Investigations On Preparation And Structure Of NZP-type Phosphate Ceramic Waste Forms Incorporated With Sr And Ce

The treatment and disposal of high radioactive waste liquid is a difficult problem due to its variety of nuclides,high radiation toxicity and high heat release rate.At present,the safer disposal method is to solidify it in a material with stable properties before carrying out deep geological burial.Due to its rich ion substitution,stable structure,excellent physical and chemical properties,NZP material has the ability to incorporate a wide variety of multivalent radionuclide.In this paper,in order to immobilize fission product and actinides of radioactive liquid waste simultaneously and stably,NZP waste matrix was prepared by microwave-assisted solid-phase method by adding ZnO as sintering aid,in which Sr²⁺and Ce⁴⁺were used to simulate divalent fission nuclides and tetravalent lanthanide nuclides respectively.The phase composition and microstructure of the waste form were investigated by XRD,Rietveld structure refinement,IR and Raman;the micro-morphology and elemental distribution of the samples were observed by SEM-EDS;the mechanical properties of the samples were analyzed by density???and Vickers hardness?Hv?;and the chemical stability of the solidified body was evaluated by PCT leaching method.In this paper,the following work has been carried out on the preparation process,phase composition and microstructure of cured solids.?1?Microwave sintering preparation and densification of NZP samples:The phase composition,morphology and mechanical properties of the samples prepared by different ZnO contents and different microwave sintering temperatures were systematically studied.The results showed that,for the formation of pure NZP phase,the microwave-assisted solid-state reaction method was greatly superior to the traditional solid-state reaction method,by which could prepare pure NZP phase in the sintering temperature range of 900¹200 ^oC.The density and hardness of the samples were significantly improved by the addition of 1wt%ZnO.The as-prepared sample exhibited the relatively homogeneous and dense microstructure.?2?Preparation and characterization of Sr-doped NZP ceramic solidified body:Sr-doped NZP samples were prepared according to the formula Na_1-2xSr_xZr₂?PO₄?₃?x=0,0.1,0.2,0.25,0.3,0.4,0.5?.The results showed that the phase composition gradually changes from NZP to SrZP,and the space group changes from R3?c to R3?with the increase of the content of Sr element.Moreover,the prepared samples had smooth grain section,compact structure and uniform distribution of elements.And the solid solubility of Sr element in NZP structure was large,the leaching rate of each element LR_i and mass normalized leaching rate LR_m of samples were low,which indicated that the Sr-NZP samples has superior chemical stability and durability.?3?Preparation and characterization of NZP samples cured with Sr and Ce.The results showed that the change of Ce content has little effect on the physical properties of the samples.The simulated nuclide Ce mainly exists in the CePO₄ phase of the grain boundaries,and the CePO₄ phase has good compatibility with SrZP.The prepared samples had flat cross section,compact structure,almost no pores and uniform distribution of elements in the samples.Simultaneously,the samples had good chemical stability,which normalized leaching rate of various elements were only on the order of 10^-310^-7.In this paper,microwave-assisted solid-phase reaction was used to solve the problem of time-consuming synthesis and high preparation temperature of NZP-type materials by adding a proper amount of ZnO as a sintering aid.The prepared NZP-type solidified body has compact structure,good mechanical properties and chemical stability.The research results in this paper can provide theoretical guidance and technical support for NZP materials curing of a wide variety of multivalent radionuclide,and promote the development of ceramic solidification technology for high-level radioactive waste.