Study on Formation and Aqueous Durability of Monazite Phosphate Glass-Ceramics for Nuclear Waste Immobilization

The immobilization of specific nuclear wastes using phosphate glass-ceramics is one of the most promising technologies; it is essential to simplify the synthesis process and identify the key factors. A monazite phosphate glass-ceramic waste form was synthesized via a simplified melt cooling process using iron-boron-phosphate glass as the matrix and Ce as a simulated nuclide. The main formation process involved the decomposition of H3BO3 below 150°C and the formation of H4FeP2O7 from the reaction between NH4H2PO4 and Fe2O3 accompanied by the appearance of an amorphous phase at around 300°C. The formation of the phosphate phases, including Fe(PO3)3 and CePO4 (monazite) at 750°C, and the gradual dissolution of these phosphate phases into the resulting phosphate glass melt at 900°C to 1000°C.Ultimately, the formed monazite phase persisted as the crystalline phase due to its slow dissolution in the melt, and the mixture was quenched to obtain the glass-ceramic waste form. Moreover, the process was facilitated by reducing the particle size of the raw materials; one of the key factors was the heat treatment temperature. The prepared glass-ceramic waste form exhibited good aqueous durability (~10−6 g·m−2·d−1) and thermal stability. These findings imply that certain phosphate glass-ceramics could be synthesized via a simple melt cooling process for nuclear waste immobilization.