Formation of lanthanide phosphates in molten salts and evaluation for nuclear waste treatment

Abstract

The formation of phosphates of the lighter lanthanides (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Dy) by reaction with alkali metal phosphates has been studied in a LiCl–KCl eutectic melt between 450 and 650 °C and in a NaCl–KCl equimolar melt at 750 °C and under an inert atmosphere. Alkali metal ortho-, meta- and pyrophosphate precursors have been employed. The reaction results in the formation of single lanthanide or double alkali metal–lanthanide orthophosphates, the former favoured in LiCl–KCl melts and the latter in NaCl–KCl melts. The mean crystallite size of precipitated phosphates was evaluated from X-ray powder diffraction patterns, and found to be within 300–450 Å. Increasing the melt temperature results in increasing the crystallite size of phosphate phases. This technique offers an attractive means of removing lanthanide fission products from chloride melts arising from pyrochemical reprocessing of spent nuclear fuels. LiCl–KCl based melts are recommended because the bulk of phosphate waste precipitated is smaller since normal rather than double phosphates are formed therein.