Fission product release in high-burn-up UO2 oxidized to U3O8

Results of oxidation experiments on high-burn-up UO₂ are presented where fission-product vaporisation and release rates have been measured by on-line mass spectrometry as a function of time/temperature during thermal annealing treatments in a Knudsen cell under controlled oxygen atmosphere. Fractional release curves of fission gas and other less volatile fission products in the temperature range 800-2000 K were obtained from BWR fuel samples of 65 GWd t⁻¹ burn-up and oxidized to U₃O₈ at low temperature. The diffusion enthalpy of gaseous fission products and helium in different structures of U₃O₈ was determined.     

Decomposition and multiphase reactions in the system UO2(NO3)2·6H2O-Ni(NO3)2·6H2O at elevated temperatures

Solid state reactions between uranyl nitrate hexahydrate and nickel nitrate hexahydrate in mixtures of various ratios have been studied at elevated temperatures. The binary system of uranyl nitrate hexahydrate and nickel nitrate hexahydrate was found to form a eutectic of composition 53 mol% uranyl nitrate hexahydrate and 47 mol% nickel nitrate hexahydrate at 40 °C. The overlap of evolution of nitric oxide (NO) and water vapour above 230 °C confirmed the presence of hydroxynitrates of uranium and nickel as intermediate products. These hydroxynitrates began to react above 280 °C to form nickel uranate, NiU₃O₁₀, in the case of mixtures containing 75 mol% uranyl nitrate hexahydrate. When the proportion of uranyl nitrate hexahydrate in the mixture was higher than 75 mol%, U₃O₈ formed along with nickel uranate. For the mixtures containing uranyl nitrate hexahydrate lower than 75 mol%, NiO was observed to form along with NiU₃O₁₀.     

Calorimetric measurements on (U,Th)O2 solid solutions

Enthalpy increments of urania - thoria solid solutions, (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ were measured by drop calorimetry in the temperature range 479 - 1805 K. Heat capacity, entropy and Gibbs energy function were computed. The heat capacity measurements were carried out also with differential scanning calorimetry in the temperature range 298 - 800 K. The heat capacity values of (U_0.10Th_0.90)O₂, (U_0.50Th_0.50)O₂ and (U_0.90Th_0.10)O₂ at 298 K are 59.62, 61.02, 63.56 J K⁻¹ mol⁻¹, respectively. The results were compared with the data available in the literature. From the study, the heat capacity of (U,Th)O₂ solid solutions was shown to obey the Neumann - Kopp’s rule.     

Reactivity feedbacks of a material test research reactor fueled with various low enriched uranium dispersion fuels

The reactivity feedbacks of a material test research reactor using various low enriched uranium fuels, having same uranium density were calculated. For this purpose, the original aluminide fuel (UAl_x–Al) containing 4.40 gU/cm³ of an MTR was replaced with silicide (U₃Si–Al and U₃Si₂–Al) and oxide (U₃O₈–Al) dispersion fuels having the same uranium density as of the original fuel. Calculations were carried out to find the fuel temperature reactivity feedback, moderator temperature reactivity feedback, moderator density reactivity feedback and moderator void reactivity feedback. Nuclear reactor analysis codes including WIMS-D4 and CITATION were employed to carry out these calculations. It was observed that the magnitudes all the respective reactivity feedbacks from 38 °C to 50 °C and 100 °C, at the beginning of life, of all the fuels were very close to each other. The fuel temperature reactivity feedback of the U₃O₈–Al was about 2% more than the original UAl_x–Al fuel. The magnitudes of the moderator temperature, moderator density and moderator void reactivity feedbacks of all the fuels, showed very minor variations from the original aluminide fuel.     

Phase study in the Na-Sr-U-O system: Characterization of new phase

In continuation with the earlier work for phase studies of Rb-Sr-U-O and Cs-Sr-U-O systems, the subsolidus phase relations in Na-Sr-U-O quaternary system were determined at 850 °C in air atmosphere. A novel quaternary phase Na₈Sr₂U₆O₂₄ in the Na-Sr-U-O system was synthesized by heating the respective oxides at 850 °C in air. XRD data of Na₈Sr₂U₆O₂₄ was indexed on a cubic system with lattice parameter a = 0.8326 nm and was found isostructural with K₈Sr₂U₆O₂₄ and Rb₈Sr₂U₆O₂₄. A pseudo-ternary phase diagram of Na₂O-SrO-UO₃ was drawn using the new quaternary compound and various phase fields were established by X-ray powder diffraction analysis. The structure of Na₈Sr₂U₆O₂₄ was derived from the powder data and structural parameters were refined by the Rietveld profile method.     

