Fuel Chemistry Division

In this communication, we report the oxidation and reduction behavior of fluorite type solid solutions in U-Zr-O. The maximum solubility of ZrO 2 in UO 2 lattice could be achieved with a mild oxidizing followed by reducing conditions. The role of valency state of U is more dominating in controlling the unit cell parameters than the incorporated interstitial oxygen in the fluorite lattice. The controlled oxidation studies on U-Zr-O solid solutions led to the delineation of a new distorted fluorite lattice at the U:Zr ¼ 2:1 composition. The detailed crystal structure analysis of this ordered composition Zr 0.33 U 0.67 O 2.33 (ZrU 2 O 7 ) has been carried from the powder XRD data. This phase crystallizes in an orthorhombically distorted fluorite type lattice with unit cell parameters: a ¼ 5.1678(2), b ¼ 5.4848(2), c ¼ 5.5557(2) Å and V ¼ 157.47(1) Å 3 (Space group: Cmcm, No. 63). The metal ions have distorted cubical polyhedra with anion similar to the fluorite structure. The excess anions are occupied in the interstitial (empty cubes) of the fluorite unit cell. The crystal structure and chemical analyses suggest approximately equal fractions of U 4+ and U 6+ in this compound. The details of the thermal stability as well as kinetics of formation and oxidation of ZrU 2 O 7 are also studied using thermogravimetry.

Phase relations and defect structures in (U 0.1 Th 0.9 )O 2 -GdO 1.5 quaternary system were studied at 1673 K. Different phases and their compositions, obtained on heating the oxides in different atmospheres were examined by powder X-ray diffraction and chemical analysis, respectively. The region of existence of fluorite type solid solutions was determined. The cubic lattice parameter of the solid solution, [(U 0.1 Th 0.9 ) 1Ày Gd y ]O 2+x , in single phase region (0 6 y 6 0.4) could be expressed in a linear equation of x and y as: a 0 (Å) = 5.581 À 0.228x À 0.198y for x < 0 O/U analysis and simple ionic model of the quaternary solid solutions indicate the oxidation of U +4 to a mixture of U +5 and U +6 . High temperature XRD studies of FCC structured [(U 0.1 Th 0.9 ) 1Ày Gd y ]O 2+x showed positive thermal expansion between 298 K and 1273 K.

Pyrohydrolysis method was studied for the separation of Cl and F from U-Zr and U-Pu-Zr alloy nuclear fuels and determination by ion chromatography. A detailed study was performed on pyrohydrolysed products of U-Zr alloy, obtained on heating in moist argon and oxygen atmospheres using X-ray powder diffraction (XRD) and thermogravimtric methods. Using oxygen as the carrier gas pyrohydrolysis performed at 900 ∘ C was complete within 30 min. It was found that, moist oxygen as carrier gas only could open the matrix completely within stipulated time.

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Oxidation of sintered thorium-plutonium mixed oxide pellets was studied by thermogravimetry in the temperature range 400–1200 K. The kinetics of oxidation was found to be controlled by oxygen chemical diffusion with activation energies in the range 25–50 kJ mol−1. Oxygen chemical diffusion coefficients are reported for Pu0.3Th0.7O1.94.

Ž . X-ray photoelectron spectroscopy XPS studies are performed on some mixed oxides of uranium with alkali and alkaline Ž . earth metals Na, Li, Rb, Tl, Sr, Ba to investigate the presence of multiple valence states of uranium in these compounds. Photoelectron peak positions, peak widths and satellite positions were measured to identify the chemical states. The analysis of the shake up satellites and the deconvolution of the principal peaks are used to quantify the chemical states. The Ž . domination of U V chemical state is inferred in Na U O and Sr U O . q 1998 Elsevier Science B.V. All rights reserved. 2 3 9 2 3 10

Compounds SrU(PO 4) 2 (SUP) and BaU(PO 4) 2 (BUP) were synthesized by solid state method in high purity argon atmosphere at 1473 K. Structures of both the compounds were derived using Rietveld method from powder Xeray diffraction (XRD) data. Compound SUP is orthorhombic and has ordered cheralite type structure with alternate layers of SrO 10 and UO 8 polyhedra. Compound BUP is monoclinic and has a non cheralite structure with Ba and U in 14 and 8 coordinations, respectively. Thermogravimetric (TG) curves obtained by heating both the compounds in air confirm the oxidation of U(IV) to U(VI) between 600 and 1573 K. High temperature XRD data collected in vacuum up to 1473 K indicated positive thermal expansion for both the compounds. Heat capacity (C p) measurements were also carried out for both the compounds by heat flow Differential Scanning Calorimeter (DSC) in argon atmosphere from 300 to 873 K.

Pyrohydrolysis method was studied for the separation of Cl and F from U-Zr and U-Pu-Zr alloy nuclear fuels and determination by ion chromatography. A detailed study was performed on pyrohydrolysed products of U-Zr alloy, obtained on heating in moist argon and oxygen atmospheres using X-ray powder diffraction (XRD) and thermogravimtric methods. Using oxygen as the carrier gas pyrohydrolysis performed at 900 ℃ was complete within 30 min. It was found that, moist oxygen as carrier gas only could open the matrix completely within stipulated time. This was attributed to the formation of α-U3O8, which was not possible in moist argon heating for 30 min. The pyrohydrolysis in moist Ar required 60–90 min for complete removal of halides. The method was then successfully extrapolated to U-Pu-Zr alloys. The results showed recovery up to 90% or above for the optimized conditions.

