Immobilization of radioactive fluoride waste in aluminophosphate glass: a molecular dynamics simulation

Molecular dynamics (MD) simulations were conducted to investigate the structural and chemical environment of aluminum in aluminophosphate glasses. Such glasses have the potential for application in the disposal of radioactive fluoride waste from molten salt reactors (MSR). Due to the risks of studying these materials and the limitations of realistic research conditions, MD simulations were used as an alternative method to study the vitrification of radioactive fluoride waste. In the past decades, aluminophosphate glasses have been studied and they exhibit certain favorable properties for high-level radioactive waste management. This work focuses on the effects of fluorine addition on structural changes in the glass. We observed that glass with composition P2O5-Al2O3-Na2O-CaO exhibited a good performance in immobilizing fluoride at low F concentrations (approximately under 25 mol%). Significant changes were observed where PO3F2 units replaced PO3F units in the glass. The four-coordinated AIO(4) units were increasingly converted into five-coordinated [A10(x)F(y)] with the increasing F content. The radionuclide Sr in the simulation had the tendency to form six-coordinated octahedrons in the glass. We conclude that the structural changes resulting from the fluoride waste added to aluminophosphate glasses does not adversely affect their chemical stability at relatively low F concentrations, i.e., under 25 mol%. Hence, the use of phosphate glasses is a potential alternative method of fluoride waste disposal.