Kinetics of noncatalytic, nonisothermal, gas‐solid reactions: Hydrofluorination of uranium dioxide

A single‐pellet, stirred‐tank reactor was used to measure temperatures, compositions, and rates of reaction for the hydrofluorination of UO2.14 at atmospheric pressure. By correlating the rate with conditions at the reacting surface, the activation energy of the first‐order reaction was found to be 6,070 cal./g.‐mole. Since the product forms an unreactive layer around the core of UO2.14, and since the heat of reaction is high, significant concentration and temperature differences develop between the gas phase and the reaction surface. The temperature and rate data were used to calculate the effective diffusivity and thermal conductivity of the product layer under reaction conditions. An adaptation of a pseudosteady state method was developed for predicting the conversion and temperature history of a pellet. Using rate constants for reaction and mass and energy transfer derived from the experimental data, this method gave good results. Predictions with estimated rate constants were poor, indicating that an accurate value was necessary for the effective diffusivity in the product layer.