Characterization of the Kinetics of NF3-fluorination of NpO2

Solid NpO2 has been contacted by gaseous NF3 under isothermal conditions at 450°C, 475°C, and 500°C; and the resulting reactions have been monitored using thermogravimetric analysis. In each case, at least two sequential reactions are clearly observed. The first reaction is fluorination of NpO2 to NpF4 and the second is oxidation and fluorination of NpF4 to NpF6. Careful observation of the experimental reaction curves reveals evidence of several physical and chemical mechanisms occurring sequentially and at times simultaneously. As such, a mathematical modeling approach utilizing a combination of sequential and parallel fundamental gas-solid reaction mechanisms (chemical reaction, diffusion, and phase boundary) is, in general, found to provide representative reaction curves that are in good agreement with experimental reaction curves. The correspondence of fundamental reaction mechanisms with distinctive characteristics of the experimental reaction curves (maximums and inflection points) provides insight into the physical and chemical nature of each reaction being monitored.