Modeling the UO2 reduction process

Abstract

Methods of molecular dynamics and DFT calculations have been used to study the reduction mechanisms of UO2 as the most representative part of spent nuclear fuel to metallic uranium. It is shown that the critical softening of the combined modulus of elasticity C11-C12 to zero is the reason for the destruction of the UO2 crystal as a result of the removal of oxygen from it. This destruction is accompanied by an order-disorder phase transition in the oxygen subsystem of the crystal under consideration. DFT calculations indicate a continuous decrease in the band gap as oxygen is removed from the UO2 crystal. When the system reaches the composition U2O3, the band gap disappears and the system becomes electrically conductive. The appearance of the dielectric-conductor transition explains the realization of the FFC Cambridge process during the recovery of spent nuclear fuel. The passage of Li+ and Cl– ions of the LiCl melt through cylindrical channels in a UO2 crystal with cross-sectional radii from 0.25 up to 2 nm has been studied. The strength of the external electric field required for the passage of these channels decreases with an increase in the channel cross section, and the number of Cl– ions entering the channel increases. On the walls of the channels that pass ions with charges of both signs, colonies of adsorbed Cl– and Li+ atoms appear separated from each other, between which strong electric fields are formed. The existence of such fields can cause Li+ ions to move deep into the material being reduced.