Thermal Analysis of Ammonium Diuranate

The kinetics of decomposition of gelled UO 3 microspheres has been studied by thermal analysis (DTA and TG) and by X-ray examination. UO3 microspheres prepared via/~ hydrolysis process were used in the study. The DTA and TG curves were obtained in various atmospheres at different heating rates. The activation energies for the various stages of decomposition were calculaled using the Carrol and Manche method.

Section: Resultssupporting

confidence: 89%

Kinetics of thermal decomposition of gelled UO3 microspheres

Bishay

Abdel-Halim

Abadir

1987

“…Washing-with hot water (UN spheres) shifts the decomposition towards higher temperatures. This result is in good agreement with that of Turcanu [7]. The microspheres incorporate urea and HMTA, when the resulting gases will have to penetrate a barely permeable membrane, reducing the gas release rate.…”

Section: Resultssupporting

confidence: 88%

“…The DTA curves obtained are similar to those published earlier [6,7]. The results show certain stages of decomposition, each involving a loss in weight.…”

Section: Resultssupporting

confidence: 85%

“…The contribution of the organic compounds (included in the microspheres) to the volume of eliminated volatile compounds will be particularly important, with direct influences on the porosity and on the chemical composition [11,12]. Turcanu et al [7] suggested a general formula UO3"0.66NH3"4H20 for the unwashed microspheres, and stated that the water is completely eliminated at 460 ~ .…”

Section: Resultsmentioning

confidence: 99%

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Effects of washing conditions on the thermal decomposition behaviour of gelled UO3 microspheres

Bishay

Halim

Hammad

et al. 1987

DTA, TG and DTG curves obtained in various atmospheres using different heating rates were used together with X-ray examinations to study the thermal decomposition mechanisms of two types of gelled UO 3 microspheres: ammonia-washed (UN) and hot water-washed (UH) microspheres.The kinetics of the thermal decompositions were studied. The specific reaction rate constant k r for the decomposition of UO 3 to U30 8 could be expressed in terms of the activation energy and the pre-exponential factor by the expressions: RTDense UO z microspheres for reactor fuels are prepared by wet chemical processes. There are several wet chemical methods that differ considerably from each other. They are generally known as "sol-gel" processes [1,2].They have in common the fact that a solution of uranium (uranyl nitrate) is dispersed into droplets and thereafter solidified by chemical precipitation on reaction. These gel particles are processed into the required ceramic microspheres, mainly by thermal treatment.To achieve good spherical kernels with high density, the precipitation must always be gel-like instead of crystalline, since only a gel structure keeps the particles spherical during drying (via uniform shrinkage) whereas a crystalline precipitate results in imperfect spherical kernels.The wet chemical processes are commonly classified into external [3] and internal [4] gelation, according to the made of precipitation.The aim of the present investigation was to study the effects of washing on the dissooiation mechanism of gelled microspheres and to study some related kinetic parameters.

“…The first step, at 220 + 5 ~ is characteristic of the removal of incorporated ammonia. The second step, at 280+ 7 ~ , is [7]. According to Haas et al [8] and F6rtmann [9], UO3 gel microspheres have compositions of the form UO 3 "xNH 3 9 yH20, where the values of x and y depend on the preparation conditions.…”

Section: Resultsmentioning

confidence: 99%

Thermal and X-ray analysis studies of UO3 gel microspheres

Abdel-Halim

Ali

Afifi

et al. 1987

The thermal decomposition behaviour of UO 3 gel microspheres has been studied. Thermal analysis involved DTA, TG, DTG and X-ray examination.The effects of washing the UO 3 gel microspheres with hot water and with ammonia solution were investigated, as were the effects of soaking the microspheres in ammonia solution before or after washing on the crystallite size.The results indicate that the thermal decomposition of UO3 gel microspheres includes five steps: the first two for dehydration, the third for ammonia removal, the fourth for ammonia oxidation and the last step for UO 3 recrystallization. This paper describes a study of the thermal analysis and X-ray diffraction of UO3 gel microspheres prepared by hydrolysis [1]. Hydrolysis is one of several sol-gel processes [2-4] which have been developed for the fabrication of UO3 microspheres can be used as fuel kernels in coated particle fuel for high-temperature gas cooled reactors. For the application of UO3 microspheres in light water and fast breeder reactor fuel elements, two different fuel forms are being considered. Sphere-pack fuel rods are formed by the vibrocompactions of microspheres and pelletizing of the microspheres.Hydrolysis is a very simple process based on the rapid solidification of droplets from a fairly concentrated uranyl nitrate solution (about !.4 M) containing urea and hexamethylenetetramine at temperatures of about 90 ~ in silicone oil. The resulting microspheres are deep-orange and fully transparent. After washing in CC14 to remove silicone oil, the kernels are either washed with hot water for 10 rain at 95 ~ or with 3% ammonia solution for 2 hours at 25 ~ Hot water-washed microspheres are opaque and yellow, while ammonia-washed microspheres are transparent and orange.