Internal Gelation Research Articles

Preparation of uranium nitride in the form of microspheres

Uranium nitride microspheres were fabricated by internal gelation and carbothermic reduction. The influence of the thermal treatment and the reaction atmosphere on the chemical composition and the structural parameter of the spheres were systematically investigated. High density (> 95% TD) spheres of 800 μm diameter were obtained by reacting in argon-hydrogen followed by nitrogen-hydrogen. Porous spheres with a distinct pore and grain structure and low crushing strength as feed for pellets have been fabricated in nitrogen and nitrogen-hydrogen atmosphere. Special emphasis was put on a reliable determination of nitrogen and on X-ray diffraction for the chemical composition and on the correlation of the crushing strength to structural parameters.

Gel supported precipitation for rare earth containing ceramics

Sol gel techniques such as water in oil emulsion, internal gelation and gel supported precipitation developed in the early sixties for nuclear fuel microspheres may find useful applications in preparing various types of rare earth containing ceramics, both as finished microsphere products or as precursor powders suitable for the preparation of monolithic products by pressureless and/or hot pressing sintering. The preparation of ZrO 2-Y 2O 3 and YBa 2Cu 3O x powders by gel supported precipitation is highlighted in this paper.

Studies of the Preparation of Thoria and Thoria-Urania Microspheres Using an Internal Gelation Process

An internal gelation process has been studied for the preparation of thoria microspheres. The gelation behavior of an acid-deficient thorium nitrate solution (NO3/Th4+ = 3.0) with hexamethylenetetr...

Fabrication of high density UO 2 fuel pellets involving sol-gel microsphere pelletisation and low temperature sintering

Powder-free sol-gel microsphere pelletisation and low temperature (1473 K) oxidative sintering processes were used in combination for fabrication of high density (≥ 96% TD) UO 2 fuel pellets for pressurised heavy water reactors. The “internal gelation of uranium” process of BARC was modified for preparation of hydrated gel-microspheres of UO 3 containing “carbon black” pore former. The gel-microspheres were subjected to controlled air-calcination at 973 K, followed by hydrogen reduction to obtain porous, dust-free and free-flowing UO 2 microspheres suitable for direct pelletisation at 225 MPa. Oxidative sintering of these pellets at 1473 K in CO 2 atmosphere followed by Ar+H 2 treatment led to high density (≥96%TD) UO 2 pellets having equiaxed grains of < 10 μm and uniformly distributed “closed” spherical pores in the diameter range of 2–5 μm. Resintering of these pellets at high temperature (1973 K) for 8 hours in Ar+8% H 2 atmosphere did not show any significant change in pellet dimension or grain size. The UO 2 pellets prepared by sol-gel microsphere pelletisation route had higher thermal conductivity compared to pellets of equivalent density prepared by the “powder-pellet” route. UO 2 pellets of large grain size (45–55 μm) and high density could be obtained with TiO 2 dopant and high temperature sintering in Ar+H 2 atmosphere. TiO 2 dopant was not effective for low temperature oxidative sintering.

Urea and urotropine conversion in the preparation of microspherical metal oxide particles by internal gelation

The results of studies on urea and urotropine product distribution in solutions formed in microspherical nuclear fuel preparation by internal gelation are presented. It is shown that urea, urotropine and formaldehyde, the urotropine hydrolysis product, undergo various conversions in solutions, resulting in methylolurea and urea-formaldehyde resins formation. Results on the influence of acidity on the process as a function of time are presented. The organic content in the resulting solutions and microspheres has been determined.

State and tendency of developing physical-chemical methods for making actinide microspherical particles

To obtain actinides oxides as microspheres, the internal gelation method is suggested. As a dispersive medium, we propose to use hydrocarbons containing fluorine and having a density of 2000 kg/m3. These substances are chemically inert, fire-proof, non-toxic and resistant to γ-radiolysis. Using a fluorohydrocarbon-containing liquid creates in the gelation column a vertical temperature gradient with minimal temperature in the zone of working solution dispersion and maximal one in the upper part of the column. This article treats the mechanism and model of a drop-forming process to control the process of microspheres formation, in which a working solution is dispersed in a dispersive medium.

