Chemical and environmental stability of monazite-cheralite solid solutions Ln_1-2xCa_xTh_xPO₄ (Ln = Pr, Nd; x = 0–0.15): A thermodynamic study

Monazite-cheralite ceramics are a promising waste form for actinides. To elucidate the long-term behavior of this matrix in aqueous solutions, this study measured thermodynamic data for Th-rhabdophanes Ln_1-2xCa_xTh_xPO₄·nH₂O (with Ln = Pr, Nd; x = 0–0.15) and the associated anhydrous monazite-cheralites Ln_1-2xCa_xTh_xPO₄. Solubility experiments at 298 K and high temperature oxide melt solution calorimetry were combined for calculation of ΔG_f^°, ΔH_f^° and S_m^° 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.