Sr/Ce co-immobilization evaluation and high chemical stability of novel Sr0.5Zr2(PO4)3–CePO4 composite ceramics for nuclear waste forms

Sodium zirconium phosphate (labeled as NZP)-monazite-type (1-x)Sr_0.5Zr₂(PO₄)₃–xCePO₄ (x = 0–1.0) composite ceramics, which were designed to simultaneously immobilize simulated fission nuclide Sr and variable valence actinide nuclide Ce, were in situ prepared by one-step microwave sintering technique. The feasibility of Sr/Ce co-immobilization was evaluated via an investigation on the phase evolution, microstructure, density, Vickers hardness, and chemical stability of the composite ceramics. The Ce valence state in the composite ceramics was further ascertained by X-ray photoelectron spectroscopy. It was shown that the Sr/Ce co-immobilized composite ceramics only consisted of Sr_0.5Zr₂(PO₄)₃ and CePO₄ crystalline phases that were compatible well to each other. Sr and Ce were independently incorporated into Sr_0.5Zr₂(PO₄)₃ phase and CePO₄ phase, respectively. The valence state of Ce in composite ceramics existed in trivalent state. And the existence of CePO₄ phase caused the grain refinement and facilitated the densification of the composite ceramics. The composite samples all showed a highly uniform and dense microstructure, whose relative density was higher than 95% and Vickers hardness could attain 774 HV1. Importantly, the series of Sr_0.5Zr₂(PO₄)₃–CePO₄ composite ceramics exhibited higher chemical stability than that of the monophase Sr_0.5Zr₂(PO₄)₃ or CePO₄ ceramics, in which the normalized leaching rates of Sr and Ce were below 10⁻⁴ g·m⁻²·day⁻¹ and 10⁻⁷ g·m⁻²·day⁻¹ order of magnitude, respectively. The NZP-monazite-type composite ceramics has the potential to be a host for the disposal of high-level nuclear wastes containing multiple radionuclides.