S. Anthonysamy

PVA aided microwave synthesis: A novel route for the production of nanocrystalline thoria powder

Abstract A new method is reported for the preparation of nanocrystalline thoria powder that yields pellets of high density. The powder is prepared by denitration of an aqueous solution of thorium nitrate and polyvinyl alcohol (PVA) using microwave heating. The powder was characterized for its surface area and crystallite size by both X-ray line broadening and transmission electron microscopy. The reactivity of the powder was estimated by determining the density of the compacts prepared from these powders and sintered at 1573 K in air. For the sake of comparison, powders obtained by direct denitration were also characterized as above. The X-ray crystallite size for the calcined powders obtained by both routes is found to be ∼10 nm and found to be in agreement with the crystallite size obtained by conventional transmission electron microscopy. The density of pellets prepared from the powder produced by denitration in the presence of PVA was 92–93% of the theoretical density and that obtained with direct denitrated powder was ∼85% of theoretical density.

Gibbs energies of formation of chromium carbides

The carbon potentials corresponding to the two-phase mixtures Cr + Cr23C6, Cr23C6 + Cr7C3, and Cr7C3 + Cr3C2 in the binary system Cr-C were measured in the temperature range 973 to 1173 K by using the methane-hydrogen gas equilibration technique. Special precautions were taken to prevent oxidation of the samples and to minimize thermal segregation in the gas phase. The standard Gibbs energies of formation of Cr23C6, Cr7C3, and Cr3C2 were derived from the measured carbon potentials. These values are compared with those reported in the literature. The Gibbs energies obtained in this study agree well with those obtained from solid-state cells incorporating CaF2 and ThO2(Y2O3) as solid electrolytes and sealed capsule isopiestic measurements reported in the literature.

Combustion synthesis of urania-thoria solid solutions

Abstract The feasibility of the combustion synthesis using citric acid as the fuel was studied in order to prepare a homogeneous mixture of uranium dioxide–thorium dioxide feed powder that could be cold compacted without binder or lubricant and sintered to a high density (⩾95% theoretical density) at relatively low temperatures (⩾1573 K). Systematic studies were carried out to optimize various process parameters such as fuel-to-oxidant mole ratio, method of heating and sintering temperature. The powders were characterized for their carbon content, specific surface area, particle size distribution and bulk density. The crystallite size of the powders was determined by the X-ray line-broadening technique. The microstructure and surface morphology of the powders were studied using scanning electron microscopy. The reactivity of the calcined powders was determined by measuring the density of the sintered compacts prepared from them. The performance of citric acid as a combustion fuel was compared with that of poly vinyl alcohol.

Calorimetric studies on urania–thoria solid solutions

Abstract The enthalpy increments of (U y Th 1− y )O 2 solid solutions with y =0.1, 0.5 and 0.9 were measured by using a drop calorimeter of isoperibol type. The experimental data cover the temperature range 473–973 K relative to 298 K. Other thermodynamic functions such as heat capacity, entropy and free energy function of these solid solutions were derived from the measured enthalpy increment values. The results indicate that the enthalpies of (U y Th 1− y )O 2 solid solutions in the temperature range 473–973 K obey the Neumann–Kopp molar additivity rule.

Microwave synthesis of solid solutions of urania and thoria: a comparative study

Abstract Recently a new route for the production of thoria powder that yields pellets of high density had been communicated. In continuation of this study, the method as applied to the preparation of (U, Th)O 2 powders with various compositions is reported here. The powders were characterised for crystallite size by using XRD and transmission electron microscopy and BET surface area. As observed in the case of thoria, the powders obtained in this study were found to be nanocrystalline. The sinterability of the powder was determined by measuring the density of the sintered pellets prepared from these powders. The sintering was carried out at 1573 K in argon atmosphere. The feasibility of calcining and sintering of the solid solutions by using microwave heating was also studied.

