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

Abstract

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).