Design and synthesis of high-entropy A2B2O7-type pyrochlore ceramics for the immobilization of molten salt radwastes

Abstract

Pyrochlore-based high-entropy ceramics can encapsulate multiple elements with different valences, making them promising for immobilizing high-level radioactive nuclides in molten salt radwaste. This approach addresses key challenges in dry reprocessing and molten salt reactor waste management. In this study, a single-phase pyrochlore-based high-entropy ceramic is synthesized to incorporate NdF3 and CeF3 as simulated molten salt radwastes. Various synthesis components have been explored, and the ceramics were characterized using x-ray diffraction (XRD) and Raman spectroscopy. Comparative analysis revealed that the size disorder is the critical factor to synthesize single-phase pyrochlore-based high-entropy ceramics. The effects of elemental ratio and the presence of variable valence elements on the synthesis of single-phase pyrochlore-based high-entropy ceramics are elucidated through the concept of size disorder. Two single-phase pyrochlore compositions have been successfully synthesized: #HE4 (Nd2(Ti0.25Zr0.25Hf0.25Sn0.25)2O7) and #HECe0.1 (Nd2(Zr0.3Hf0.2Sn0.2Nb0.2Ce0.1)2O7). These samples were tested with a 7-day leaching experiment using the Product Consistency Test Method PCT-B. The normalized release rates of all elements in #HE4 ranged from 10-7 to 10-9 g·m-2·d-1, indicating excellent chemical durability. These findings suggest that medium- and high-entropy pyrochlore-based ceramics are effective for immobilizing radioactive nuclides in molten salt radwastes.