Determination of the Energy Characteristics of the Reactions UF<sub>6</sub> ↔ UF5 + F and UF<sub>6</sub> ↔ UF<sub>4</sub> + F<sub>2</sub>
We used the quantum-mechanical methods to estimate the energy barriers of the dissociation and recombination reactions UF<sub>6</sub> ↔ UF<sub>5</sub> + F and UF<sub>6</sub> ↔ UF<sub>4</sub> + F<sub>2</sub>. The energy characteristics of these reactions are found to be strongly asymmetric: the dissociation reaction barriers exceed the recombination reactions barriers by more than 4 eV. The equilibrium atomic configurations of F<sub>2</sub>, UF<sub>4</sub>, UF<sub>5</sub>, and UF<sub>6</sub> have been determined using precision quantum mechanical calculations. The U-F bond lengths obtained as a result of the calculations are in good agreement with experimental data. It was found that the decay reaction UF<sub>6</sub> ↔ UF<sub>5</sub> + F is either barrier-free, or the energy barrier for such a reaction is less than the resolving power of the method (~ 0.1 eV). For the decay UF<sub>6</sub> ↔ UF<sub>4</sub> + F<sub>2</sub>, there is an energy barrier with a height of about 0.3 eV. An initial approximation was proposed for the arrangement of UF<sub>6</sub> atoms in order to find the saddle points of the UF<sub>6</sub> dissociation reactions. In this initial configuration, all 7 atoms of the UF<sub>6</sub> molecule are located in the same plane. The F atoms are located at the vertices of a regular hexagon, and the U atom is at the center of such a hexagon. The results of this work can be used to determine the constants of thermal reactions of dissociation and recombination UF<sub>6</sub> ↔ UF<sub>5</sub> + F and UF<sub>6</sub> ↔ UF<sub>4</sub> + F<sub>2</sub>. These constants are necessary for modeling the physicochemical processes occurring during the enrichment of spent nuclear fuel (SNF).