The reaction of hydrogen with uranium powder was investigated at 13.3 and 26.6 kPa between 50 and 250°C. The reaction order was independent of temperature but varied from 2/3-order at 13.3 kPa to 1st-order at 26.6 kPa. Increasing temperatures resulted in decreasing reaction rates over the temperature range studied. A reaction mechanism with adsorption as the rate controlling step is proposed to explain the temperature behavior. Decomposition of the hydride was found to follow a zero-order rate process.
The kinetics of the reactions between uranium metal, H2, HD and D2 have been examined. There is strong evidence that the reaction involves several steps of which activated adsorption of the gas by the metal is the rate governing process. Rate constants have been determined for the over-all processes in the temperature range 27.5 deg to 75 deg C. Energies of activation which are of reasonable magnitude have been calculated being 10,200 calories for the process involving D2, 8,500 calories for the process involving HD and 8,200 calories for the process involving H2. From a consideration of the zero point energies for these three molecular species of hydrogen, these results follow in the right order. However because of the adsorption step and since it is not possible to determine the heat of adsorption it is impossible to state that the observed activation energies are the true activation energies. Since the heat of adsorption during the process must be considered in the final interpretation of the results, the true activation energies are probably less positive than those observed. The adsorption step in the processes also accounts for the fact that the ratios of the rates of the reactions, which may be expected to differ inversely as the square roots of the reduced masses of the gas molecules, deviate somewhat from the theoretical values. Heterogeneous reactions such as those reported by this thesis do not lend themselves readily to kinetics experiments. Because of the reactivity of the uranium-hydrogen system it was not possible to determine to what extent the heat of adsorption enters into the values obtained for the observed activation energies. However other metal-hydrogen systems which do not interact so rapidly should better lend themselves to studies of this nature.
The kinetics of the reduction of some powdered uranium oxides by hydrogen and carbon monoxide was studied by means of a static method. Reductant pressures of about one millimeter of mercury were used, and reaction temperatures ranged from 590 to 950 deg C. It was found that for all oxide compositions and for reduction by carbon monoxide as well as by hydrogen, the reduction was controlled by a surface process rate expression (equation provided in paper).
