Abstract
Monazite-cheralite ceramics are a promising waste form for actinides. To elucidate the longterm behavior of this matrix in aqueous solutions, this study measured thermodynamic data for Thrhabdophanes Ln 1-2x Ca x Th x PO 4 •nH 2 O (with Ln = Pr, Nd; x = 0-0.15) and the associated anhydrous monazite-cheralites Ln 1-2x Ca x Th x PO 4. Solubility experiments at 298 K and high temperature oxide melt solution calorimetry were combined for calculation of , and of Th-rhabdophanes and associated monazite-cheralites. Standard solubility constants were employed in a geochemical simulation using the PHREEQC software, the results of which confirmed the high chemical stability of the monazite-cheralite phases and supported their use as a specific conditioning matrix for the long-term immobilization of actinides.