Investigation On The Chemical Durability Of Zirconolite-Hollandite For Trivalent Analog Nuclide Nd And Fission Product Cs Immobilization

The radioactivity of high-level radioactive waste is mainly derived from actinide nuclides and long-lived fission products in its components,and it is necessary to immobilize actinide nuclides and long-lived fission products before geological disposal.Synroc has received extensive attention due to the excellent stability and nuclide loading,but due to the differences in physicochemical properties of various nuclides,it is difficult for the single mineral phase to immobilize actinide nuclides and fission products simultaneously.The simultaneous lattice immobilization of multiple nuclides can be realized through multi-mineral phase aggregation.As the main mineral phases of Synroc-C,zirconolite and hollandite are mainly used to immobilize actinide nuclides and fission products Cs,respectively.The zirconolite-hollandite assemblage can simultaneously immobilize actinide nuclides and fission products,which can improve the disposal efficiency of radioactive waste and reduce the harm caused by nuclide release to the environment.The starting point of this study is to elucidate the phase structure evolution and stability by assemblage mineral immobilize various nuclides.Zirconolite was used as the host phase of the trivalent actinides,and hollandite was used as the host phase of the fission product Cs,two mineral phases were combined to realize the simultaneous lattice fixation of the actinides and fission products.The lanthanide Nd³⁺is used to simulate trivalent actinides,and Nd is introduced into the A site of zirconolite to partially substitutes Ca²⁺and Zr⁴⁺.With ¹³³Cs and ¹³³Ba as the simulated nuclides of ¹³⁷Cs and daughter ¹³⁷Ba,Cr³⁺partially substitutes Ti⁴⁺at B-site of hllandite phase to adjusts the charge imbalance of crystal structure caused by the substitution of Ba²⁺by Cs⁺at A-site.High temperature solid state method was used to fabricate the samples.By means of XRD,FTIR,Raman,SEM,TEM and other tests,the phase structure and chemical stability of single-phase and multiphase ceramics were investigated.（1）High density and crystallography of Ca_1-x/2Nd_xZr_1-x/2Ti₂O₇（0≤x≤0.4）zirconolite,(Cs_xBa_1.2-x)(Cr_2.4-xTi_5.6+x)O₁₆（0.0≤x≤0.8）hollandite and zirconolite-hollandite assemblage minerals were prepared by high temperature solid state method at 1250℃for 4h,Nd and Cs evenly distributed in zirconolite and hollandite,respectively.（2）Rietveld refinement results show that the zirconolite has monoclinic structure,and hollandite has tetragonal structure.The change of nuclide content and phase ratio does not cause lattice distortion of host mineral phase,zirconolite and hollandite have excellent structural stability.（3）Zirconolite,hollandite and zirconolite-hollandite assemblage minor have excellent chemical durability in neutral liquid environment,the normalized leaching rates of Nd and Cs are 10^-5g·m^-2·d^-1and 10^-3g·m^-2·d^-1,respectively.However,in acidic liquid environment,the mineral phase crystal grains are seriously corroded,and the leaching resistance of zirconolite and hollandite decreases.