Microwave sintering of 8 mol% yttria-zirconia (8YZ): An inert matrix material for nuclear fuel applications

This study focused on reducing overall processing time and temperature for fully stabilized zirconia, an inert matrix material candidate, to minimize the loss of actinides (that will be incorporated into the matrix material), while maintaining at least 90% theoretical density (TD). The effects of different processing routes on bulk density and microstructure were evaluated. The results obtained by adopting microwave sintering for 8 mol% Y₂O₃-ZrO₂ were compared to conventional sintering. A 20 min soak time at 1300 °C resulted in pellets with 90% TD for microwave-processed samples, compared to 77% TD for pellets processed conventionally. A similar density was obtained at lower temperature (1200 °C) by increasing the soak time to 100 min in microwave processing. This time and temperature resulted in 60% TD conventionally processed pellets. Compressive strength values obtained for a 1300 °C (20 min soak time) microwave-processed sample were higher (1600 MPa) as compared to a conventionally processed sample (1300 MPa).     

Thermodynamic properties of ThxU1−xO2 (0 < x < 1) based on quantum-mechanical calculations and Monte-Carlo simulations

Th_xU_1−xO_2+y binary compositions occur in nature, uranothorianite, and as a mixed oxide nuclear fuel. As a nuclear fuel, important properties, such as the melting point, thermal conductivity, and the thermal expansion coefficient change as a function of composition. Additionally, for direct disposal of Th_xU_1−xO₂, the chemical durability changes as a function of composition, with the dissolution rate decreasing with increasing thoria content. UO₂ and ThO₂ have the same isometric structure, and the ionic radii of 8-fold coordinated U⁴⁺ and Th⁴⁺ are similar (1.14 nm and 1.19 nm, respectively). Thus, this binary is expected to form a complete solid solution. However, atomic-scale measurements or simulations of cation ordering and the associated thermodynamic properties of the Th_xU_1−xO₂ system have yet to be determined. A combination of density-functional theory, Monte-Carlo methods, and thermodynamic integration are used to calculate thermodynamic properties of the Th_xU_1−xO₂ binary (ΔH_mix, ΔG_mix, ΔS_mix, phase diagram). The Gibbs free energy of mixing (ΔG_mix) shows a miscibility gap at equilibration temperatures below 1000 K (e.g., E_exsoln = 0.13 kJ/(mol cations) at 750 K). Such a miscibility gap may indicate possible exsolution (i.e., phase separation upon cooling). A unique approach to evaluate the likelihood and kinetics of forming interfaces between U-rich and Th-rich has been chosen that compares the energy gain of forming separate phases with estimated energy losses of forming necessary interfaces. The result of such an approach is that the thermodynamic gain of phase separation does not overcome the increase in interface energy between exsolution lamellae for thin exsolution lamellae (10 Å). Lamella formation becomes energetically favorable with a reduction of the interface area and, thus, an increase in lamella thickness to >45 Å. However, this increase in lamellae thickness may be diffusion limited. Monte-Carlo simulations converge to an exsolved structure [lamellae || ] only for very low equilibration temperatures (below room temperature). In addition to the weak tendency to exsolve, there is an ordered arrangement of Th and U in the solid solution [alternating U and Th layers || {1 0 0}] that is energetically favored for the homogeneously mixed 50% Th configurations. Still, this tendency to order is so weak that ordering is seldom reached due to kinetic hindrances. The configurational entropy of mixing (ΔS_mix) is approximately equal to the point entropy at all temperatures, indicating that the system is not ordered.     

Fabrication method for UO2 pellets with large grains or a single grain by sintering in air

The effects of a powder treatment, the sintering temperature and the sintering time on the grain growth of UO₂ pellets were investigated in air to obtain UO₂ pellets with large grains. Air could be used for sintering because an oxidation path above 1803 K does not pass through a two-phase (UO_2+x + U₃O_8−z) region. The UO₂ pellets sintered by the CO₂-air-CO₂-H₂ process consisted of a single grain or some large grains in the order of several millimeters.     

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.     

Developing uranium dicarbide-graphite porous materials for the SPES project

Uranium carbide dispersed in graphite was produced under vacuum by means of carbothermic reduction of different uranium oxides (UO₂, U₃O₈ and UO₃), using graphite as the source of carbon. The thermal process was monitored by mass spectrometry and the gas evolution confirmed the reduction of the U₃O₈ and UO₃ oxides to UO₂ before the carbothermic reaction, that started to occur at T > 1000 °C. XRD analysis confirmed the formation of α-UC₂ and of a minor amount of UC. The morphology of the produced uranium carbide was not affected by the oxides employed as the source of uranium.     

Cuboctahedral oxygen clusters in U3O7

When UO₂ is oxidised to U₃O₇, the positions in the crystal lattice of all the uranium atoms and of about 70% of the oxygen atoms are hardly affected. The remaining oxygen atoms occupy new sites which are located 310 pm along 〈1 1 0〉 vectors from the holes in the fluorite framework of UO₂. These results, which are based on the analysis of neutron diffraction powder data, are consistent with the concept that excess oxygen in U₃O₇ is accommodated in cuboctahedral anionic clusters.     

