Study of mass and momentum transfer and their effect on the direct fluorination of uranium oxide

The mechanism for the fluorination of solid U3O8 to gaseous UF6 was found to be a two-step process with solid UO2F2 as an intermediate. The highest particle temperatures were found to be associated with the initial reaction step to UO2F2; it was recommended that these temperatures be maintained below 17000F. The chemical equilibrium constant for the fluorination of PuF4 to PuF6 was found to be unexpectedly low at typical flame tower temperatures. Although not confirmed, there is an indication in the literature that a similar equilibrium constant is associated with the fluorination of NpF4 and other transuranic molecules. It was recommended that uranium oxides which are significantly contaminated with transuranics should not be processed through a direct fluorination reactor such as the UF6 flame tower. Reaction rate equations were developed for the fluorination of U3O8, UF4, PuF4 and NpF4. During the course of the development, a significant discrepancy was found in the literature for the activation energy of the fluorination of U3O8. Equations were developed for both a high and low limit rate constant for the fluorination of U3O8. A variey of momentum, heat and mass transfer equations were developed for both oxide particles and the gas phase within the flame tower. Equations were developed to estimate the physical and transport properties of each gaseous component and the gas mixture as a whole. These properties and the transport equations were used to estimate the reaction time and distance for oxide particles with both the low and high limit reaction rate constant. The procedures used to perform these calculations is limited to constant temperature and an oxide feed comprised of a single particle size. The results indicate that above 10000F the mass transfer of reactants and products becomes increasingly important to the overall rate of the reaction