Physico-Chemical Characteristics of Uranium Oxide Microspheres Produced by Internal Gelation

The developed Sol-Gel Hydrolysis Process was applied to produce uranium oxide microspheres suitable for use as nuclear fuel. Different preparation conditions of washing, feed solution aging and microsphere soaking were tried to produce dense UO3- and UO2-micospheres. The effect of reduction time and temperature on the density and porosity of the microspheres was carried out to optimize the preparation parameters. The total surface area, total pore volume and pore radius of UO3- and UO2-microspheres were measured through N2 gas adsorption and application of the BET-equation at 77 K. The effect of the preparation conditions on crystallite size of the samples was made through ray diffraction analysis. Examination of the prepared uranium oxides by differential thermal analysis and thermal gravimetric analysis was carried out.

Characterization of sintering of gelled UO3 microspheres

UO3 microspheres, prepared using an internal gelation process, were reduced and sintered. Hydrogen and argon atmospheres were used at temperatures ranging from 1100 °C to 1400 °C for different durations.It was found that the sintered density is higher for spheres réduced at 700 °C than at any other reduction temperature. Lower densities were achieved in argon atmosphere than in hydrogen for the same sintering and reduction temperature. More than 98% Th.D. could be obtained by calcining the UO3 spheres at 400 °C for 1 hr, reducing at 700 °C and then sintering at 1400 °C for 2 hrs in hydrogen atmosphere.

Application of emanation thermal analysis for characterization of intermediate products of urania and thoria ceramics

Emanation thermal analysis (ETA) is demonstrated as a tool for checking the processes taking place in the preparation of urania ceramics from uranyl gel and urania powder. By means of this method solid phases arizing during heat treatment of the uranyl gel microspheres prepared by internal gelation technique were characterized. The differences in thermal behaviour of various uranyl gel samples were shown, the samples differing in the concentration of uranium and gelation additives (carbamide and urotropine), in the way of drying, washing and aging of the gel. The differences in the sinterability of urania powders (delivered commercially) were tested by means of ETA during the heat treatment in reducing atmosphere. For thoria powder, obtained by the decomposition of thorium oxalate, the kinetics of the changes taking place at early sintering stages, was determined by means of emanation thermal analysis. The emanation thermal analysis has been recommended as the method for characterization of intermediate products of urania and thoria ceramics.

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

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.

Immobilization of yeast cells by internal gelation of alginate

A method for immobilizing yeast cells in alginate particles by internal gelation was investigated. The alginate is gelled in situ by the liberation of Ca ions from insoluble calcium compounds through the pH lowering which accompanies d-glucono-1,5-lactone hydrolysis in the presence of solution. With the internal gelation method gel particles of any shape, and novel types of fermentation reactors, can be prepared. The characteristics of the internally gelled, immobilized cell particles were evaluated using a standardized fermentation procedure. The new gelation method exhibited promising characteristics with respect to fermentation rate and gel strength .

A new principle for immobilized yeast reactors based on internal gelation of alginate

Internal gelation of alginate byin situ liberation of Ca2+ ions permitted production of a one-piece continuous immobilized yeast cell reactor unit having vertical internal flow channels. The reactor showed good characteristics with respect to gel stability, gas-release and fermentation stability over the 3.5 month test period.