We have determined the thermo-physical (elastic modulus, specific heat, thermal expansion and thermal conductivity) and transport (ionic conductivity) properties of ThO 2-5wt.%CeO 2 mixed oxide (MOX) using a combined experimental and theoretical methodology. The specific heat, ionic conductivity and elastic properties of ThO 2-5wt.%CeO 2 pellets prepared by conventional powder metallurgy (POP) and coated agglomerate pelletization (CAP) routes (sintered in both air and Ar-8%H 2 atmosphere) are compared with respect to homogeneity (CeO 2 distribution in ThO 2 matrix), microstructure, porosity and oxygen to metal ratio. The effects of inhomogeneity and pore distribution on thermal expansion and thermal conductivity of the mixed-oxide pellets are identified. Molecular dynamics (MD) simulations using the Coulomb-Buckingham-Morse-many-body model based interatomic potentials are used to predict elastic properties in the temperature range between 300 and 2000 K and thermodynamic properties, viz., enthalpy increment and specific heats of ThO 2. Finally, the thermal expansion coefficient and thermal conductivity of ThO 2 and (Th,Ce)O 2 mixed-oxides obtained from MD are compared with available experimental results.

Structure D 2000 Studies on Structural and Thermal Expansion Behavior of Alkali-Thorium Phosphates.-The compounds MTh2(PO4)3 (M: Rb, Cs) are isostructural to KTh2(PO4)3 and crystallize in the monoclinic space group C2/c with Z = 4 (powder XRD). The structures consist of a [Th2(PO4)3]∞ framework of edge-and corner-sharing PO4 tetrahedra and ThO9 polyhedra. Rb and Cs atoms sit on a twofold axis in large tunnels running along the c-axis. Due to flexibility in the structure, these materials can be considered as a potential crystalline phosphate matrix for the storage of nuclear waste. The thermal expansion behavior of MTh2(PO4)3

Solid state reactions of Na 2 U 2 O 7 and K 2 U 2 O 7 in air at 1273 K, formed mixed uranates of type (K x Na 1−x)U 2 O 7. Reduction of these mixed uranates formed (K x Na 1−x)UO 3 perovskite type of phase, having cubic structure for 0.5 ≤ x ≤ 1.0 and orthorhombic structure for 0 ≤ x < 0.5. The kinetics of oxidation of (K x Na 1−x)UO 3 in air, studied by thermogravimetry showed that the reaction is diffusion controlled with an activation energy in the range 135-202 kJ/mol. The enthalpy of oxidation of these perovskite phases was determined from DTA peak area measurement and the values are in the range 40-90 kJ/mol. High temperature X-ray diffraction studies showed that the % thermal expansion increases with increase in sodium concentration in KUO 3 phase.

Solid state reactions of UO 2 and ZrO 2 in mild oxidizing condition followed by reduction at 1673 K showed enhanced solubility up to 35 mol% of zirconium in UO 2 forming cubic fluorite type Zr y U 1Ày O 2 solid solution. The lattice parameters and O/M (M = U + Zr) ratios of the solid solutions, Zr y U 1Ày O 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: a 0 (nm) = 0.54704 À 0.021x-0.030y. The oxidation of these solid solutions for 0.1 6 y 6 0.2 resulted in cubic phase MO 2+x up to700 K and single orthorhombic zirconium substituted a-U 3 O 8 phase at 1000 K. The kinetics of oxidation of Zr y U 1Ày O 2 in air for y = 0-0.35 were also studied using thermogravimetry. The specific heat capacities of Zr y U 1Ày O 2 (y = 0-0.35) were measured using heat flux differential scanning calorimetry in the temperature range of 334-860 K.

The quaternary Tl-Pu-MoO system was investigated for the first time and two new quaternary compounds Tl 2 Pu(MoO 4) 3 and Tl 4 Pu(MoO 4) 4 were synthesized by solid state reactions of Tl 2 MoO 4 with Pu(MoO 4) 2 in 1:1 and 2:1 molar proportions at 773 K and 823 K, respectively. X-ray powder diffraction data of these compounds were indexed on orthorhombic system. Thermogravimetric (TG) curves of Tl 2 Pu(MoO 4) 3 and Tl 4 Pu(MoO 4) 4 were recorded in air and no significant weight changes were observed up to 973 K and 1033 K, respectively. Differential thermal analysis (DTA) curves of Tl 2 Pu(MoO 4) 3 and Tl 4 Pu(MoO 4) 4 showed endothermic peaks due to the melting of the compounds at 838 K and 1013 K, respectively. Non-isothermal kinetics of decomposition of Tl 2 Pu(MoO 4) 3 and Tl 4 Pu(MoO 4) 4 in air was studied using thermogravimetric technique. Tl 2 Pu(MoO 4) 3 and Tl 4 Pu(MoO 4) 4 when heated up to 1673 K decomposed to Tl 2 O and MoO 3 in the gaseous form, giving solid PuO 2 as an end product. The reactions followed unimolecular nucleation and growth mechanism with activation energies of 106 kJ/mol and 157 kJ/mol, respectively.

