Phase tailoring and temperature-induced phase evolution of SrZP-(Ce, Nd, Sm)PO4 dual-phase ceramic waste forms

In this study, a series of (1- x )Sr 0.5 Zr 2 (PO 4 ) 3 - x (Ce 1-2 y Nd y Sm y )PO 4 (abbreviated as SrZP-(Ce, Nd, Sm)PO 4 , x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1; y = 0, 1/12, 2/12, 3/12, 4/12, 5/12, 6/12) dual-phase ceramic waste forms were fabricated through a one-step microwave sintering technique. The phase evolution, micromorphology, and densification behavior of these ceramics were systematically investigated using XRD, Raman spectroscopy, SEM-EDS, and density measurement. The results confirmed that the dual-phase ceramics exclusively consisted of SrZP and (Ce, Nd, Sm)PO 4 phases, which coexisted stably and independently, enabling simultaneously effective immobilization of simulated fission products (Sr 2+ ) and actinides radionuclides (Ce 3+ , Nd 3+ , and Sm 3+ ). The ceramics exhibited a homogeneous distribution of both phases, a compact micromorphology, and relative densities achieving a maximum of 98.9%. It was demonstrated that alterations in the phase content ( x value) induced only negligible variations in the lattice parameters of both crystalline phases, indicating minimal structural perturbations. Rietveld refinement analysis revealed a gradual reduction in the lattice parameters of the (Ce, Nd, Sm)PO 4 phase with increasing y value, consistent with the anticipated immobilization of the simulated actinide radionuclides. In addition, by integrating XRD and Raman analyses, the temperature-induced phase evolution of the representative sample SrZP-Ce 1/3 Nd 1/3 Sm 1/3 PO 4 was comprehensively characterized. The results indicated that the Ce 1/3 Nd 1/3 Sm 1/3 PO 4 phase preferentially formed over the SrZP phase during the dual-phase ceramic preparation. Furthermore, the Kissinger model based on TG-DSC results at various heating rates ( β T = 10, 15, 20, and 25 K/min) provided additional thermodynamic evidence, in which the Ce 1/3 Nd 1/3 Sm 1/3 PO 4 phase possessed a lower apparent activation energy (353.35 kJ/mol) than the SrZP phase (501.33 kJ/mol). Besides, the SrZP-Ce 1/3 Nd 1/3 Sm 1/3 PO 4 dual-phase ceramic waste form demonstrated high chemical stability, with normalized leaching rates of all elements remaining relatively low ( LR Sr ∼ 10 −4 g·m −2 ·d −1 , LR Ce ∼ 10 −7 g·m −2 ·d −1 , LR Nd ∼ 10 −7 g·m −2 ·d −1 , and LR Sm ∼ 10 −6 g·m −2 ·d −1 ).