Chemical Flowsheet Conditions for Preparing Urania Spheres by Internal Gelation

Small, ceramic urania spheres can be prepared for use as nuclear fuel by internal chemical gelation of uranyl nitrate solution droplets. Decomposition of hexamethylenetetramine (HMTA) dissolved in the uranyl nitrate solution releases ammonia to precipitate hydrated UO/sub 3/. Previously established flowsheet conditions have been improved and modified at ORNL and have been applied to prepare dense UO/sub 2/ spheres with average diameters of 1200, 300, and 30 ..mu..m. Acid-deficient uranyl nitrate (ADUN) solutions up to 3.4 M in uranium with NO/sub 3//sup -//U mole ratios of 1.5 to 1.7 are prepared by dissolution of U/sub 3/O/sub 8/ or UO/sub 3/. Continuous mixing of metered, cooled ADUN containing urea and HMTA solutions provides a smooth, regulated flow of the temperature-sensitive feed solution. The gelation times for solution drops in organic liquids at 45 to 95/sup 0/C depend on both the chemical reaction rates and the rates of heat transfer. The gel properties vary with temperature and other gelation variables. Gelation conditions were determined which allow easy washing, drying, firing, and sintering to produce dense UO/sub 2/ spheres of all three sizes. The 1200- and 300-..mu..m UO/sub 2/ spheres were pepared by gelation in trichloroethylene at 50 to 65/sup 0/C; 2-ethyl-l-hexanol wasmore » used as the gelation medium to prepare 30-..mu..m UO/sub 2/ spheres. Washing and drying requirements were determined. The gel dried to 225/sup 0/C contains about 95% UO/sub 3/; the remaining components are H/sub 2/O, NH/sub 3//sup -/, which are volatilized during firing to UO/sub 2/.« less

Emanation Thermal Analysis—Results from Characterizing Intermediate Steps and Intermediate Products for Urania Spheres

Emanation thermal analysis (ETA) is demonstrated as a tool for the objective characterization of solid phases arising during treatment of the uranyl gel microspheres up to the final UO2 product. The method is able to monitor very sensible differences in the morphology, surface area, and structure of a solid, as well as their changes when heated. The ETA results for seven samples are discussed and correlated with results from other independent methods (differential thermal analysis, thermogravimetry, thermodilatometry, and density measurements). Various preparation conditions of the internal gelation technique, such as uranium concentration, concentration of gelation additives, the way of leaching, etc., are reflected. The ETA is proposed as a method for simulating thermal processes of UO2 fuel preparation.

Determination of the region of existence of U(VI) gels prepared by internal gelation

Abstract Regions of U(VI) gel formation at 90°C were determined for solutions with variable contents of uranyl nitrate, urotropine ( hexamine  hexamethylene tetramine or HMTA), and urea and for three distinct values of the (NO 3 − /U 2 2+ ) ratio with the aim of better understanding the process of internal gelation. The mechanism of the gel phase formation depends on urea concentration in the initial solution. At urea concentrations below 3 mole/1 the spatial gel lattice is formed by chaining colloid particles of hydrolytic products of hexavalent uranium. At higher urea concentrations, the formation of an organic skeleton of urea-formaldehyde resin participates in the solidification reaction. The acidic medium necessary for the copolymeration reactions is held constant by the buffering effect of hydrolyzed uranyl ions. The latter mechanism of solidification is suppressed in acid-deficient solutions from which the small effect of high amounts of urea on the form of the gel region follows with systems in which (NOjVUOl) is lower than 2.

Crystal habit and phase attribution of U(VI) oxides in a gelation process

The effects of key process variables on the product characteristics of urania microspheres prepared by an internal gelation method are determined by studies of the crystal structure and morphology of urania in individual gel microspheres. Three compounds in the system UO 3NH 3H 2O are identified, and their characteristic crystal morphologies are determined. The results show that information obtained in this way provides a sound basis for the attribution of phases, for evaluating process conditions and for optimizing process variables.

Uranyl prepared by the internal gelation. Part I. The determination of the gelation regions in uranyl nitrate systems

In the present paper preparation conditions for the formation of various types of uranyl gel, using the internal gelation route, are described. Various types of the gel can be prepared differing in their physico-mechanical properties. The research parameters were selected with respect to practical preparation conditions. The results of the paper represent an attempt to investigate systematically some of the basic chemistry problems of the gel process.

Fabrication of (Pu-U) oxide microspheres using an internal gelation method

This article describes the application of an internal gelation sol-gel method for the preparation of (U, Pu)O 2 microspheres. A mixture of PuO 2 sol, a concentrated denitrated uranyl nitrate solution, urea and hexamethylenetetramine are injected into a glass column containing paraffin oil and tetrachloroethylene in an upward flow. The reaction conditions for the different Pu/U ratios are given as well as the optimum washing conditions. The drying and thermal treatment of the spheres are discussed and densities and chemical compositions of the spheres are given. Some sphere cross sections are presented.