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Sodium zirconium phosphate (labeled as NZP)-monazite-type (1-x)Sr0.5Zr2(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 Sr0.5Zr2(PO4)3 and CePO4 crystalline phases that were compatible well to each other. Sr and Ce were independently incorporated into Sr0.5Zr2(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 Sr0.5Zr2(PO4)3–CePO4 composite ceramics exhibited higher chemical stability than that of the monophase Sr0.5Zr2(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. [ABSTRACT FROM AUTHOR]

Sodium Zirconium Phosphate (NaZr 2 (PO 4) 3 , hereinafter NZP) and monazite are both potential materials for immobilization of nuclear waste. In this work, novel (1- x)Sr 0.5 Zr 2 (PO 4) 3 - x NdPO 4 composite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) for simultaneously immobilizing the simulated fission product (FP) Sr and trivalent minor actinide (MA) Nd were prepared by one-step microwave sintering technique, in which Sr and Nd were immobilized into NZP and monazite type structures, respectively. The phase evolution and microstructure of the samples were investigated by X-ray diffraction (XRD), Raman, and backscattering scanning electron microscopy (BSE). The results showed that the expected composite ceramics were successfully obtained by one-step microwave sintering at 1050 °C for 2 h. The as-prepared samples consisted of Sr 0.5 Zr 2 (PO 4) 3 and NdPO 4 phases, and the content of the two phases varied regularly as x changed, generally conforming to the designed nominal chemical composition. Importantly, the composite ceramics presented the homogenous and dense microstructure. The relative density of the composite ceramics was more than 95%, meanwhile, the Vickers-hardness of the samples was higher than 600 MPa. It was indicated that NZP-monazite type composite ceramics could be a potential matrix for the simultaneous immobilization of actinide and fission product. [ABSTRACT FROM AUTHOR]

• Borosilicate glass immobilizing simulated actinides were synthesized. • The crystallization behavior of pressed powder and green pellet was investigated. • The relationship between the content of Ce3+/Ce4+ and chemical stability were revealed. • Borosilicate glasses nuclear waste forms have good chemical stability. A series borosilicate glasses containing three alkaline earth metals (Ca, Mg, Ba) and immobilized with ceria from low content (3 wt%) to exceed the solubility limit (20 wt%) were prepared by the solid-state melt method. The results of phase separation revealed by SEM, XRD and EDX show that simulated waste CeO 2 loading was below 13 wt%, and the excess part was tightly connected with glass. The controlled heat treatment of the quenched blocks and the green pellets was conducted to explore the effect of ceria content of the system on the crystalline state. The solidified waste forms with a ratio of Ce3+/Ce4+ close to 1 have the lower leaching rate after 28 d (LR Si =9.84 × 10−4 g m−2 d−1, LR Ca =6.68 × 10−4 g m−2 d−1) and LR Ce of all samples is always maintained in the order of magnitude of 10−5 g m−2 d−1, suggesting that cerium doped multiple alkaline earths borosilicate glasses have higher chemical stability. [ABSTRACT FROM AUTHOR]

Sodium zirconium phosphate (NaZr 2 (PO 4) 3 , NZP) family ceramics were considered as a potential host matrix for the immobilization of nuclear waste. In this study, the dense Sr-incorporated NZP ceramics with the formula of Na 1-2 x Sr x Zr 2 (PO 4) 3 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, and 0.5) were fabricated by microwave-assisted solid-state reaction method. The phase evolution, densification behavior and chemical stability of the as-prepared samples were systematically investigated by XRD, Rietveld refinement, Raman, SEM-EDS and PCT method. The results demonstrated that Na in the NZP lattice could be substituted by the simulated fission product Sr, and the phase composition gradually changed from NZP to Sr 0.5 Zr 2 (PO 4) 3 (SrZP) with the increase of Sr content. The samples presented high densification and uniform elements distribution after microwave sintering at 1100 °C for 2 h. Moreover, the as-prepared samples exhibited the superior chemical stability. The normalized elemental leaching rate of Sr, Na, P, and Zr were 10−3, 10−4, 10−3, and <10−7 g⋅m−2⋅d−1, respectively. [ABSTRACT FROM AUTHOR]

• The detoxified Cr-added FMB geopolymer was prepared by hydrothermal processing. • The detoxified Cr can be fixed in the skeleton structure of FMB geopolymer. • The sample still possesses higher compressive strength after hydrothermal processing. • The products exhibit excellent Cr(VI) leaching toxicity. The detoxification and disposal of hexavalent chromium [Cr(VI)] is of real importance to the ecological environment protection and human being health. In this paper, Cr(VI) was in advance reduced to trivalent chromium [Cr3+ by Fe2+ and was immediately immobilized in a "fly ash-metakaolin" based (FMB) geopolymer through alkaline activation and hydrothermal processing. The phase structure and microstructure of the as-prepared detoxified Cr-added FMB geopolymer were investigated by XRD, FT-IR and SEM. The compressive strength and Cr(VI) leaching toxicity of the samples were also systematically investigated. The results showed that the phase compositions of the samples were mainly composed of amorphous phase, several weak crystalline phases of quartz, gismondite and sodium chloride after hydrothermal processing. The detoxified Cr can be fixed as the form of chemical bonding in the skeleton structure of FMB geopolymer. In addition, the compressive strength test results revealed that the compressive strength was mainly affected by the contents of Cr addition and reducing agent. Importantly, the extremely low leaching concentration of Cr(VI) suggested that the Cr-added FMB geopolymer prepared by hydrothermal processing possessed excellent Cr(VI) leaching toxicity, and the reduced Cr3+ was difficultly reoxidized to Cr(VI) after being immobilized in the FMB geopolymer. [ABSTRACT FROM AUTHOR]