KINETIC POSSIBILITIES OF CONTROLLED TRANSFORMATION RATE THERMAL-ANALYSIS (CRTA) - APPLICATION TO THE THERMOLYSIS OF HEXAHYDRATED URANYL-NITRATE

In the first part, this paper reviews several ways to derive kinetical results from Controlled transformation Rate Thermal Analysis (CRTA) experiments: applying the rate-jump method to measure the activation energy, determining the reaction mechanism simply from the shape of the curve and finally deriving both the activation energy and the reaction mechanism from a single CRTA experiment. Application to the 5 steps of the thermal analysis of UO2(NO3)2(H2O)2.4H2O shows that the layered structure of the hydrate leads to 4 dehydration steps essentially following a mechanism of nucleation and 2-dimensional growth whereas the denitration step seems to be controlled by a double mechanism of diffusion and desorption. The first 4 water molecules to leave are in the same starting state but evolve in 2 steps, well separated by CRTA and involving 3 and 1 molecule, respectively, which is understood by structural considerations.