V.N. Vaidya

A study of chemical parameters of the internal gelation based sol-gel process for uranium dioxide

Abstract Internal gelation process is one of the important sol-gel routes for the preparation of spherical particles of fuel materials. Successful preparation of defect free fuel particles has been reported only with a narrow range of feed solution compositions. Investigations have been carried out to study the gelation behaviour of solutions containing uranyl nitrate, hexamethylene-tetramine (hexa) and urea with a view to defining the regions of possible interest to the process. A gelation field diagram has been constructed defining regions where a single phase gel can be readily obtained. A number of compositions from this gelation field diagram have been used for the preparation of UO 2 microspheres and it was observed that good spherical particles could be obtained with uranium concentrations ranging from 0.7 to 1.5 molar. The mole ratio (hexa, urea)/uranium for obtaining good particles decreased with increasing uranium concentration.

Synthesis of yttrium aluminium garnet by the glycerol route

Abstract Yttrium aluminium garnet (YAG), represented by the chemical formula Y 3 Al 5 O 12 , was synthesised from the stoichiometric mixture of the metal nitrate solution by boiling with glycerol. For a fixed batch size, the glycerol amount was systematically varied to study its effect on the specific surface area and phase purity of the YAG powder. The quantity of glycerol was optimised to get the desired powder characteristics. Using this technique, single phase YAG powder of high specific surface area was obtained at 1000 °C. Characterisation was done by thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), particle size analysis and by measuring the specific surface area, as well as the carbon content, at various stages, to ascertain the quality of the product.

Synthesis of yttrium aluminium garnet by the gel entrapment technique using hexamine

Abstract A novel technique has been developed for the preparation of yttrium aluminium garnet (YAG), represented by the chemical formula Y3Al5O12, by achieving the homogeneous precipitation of the constituent metal hydroxides from the stoichiometric mixture of the metal nitrate solution by the addition of hexamethylene tetramine solution. Aluminium ions form a gel at pH 4, entrapping all the liquid, followed by the precipitation of yttrium ions at a higher pH. In spite of the sequential precipitation, the resultant mass is homogeneous because of the gel entrapment. Using this technique, singlephase YAG powder of high specific surface area (80 m2 g−1) was obtained at 810 °C. Characterisation was done by thermogravimetry, differential thermal analysis, X-ray diffraction and scanning electron microscopy to ascertain the high quality of the product. The technique was subsequently used for the production of highpurity neodymium-doped YAG powder with excellent microhomogeneity on an increased batch size of 1 kg.

Fabrication of UO2 pellets by gel pelletization technique without addition of carbon as pore former

Abstract High density UO 2 pellets for pressurised heavy water reactor fuel were fabricated by a gel pelletization technique from soft UO 2 microspheres prepared by an internal gelation process. Chemical and heat treatment parameters were determined for obtaining soft UO 2 microspheres with optimum properties required for making defect-free, high density, UO 2 pellets. Investigations reveal that a high uranium concentration in the feed broth during the preparation of UO 3 gel particles, conversion to UO 2 via U 3 O 8 , and optimized calcination and reduction temperatures, result in UO 2 microspheres suitable for obtaining good quality green compacts. Green pellets were densified by low temperature sintering in a C0 2 atmosphere at 1200°C to obtain high density UO 2+ x pellets and reduced at 800°C to UO 2.00 in Ar + 8%H 2 Resulting pellets were free from open porosity and black berry structure.

Oxidation and hydrolysis kinetic studies on UN

Abstract The reaction of oxygen and water vapour with UN microspheres containing 0.78 and 10.9 mol% UO2 as impurity was studied under non-isothermal heating conditions in a thermobalance under different partial pressures of oxygen, a fixed pressure of water vapour in argon, and in air. Uranium mononitride was ultimately converted to U3O8, with the formation of UO2 and U2N3 as intermediates. The end product of pyrohydrolysis was UO2. The kinetic parameters were evaluated and the mechanism of the reaction was suggested. Different kinetic models were used to explain the oxidation behaviour of UN.

Kinetic study of the carbothermic synthesis of uranium monocarbide microspheres

Abstract Uranium monocarbide microspheres were synthesized by carbothermic reduction of porous uranium oxide microspheres with uniformly dispersed carbon black. Kinetics of the reduction was studied under vacuum and flowing inert gas from 1250 to 1550° C. The carbon monoxide gas concentration in the effluent stream during reduction was used to determine the rate of carbide formation. Under vacuum, reduction was found to be controlled by reaction at the reactant-product interface whereas under flowing gas conditions, the diffusion of carbon monoxide gas through the carbide layer was the rate controlling process. The activation energy was 335.1 ± 8.6 and 363.7 ± 7.6 kJ/mol for reduction under vacuum and flowing gas, respectively.

Kinetics of the carbothermic synthesis of uranium mononitride microspheres

Abstract The mononitride of uranium is an important nuclear fuel material. Kinetics of UN microspheres preparation by the reaction of carbon containing porous uranium oxide microspheres with nitrogen was studied in the temperature range of 1573 to 1823 K. Carbon monoxide concentration in the nitrogen stream during the reaction was used to determine the rate of formation of the nitride. The results show that the carbothermic synthesis of the mononitride microspheres follows a first order rate equation and the value of the energy of activation for the reaction was found to be 365.7±14.9 kJ/mol. The diffusion of carbon monoxide through the product layer appears to be the rate controlling step.

Removal of plutonium and americium from oxalate supernatants by co-precipitation with thorium oxalate

As a part of treatment of low level active waste, co-precipitation of plutonium (Pu) and americium (Am), with thorium oxalate, from oxalate supernatants generated during plutonium oxalate precipitation has been investigated. A simple method for the simultaneous removal of both Pu and Am from oxalate supernatants could be developed. This simple process achieves incorporation of these alpha active nuclides into small volumes of solid matrix from large volumes of aqueous waste.

The oxidation of uranium monocarbide microspheres

Abstract The reaction of oxygen with UC microspheres was studied as a function of heating rate, partial pressures of oxygen and sample size under nonisothermal heating conditions. The ultimate product of the oxidation was U3O8 with the formation of UO2 as an intermediate. The kinetic parameters were evaluated and conditions suggested for ignition free bulk oxidation of UC.

Kinetics and mechanism of UO2 + C reaction for UCUC2 preparation

Abstract The kinetics of the carbothermic conversion of (UO2 + C) microspheres to UC UC 2 in the temperature range of 1473 to 1973 K has been studied under nonisothermal heating conditions in vacuum and flowing argon. Reduction of UO2 microspheres, dispersed in a carbon matrix was also studied under vacuum. The carbon monoxide gas concentration in the effluent stream during reduction was used to determine the rate of carbide formation. A model representing stepwise conversion of the reactant to the product has been proposed and the probable rate controlling steps, under different reaction conditions, have been identified. The reaction of oxide with carbon under vacuum appears to be controlled by diffusion of carbon. However, under flowing gas conditions, the diffusion of carbon monoxide gas through the carbide layer was the rate-controlling process. The activation energies for the reaction carried out under different conditions varied between 332 to 373 kJ/mol.