Thermal studies on fluorite type ZryU1−yO2 solid solutions

Solid state reactions of UO₂ and ZrO₂ in mild oxidizing condition followed by reduction at 1673 K showed enhanced solubility up to 35 mol% of zirconium in UO₂ forming cubic fluorite type Zr_yU_1−yO₂ solid solution. The lattice parameters and O/M (M = U + Zr) ratios of the solid solutions, Zr_yU_1−yO_2+x, prepared in different gas streams were investigated. The lattice parameters of these solid solutions were expressed as a linear equation of x and y: a₀ (nm) = 0.54704 − 0.021x - 0.030y. The oxidation of these solid solutions for 0.1 ? y ? 0.2 resulted in cubic phase MO_2+x up to700 K and single orthorhombic zirconium substituted α-U₃O₈ phase at 1000 K. The kinetics of oxidation of Zr_yU_1−yO₂ in air for y = 0-0.35 were also studied using thermogravimetry. The specific heat capacities of Zr_yU_1−yO₂ (y = 0-0.35) were measured using heat flux differential scanning calorimetry in the temperature range of 334-860 K.     

Calorimetric investigations on stoichiometric barium and uranium oxides

Heat capacities and enthalpy increments of barium uranates: BaU₂O₇(s), Ba₂U₃O₁₁(s), Ba_2.875UO_5.875(s) and Ba₃UO₆(s) were measured using a differential scanning calorimeter and a high-temperature Calvet calorimeter. The heat capacities and enthalpy increments were measured in the temperature range 126-304 K and 299-1011 K, respectively. A set of self consistent thermodynamic functions such as entropy, Gibbs energy function, heat capacity and Gibbs energy and enthalpy of formation values for BaU₂O₇(s), Ba₂U₃O₁₁(s), Ba_2.875UO_5.875(s) and Ba₃UO₆(s) have been computed for the first time using the data obtained in the present study and other available experimental data.     

Microstructural evolution in irradiated uranium-bearing delta-phase oxides A6U1O12 (A = Y, Gd, Ho, Yb, and Lu)

Irradiation-induced microstructural evolution in uranium-bearing delta-phase oxides of A₆U₁O₁₂ (A = rare earth cations) were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Polycrystalline Y₆U₁O₁₂, Gd₆U₁O₁₂, Ho₆U₁O₁₂, Yb₆U₁O₁₂, and Lu₆U₁O₁₂ samples were irradiated with 300 keV Kr⁺⁺ to a fluence of 2 × 10²⁰ ions/m² at cryogenic temperature (∼100 K). The crystal structure of these compounds was determined to be an ordered, fluorite derivative structure, known as the delta-phase, a rhombohedral symmetry belonging to space group . Experimental results indicate that all these compounds are resistant to amorphization to a displacement damage dose of ∼60 displacements per atom. In these experiments, we sometimes observed an irradiation-induced order-to-disorder phase transformation, from an ordered rhombohedral to a disordered fluorite structure.     

Formation of columnar and equiaxed grains by the reduction of U3O8 pellets to UO2+x

The reduction of U₃O₈ pellets to UO_2+x has been investigated at 1300 °C in H₂, Ar and CO₂ gas atmospheres by TGA, SEM, and X-ray diffraction. The selected U₃O₈ pellet was prepared by sintering a U₃O₈ powder compact. The TGA results show that the reduction rate is fastest in H₂ gas, and X-ray diffraction indicates that U₃O₈ reduces to UO_2+x without any intermediate phase. The reduced pellet, UO_2+x, has a special grain structure that consists of equiaxed grains at the surface, columnar grains in the middle, and equiaxed grains in the center. The equiaxed grains and columnar grains are much smaller in H₂ gas than in Ar or CO₂ gas. The reducing gases significantly influence the morphology of the grain structure. This difference can be explained in terms of a relation between oxygen potential and critical nucleus size during the reduction.     

Synthesis, characterization and thermal expansion measurements on uranium-cerium mixed oxides

Uranium-cerium mixed oxides (U_1−yCe_y)O₂ (y = 0.2, 0.4, 0.6, 0.8) were prepared by combustion synthesis using citric acid as the fuel. Sintering of the solid solutions was carried out at 1873 K under reduced atmosphere. From the room temperature XRD patterns of the sintered samples it was found that the solid solutions form single phase fluorite structure. The room temperature lattice parameters of (U_1−yCe_y)O₂ (y = 0.2, 0.4, 0.6, 0.8) are 0.5458, 0.5446, 0.5434 and 0.5422 nm respectively. Thermal expansion of (U_1−yCe_y)O₂ (y = 0.2, 0.4, 0.6, 0.8) in the temperature range 298-1973 K was measured by high temperature X-ray diffraction (HTXRD). The coefficients of thermal expansion increase with increase in CeO₂ content in the sample and the measured data in the temperature range 298-1973 K, for (U_1−yCe_y)O₂ (y = 0.2, 0.4, 0.6, 0.8) are 18.23, 19.91, 21.59, 23.29 × 10⁻⁶ K⁻¹, respectively.     

