Investigations On Structure And Chemical Stability Of （1-x）Sr_0.5Zr₂（PO₄）₃-xCe_1-yNd_yPO₄ Composite Ceramics Waste Forms For Immobilizing Nuclear Waste

High level liquid waste（HLLW）,including various radionuclides,possess complex composition,and it has high biological toxicity and heat release rate,so selecting a suitable waste form is an important step in the disposal of HLLW.Due to the limitation of its crystal structure characteristics,some ceramic matrix is usually suitable for immobilizing certain source terms of actinides（An）or minor actinides（MA）,which has a huge limitation on the immobilization of complex HLLW containing actinides,minor actinides,and fission product（FP）in practice.Therefore,it is proposed that the composite ceramics with several crystalline phases might be the potential candidate for immobilizing HLLW with multiple radionuclides.In this work,the Sodium Zirconium Phosphate（Na Zr₂（PO₄）₃,hereinafter NZP）-monazite type composite ceramics for simultaneously immobilizing fission products and actinide radionuclides were explored.One-step microwave sintering was adopted to prepare the（1-x）Sr_0.5Zr₂（PO₄）₃-x Ce_1-yNd_yPO₄ composite ceramics（Sr,Nd and Ce were chosen as the surrogates for FP,trivalent MA and variable valence MA,respectively）.With the help of XRD（including structural refinement）,Raman,BSE-EDS and XPS,the phase evolution,crystal structure,microstructure,element distribution and the valence state of Ce of（1-x）Sr_0.5Zr₂（PO₄）₃-x Ce_1-yNd_yPO₄ composite ceramic waste forms were studied and the preparation technology of composite ceramic waste forms was improved by thermal analysis.The density and Vickers-hardness of the composite ceramic waste forms were measured by Archimedes method and Vickers-hardness tester to analyze its physical properties.The chemical stability of the composite ceramics was evaluated by PCT static leaching test.This work focuses on the preparation and the technology improvement of the composite ceramic waste forms to analyze its structure and properties.The study is as following:（1）The preparation,structure and properties of（1-x）Sr_0.5Zr₂（PO₄）₃-x Ln PO₄（Ln=Nd or Ce）composite ceramic waste forms for simultaneously immobilizing Sr and Ln were studied.The（1-x）Sr_0.5Zr₂（PO₄）₃-x Ln PO₄（Ln=Nd,Ce;x=0,0.2,0.4,0.6,0.8,1）composite ceramic waste forms were prepared by a one-step microwave sintering technique.The results showed that two kinds of composite ceramic waste forms composed of Sr_0.5Zr₂（PO₄）₃ and Nd PO₄(or Sr_0.5Zr₂（PO₄）₃ and Ce PO₄)were successfully prepared at 1050°C for 2 h.The two phases in the composite ceramic waste forms were well compatible with each other,and the content of the two phases altered regularly as x changed,which conformed to the designed nominal chemical composition.In（1-x）Sr_0.5Zr₂（PO₄）₃-x Ce PO₄ composite ceramics,the valence state of Ce ion was+3.Meanwhile,the two kinds of composite ceramic waste forms showed homogeneous and compact microstructure.The relative density and Vickers-hardness of composite ceramic waste forms were more than 95%and 600 HV1,respectively.Moreover,composite ceramic waste forms present so well in chemical stability that the addition of Nd PO₄ phase and Ce PO₄ phase in the composite ceramics improved the leaching stability of Sr_0.5Zr₂（PO₄）₃ phase without significantly affecting to the chemical stability of the waste forms.Compared with the single-phase ceramic,the composite ceramics possessed lower element leaching rates that the normalized leaching rates of Nd,Ce and Sr are～10^-7,～10^-7 and～10^-4 g·m^-2·d^-1,respectively.（2）The preparation,structure and properties of（1-x）Sr_0.5Zr₂（PO₄）₃-x Ce_1-yNd_yPO₄ composite ceramic waste forms for simultaneously immobilizing Sr,Ce and Nd were studied.The（1-x）Sr_0.5Zr₂（PO₄）₃-x Ce_1-yNd_yPO₄（y=0,0.2,0.4,0.6,0.8,1 or x=0.2,0.4,0.6,0.8）composite ceramic waste forms were prepared by a one-step microwave sintering technique.The results showed that both Ce and Nd can be involved into monazite structure to form（Ce,Nd）PO₄ monazite solid solution and its structural change has no effect on the structure of Sr_0.5Zr₂（PO₄）₃.Meanwhile,the structural change of（Ce,Nd）PO₄ monazite solid solution had no obvious impact on the compactness,hardness and chemical stability of Sr_0.5Zr₂（PO₄）₃-monazite（SrZP-monazite）composite ceramics.Moreover,Sr_0.5Zr₂（PO₄）₃phase and monazite solid solution present good compatibility.With the increase of monazite solid solution phase in the composite ceramics system,the crystal grain size is refined,and the density is improved.The relative density of monazite reaches 94%,and the Vickers hardness is about 730 HV1.In this paper,it is shown that SrZP-（Ce,Nd）PO₄ composite ceramics could be a candidate matrix for simultaneously immobilizing FP and various MA.The results in this work can provide ideas and reference for exploring novel composite ceramic waste forms with more diversified nuclides and higher stability.