Studies on the oxygen potentials of (UyTh1−y)O2+x solid solutions

Abstract The oxygen potentials of (U) y Th 1− y )O 2+ x solid solutions ( y varying from 0.54 to 0.9) were measured by using a gas equilibration method, employing H 2 + H 2 O, CO + CO 2 and CO 2 + H 2 gas mixtures. The measurements covered an oxygen potential range of −450 to −220 kJ mol −1 and the O M ratios varied between 2.000 and 2.035. The attainment of equilibrium as well as the actual oxygen potentials realised in the gas mixture were determined by using an oxygen probe based on calcia stabilized zirconia solid electrolyte. The results of the measurements indicate that the oxygen potentials of (U y Th 1− y )O 2+ x solid solutions are a function of both the uranium valence and the U/U + Th ratio. Partial molar enthalpies and entropies of solution of oxygen in the solid solutions were also computed from the temperature dependance of the oxygen potentials.

Chemical potential of carbon in the system U-Pu-C-O-N: Measurements and calculation

The carbon potential of (U,Pu) mixed carbides with Pu/(U + Pu) ratios of 0.55 and 0.70 was measured in the temperature range 973 to 1173 K by employing a methane-hydrogen gas equilibration technique. The technique was validated by measuring the Gibbs energy of formation of WC. The compatibility of the mixed carbides with the stainless steel clad was analysed by using the measured carbon potentials. The carbon potentials of mixed carbides of other compositions were calculated theoretically in order to assess their compatibility. The calculations assume ideal solution behavior for all the solid solutions present in the U-Pu-C-O-N system.

Studies on the kinetics of oxidation of urania-thoria solid solutions in air

The oxidation behaviour of (UyTh1ˇy)O2 powders (ya 0:15; 0:30; 0:72 and 0:77) was studied by means of thermogravimetry. The oxidation was carried out under both isothermal and non-isothermal conditions. The limits of oxygen solubility in terms of O/M ratios of (UyTh1ˇy)O2 solid solutions, derived from the thermogravimetric data, were 2.08, 2.15, 2.35 and 2.36 for ya 0:15; 0:30; 0:72 and 0:77, respectively. A single-step oxidation was observed for (UyTh1ˇy)O2 solid solutions with ya 0:15‐0:77. The activation energy for the oxidation of (UyTh1ˇy)O2 powders with ya 0:15 and 0.30 (the final product is single-phase), was found to be (48.5 a 2.5) kJ mol ˇ1 . The activation energy for the oxidation of (UyTh1ˇy)O2 powders with ya 0:72 and 0.77 (the final product is a bi-phasic mixture) was found to be

Reprocessing of carbide fuels: Conversion of carbide to nitride as a head end step

Abstract When irradiated carbide fuel is dissolved in nitric acid for reprocessing, many organic compounds are produced which are likely to interfere in the subsequent steps of fuel reprocessing. To avoid the production of these deleterious organic compounds, the carbide fuel needs to be converted into an oxide or nitride before dissolution. In this paper, the optimum conditions for converting the carbide into nitride are discussed and the feasibility of the removal of carbon liberated in the conversion reaction, as methane is also examined. Results of the experiments on the reaction of nitrogen/hydrogen with uranium carbide and (U, Pu) mixed carbide with Pu/(U + Pu) = 0.70, are reported.

Structural characterization of electrodeposited boron

Structural characterization of electrodeposited boron was carried out by using transmission electron microscopy and Raman spectroscopy. Electron diffraction and phase contrast imaging were carried out by using transmission electron microscopy. Phase identification was done based on the analysis of electron diffraction patterns and the power spectrum calculated from the lattice images from thin regions of the sample. Raman spectroscopic examination was carried out to study the nature of bonding and the allotropic form of boron obtained after electrodeposition. The results obtained from transmission electron microscopy showed the presence of nanocrystallites embedded in an amorphous mass of boron. Raman microscopic studies showed that amorphous boron could be converted to its crystalline form at high temperatures.