UO 2 , ThO 2 and (U x Th 1−x)O 2 (x = 0.1 and 0.5) were reacted with NH 4 NO 3 in different molar ratios up to 400 • C in air, to study the dissolution behavior. Solid state reaction of UO 2 with different moles of NH 4 NO 3 showed various intermediate products, depending on the amount of NH 4 NO 3. The end products at 400 • C were identified as U 3 O 8 and UO 3 , respectively when 1 and 1.5 mol of NH 4 NO 3 were used. When UO 2 and NH 4 NO 3 were reacted in 1:4 molar ratio, the intermediate products identified were (NH 4) 2 UO 2 (NO 3) 4 and NH 4 UO 2 (NO 3) 3 at 180 and 220 • C, respectively. XRD data of both the compounds were indexed on orthorhombic system. ThO 2 did not show any reaction with NH 4 NO 3 up to 400 • C. The reaction of (U,Th)O 2 with 4 mol of NH 4 NO 3 was studied to see the dissolution and separation of U from the mixed oxide. X-ray diffraction, thermal analysis and chemical analysis techniques were used for the characterization of the products.

Structure of monoclinic CaU(PO 4) 2 , synthesized in argon atmosphere at 1473 K, was derived from powder X−ray diffraction (XRD) data using Rietveld refinement method. Thermogravimetric (TG) Curve recorded up to 1573 K in argon atmosphere showed thermal as well as structural stability of CaU(PO 4) 2. When heated in air, TG curve of CaU(PO 4) 2 showed weight gain between 1100−1673 K due to oxidation of U(IV) to U(VI). Molar heat capacity of CaU(PO 4) 2 as well as some ternary alkaline earth phosphates were calculated using differential scanning calorimeter technique. Phase diagrams of pseudo ternary alkaline earth metal−uranium (IV)−phosphates were drawn and phase boundaries were established. All the ternary and quaternary compounds used for drawing the phase diagrams were synthesized in an argon atmosphere at 1473 K. X-ray diffraction (XRD) was used as a prime analytical tool to investigate the phase mixtures and draw phase boundaries. Phase diagram of SrO−UO 2 −P 2 O 5 and BaO−UO 2 −P 2 O 5 systems confirm the existence two quaternary compounds, (Sr/Ba)U(PO 4) 2 and (Sr/Ba) 7 U(PO 4) 6 in the systems whereas in CaO−UO 2 −P 2 O 5 system only one quaternary compound, CaU(PO 4) 2 , was identified.

Niobates of the formula Ba 2 LaNbO 6 and BaLaM 2+ NbO 6 (M 2+ =Mg, Ca) were synthesized by the conventional solid state route. Size dependence of M-cation on the bulk crystal structure of BaLaM 2+ NbO 6 was studied and compared with the Ba 2 LaNbO 6 compound. X-ray diffraction study confirmed that BaLaMgNbO 6 compound has the rhombohedral () crystal R3 structure as compared to the monoclinic (I2/m) in BaLaCaNbO 6 and Ba 2 LaNbO 6. The change in the local structure of La cation among these compounds was investigated by carrying out Photoluminescence study on 2 atom% Eu 3+ doped samples. PL study of BaLaMgNbO 6 :Eu 3+ sample indicates Eu 3+ ions occupying the distorted 12-coordinated A-site, while in Ba 2 LaNbO 6 : Eu 3+ , Eu 3+ is present at highly symmetric octahedral B-site. Upon excitation, the light emission of these compounds changes from reddishorange to red to purple in the order Ba 2 LaNbO 6 :Eu → BaLaCaNbO 6 :Eu → BaLaMgNbO 6 :Eu 3+ , due to change in Eu 3+-ions site occupancy. Lifetime study also confirmed the presence of two different Eu 3+ components at two different lattice sites and their respective emission spectra were isolated by time resoled emission spectroscopy. Further, this site selective lattice occupancy of Eu 3+ ions also gave various new insights about its radiative and non-radiative properties at different lattice sites. This works Accepted Article This article is protected by copyright. All rights reserved presents a complete structural understanding of BaLaMNbO 6 based matrices and their versatile phosphor characteristics when doped with Eu 3+ ion.

Na-U-O system were prepared and characterised by X-ray, thermal and chemical methods. Na,U, _Y02_x was found to be a solid solution of Na,O in UO, with fluorite structure with a limited solid solubility of 10 mole% Na,O. These new phases reported could lead, in addition to the modification of the phase diagrams, to a better understanding of the fuel-fission product-coolant-clad chemical interactions. The new phases were identified during studies on solid state reactions of chromates of caesium, rubidium, potassium and sodium with uranium oxides upto 1200°C in inert atmosphere. These reactions have shown that different uranates are obtained depending upon the O/U of the starting uranium oxide, with chromium separating as chromium oxide (Cr,O,).