Determination of the energy characteristics of the reactions UF6 ↔ UF5 + F and UF6 ↔ UF4 + F2

Quantum-mechanical methods are used to assess the energy barriers to dissociation and recombination reactions of UF₆ ↔ UF₅ + F and UF₆ ↔ UF₄ + F₂. 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 F₂, UF₄, UF₅ and UF₆ 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 UF₆ → UF₅ + 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 of UF₆ → UF₄ + F₂, there is an energy barrier with a height of about 0.3 eV. An initial approximation was proposed for the arrangement of UF₆ atoms in order to find the saddle points of the UF₆ dissociation reactions. In this initial configuration, all 7 atoms of the UF₆ 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 UF₆ ↔ UF₅ + F и UF₆ ↔ UF₄ + F₂. These constants are necessary for modeling the physicochemical processes occurring during the enrichment of spent nuclear fuel (SNF).