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

Abstract

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.