Rapid synthesis of Pb5(VO4)3I, for the immobilisation of iodine radioisotopes, by microwave dielectric heating

Rapid synthesis of Pb₅(VO₄)₃I, a potential immobilisation host for iodine radioisotopes, was achieved in an open container by microwave dielectric heating of a mixture of PbO, PbI₂, and V₂O₅ at a power of 800 W for 180 s (at 2.45 GHz). The resulting ceramic bodies exhibited a zoned microstructure, differentiated by inter-granular porosity and phase assemblage, as a consequence of the inverse temperature gradient characteristic of microwave dielectric heating. Liquid PbI₂ within the interior of microwave processed ceramics assisted formation of Pb₅(VO₄)₃I, and reduced inter-granular porosity. In contrast, the exterior of microwave processed ceramics comprised poorly sintered Pb₅(VO₄)₃I with the presence of minor reagent relics. Quantitative microanalysis, electron diffraction and Rietveld analysis, confirmed the synthesis of stoichiometric Pb₅(VO₄)₃I within precision. The crystal structure of Pb₅(VO₄)₃I was found to adopt space group P6₃/m with a = 10.4429(3) Å and c = 7.4865(2) Å.     

Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization

Nano-crystalline W-1%Y₂O₃ (wt.%) powder was produced by a modified solution chemical reaction of ammonium paratungstate (APT) and yttrium nitrate. The precursor powder was found to consist of particles of bimodal morphology i.e. large APT-like particles up to 20 μm and rectangular yttrium containing ultrafine plates. After thermal processing tungsten crystals were evolved from W-O-Y plate like particles. spark plasma sintering (SPS) was used to consolidate the powder at 1100 and 1200 °C for different holding times in order to optimize the sintering conditions to yield high density but with reduced grain growth. Dispersion of yttrium oxide enhanced the sinterability of W powder with respect to lanthanum oxide. W-1%Y₂O₃ composites with sub-micron grain size showed improved density and mechanical properties as compared to W-La₂O₃ composites. Sample sintered in two steps showed improved density, due to longer holding time at lower temperature (900 °C) and less grain growth due to shorter holding time at higher temperature i.e. 1 min at 1100 °C.     

Effect of Nd and Pr addition on the thermal and mechanical properties of (U, Ce)O2

The thermal conductivity, Young’s modulus, and hardness of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ (x = 0.01, 0.08, 0.12) were evaluated and the effect of Pr and Nd addition on the properties of (U, Ce)O₂ were studied. The polycrystalline high-density pellets were prepared with solid state reactions of UO₂, CeO₂, Pr₂O₃, and Nd₂O₃. We confirmed that all Ce, Pr, and Nd dissolved in UO₂ and formed solid solutions of (U, Ce, Pr, Nd)O₂. We revealed that the thermal conductivity of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ (x = 0.12) was up to 25% lower than that of x = 0.01 at room temperature. The Young’s modulus of (U_0.65−xCe_0.3Pr_0.05Nd_x)O₂ decreased with x, whereas the hardness values were constant in the investigated x range.     

Raman spectroscopy characterization of actinide oxides (U1−yPuy)O2: Resistance to oxidation by the laser beam and examination of defects

Structural changes in four (U_1−yPu_y)O₂ materials with very different plutonium concentrations (0 ? y ? 1) and damage levels (up to 110 dpa) were studied by Raman spectroscopy. The novel experimental approach developed for this purpose consisted in using a laser beam as a heat source to assess the reactivity and structural changes of these materials according to the power supplied locally by the laser. The experiments were carried out in air and in water with or without hydrogen peroxide. As expected, the material response to oxidation in air depends on the plutonium content of the test oxide. At the highest power levels U₃O₈ generally forms with UO₂ whereas no significant change in the spectra indicating oxidation is observed for samples with high plutonium content (²³⁹PuO₂). Samples containing 25 wt.% plutonium exhibit intermediate behavior, typified mainly by a higher-intensity 632 cm⁻¹ peak and the disappearance of the 1LO peak at 575 cm⁻¹. This can be attributed to the presence of anion sublattice defects without any formation of higher oxides. The range of materials examined also allowed us to distinguish partly the chemical effects of alpha self-irradiation. The results obtained with water and hydrogen peroxide (a water radiolysis product) on a severely damaged ²³⁸PuO₂ specimen highlight a specific behavior, observed for the first time.