Exploring the effects of Sr/Ce/Nd co-incorporation on the phase evolution and chemical stability of Sr0.5Zr2(PO4)3-(Ce, Nd)PO4 multiphase ceramics designed for nuclear waste forms

In this work, 0.6Sr0.5Zr2(PO4)3-0.4Ce1-xNdxPO4 (x = 0–1) multiphase ceramics were fabricated using a one-step microwave sintering technique, where Sr and Ce/Nd were utilized as the substitutes for the fission products 90Sr and actinide nuclides in high-level radionuclide wastes, respectively. The influences of Sr/Ce/Nd co-incorporation on the phase evolution, crystal structure, micromorphology, densification, and chemical stability of the as-prepared multiphase ceramic waste forms were systematically investigated. It was detected that the multiphase ceramics were composed of Sr0.5Zr2(PO4)3 (SrZP) phase and monazite phase regardless of x value, and (Ce, Nd)PO4 monazite solid solution was formed with Ce/Nd substitution. Rietveld refinement results further verified the cell parameters of (Ce, Nd)PO4 monazite solid solution regularly decreased with increasing Nd replacement, and the structural change of crystalline monazite did not affect the structure of SrZP phase. All samples presented a well-compacted and homogeneous microstructure, and their micromorphology did not change significantly with the replacement of Ce by Nd. Moreover, the relative density of the as-prepared multiphase ceramics all reached more than approximately 94.5 %, demonstrating excellent densification. Importantly, the good compatibility of SrZP and (Ce, Nd)PO4 monazite phases brought out superior chemical stability. The normalized elemental leaching rate showed no significant change for different Ce/Nd substituted samples, whose normalized leaching rates of Sr, Ce, and Nd were about 10−4 g/(m2·d), 10−7 g/(m2·d), and 10−6 g/(m2·d) order of magnitude, respectively. It is demonstrated that SrZP-(Ce, Nd)PO4 multiphase ceramics can be used as candidate materials for simultaneously immobilizing fission products and actinide nuclides of high-level nuclear wastes.