Study On The Immobilization Of High-level Radioactive Waste With Ilmenite-bearing Monzogneiss

Clean and safe nuclear energy ensures energy supply and improves the world energy structure.During the spent fuel reprocessing process,high-level liquid waste will be produced.If this type of nuclear energy application product is not properly handled,it may threaten the Earth’s ecosystem.To reduce the amount of waste,China adopts the independently developed TRPO process to separate transuranium from high-level liquid waste.However,the waste at the end of the TRPO process is still produced,which is a complex radioactive waste with a small amount of lanthanide,actinides and fission products.Considering the sustainable use of nuclear energy,the safe treatment of the end waste of TRPO waste is particularly critical.In this paper,it is treated by glass-ceramic waste form.The glass phase can immobilize other components in the end waste of TRPO process without selectivity,and the ceramic phase can stabilize and immobilize some of the lanthanide and actinides.The two phases cooperate to complete the immobilization of waste.Ilmenite-bearing monzogneiss（IBMG）was used as glass and ceramic mineral forming agent to prepare samples by traditional high-temperature melting method.The composition,thermodynamic behavior,phase composition,structure,microstructure and element distribution of the samples were analyzed by XRF,DSC,XRD,FT-IR,Raman,SEM-EDS and XPS.The density and Vickers hardness of the samples were measured to analyze their physical properties.The chemical stability of the samples was tested by PCT and MCC-1 methods,and the existence form of waste elements in the waste form was discussed.The main research contents are as follows:（1）The optimum process condition of IBMG material is 1500°C for 1 h,and the sample shows obvious glass characteristics.The glass transition temperature of IBMG material is 1000-1250°C,and there is still a small amount of unmelted Fe₃O₄crystal at1500°C.（2）The solubility of simulated trivalent actinide（Nd₂O₃）in the glass-ceramic formed by IBMG material is 29 wt.%,and a small amount of ceramic phase is neodymium-bearing feldspar Na Nd₉（Si O₄）₆O₂.The glass structural units Q²and Q³account for the largest proportion,and the degree of glass polymerization is not high.The samples retain the excellent high strength and high density of granite,and the leaching rate of Nd element is stable at the order of～10^-6g·m^-2·d^-1after 28 days.（3）The IBMG material was completely vitrified at 1500°C,and the solubility of CeO₂was 17 wt.%.The content of CeO₂affects the glass network connectivity,which decreases with the increase of CeO₂content.The density and hardness show that the waste form inherits the excellent physical properties of the rock.In the PCT leaching experiment results,the normalized leaching rates of Nd and Ce elements were stable at～10^-6g·m^-2·d^-1after 14days,showing good chemical stability.（4）Based on the previous experiments,immobilization experiments of the end waste of TRPO process were carried out under the same process conditions.When the content of waste is less than 30 wt.%,the waste form is mainly glass phase;when the content of waste is≥30 wt.%,neodymium-bearing feldspar begins to appear,and fluorite phase and iron-manganese oxide also appear one after another.Their crystallization also increases the density and hardness of the waste form.For the glass phase,in addition to some simulated elements being immobilized in the glass network space,some elements are involved in the formation of the glass network and are immobilized.For example,Ce and Fe form Si-O-Ce and[Fe O₄]-tetrahedra respectively and are connected into the network.MCC-1 static leaching test showed that the normalized leaching rates of Ce,Nd,Mn and Fe elements tended to be gentle after 21 days,and were all≤～10^-4g·m^-2·d^-1.The high waste loading and excellent stability of the waste form indicate that IBMG material can be used as a potential aluminosilicate waste form.