSYS Calvet-type high temperature calorimeter to fulfill two functions: i) load hazardous or air-sensitive samples in an airtight , sealed container; and ii) drop the samples into the calorimeter chamber using an "off-then-on" mechanism. As a case study, we used the calorimetric dropper for measurements of the enthalpy of drop solution of PuO2 in molten sodium molybdate (3Na2O • 4MoO3) solvent at 700 °C. The obtained enthalpy of-46.04 ± 3.75 kJ/mol is consistent with the energetic systematics of other actinide oxides (UO2, ThO2 and NpO2). This capability has thus laid the foundation for thermodynamic studies of other Pu-bearing phases in the future.
Ambient melting behavior of stoichiometric uranium oxides
Frontiers in Nuclear Engineering, 2024
As UO 2 is easily oxidized during the nuclear fuel cycle it is important to have a detailed understanding of the structures and properties of the oxidation products. Experimental work over the years has revealed many stable uranium oxides including UO 2 , U 4 O 9 (UO 2.25), U 3 O 7 (UO 2.33), U 2 O 5 (UO 2.5), U 3 O 8 (UO 2.67), and UO 3 , all with a number of different polymorphs. These oxides are broadly split into two categories, fluorite-based structures with stoichiometries in the range of UO 2 to UO 2.5 and less dense layered-type structures with stoichiometries in the range of UO 2.5 to UO 3. While UO 2 is well characterized, both experimentally and computationally, there is a paucity of data concerning higher stoichiometry oxides in the literature. In this work we determine the ambient melting points of all the six stoichiometric uranium oxides listed above and compare them to the available experimental and/or theoretical data. We demonstrate that a family of the six ambient melting points map out a solid-liquid transition boundary consistent with the high-temperature portion of the phase diagram of uranium-oxygen system suggested by Babelot et al.