Study On Microscopic Mechanism Of The Changes In Mechanical Properties And Leaching Behavior Of Borosilicate Glasses Irradiated By Xe Ions

In the context of“double carbon”targets,vigorously developing nuclear energy is the important strategy for the maintenance of energy security and the reform of low carbon energy.However,the treatment of radioactive waste needs to be solved urgently with the development of nuclear power.Currently,vitrification and deep geological disposal are widely used in the treatment of high-level radioactive waste（HLW）.Borosilicate glass is selected as the suitable material for solidifying the HLW because of its good thermal stability,excellent waste containment capacity,and mature technology.The most probable migration way to enter the biosphere for radionuclides inside the vitrification is the erosion of groundwater.To avoid radionuclides endangering the biosphere,it is necessary to study the changes in strong radiation fields of the mechanical properties and leaching stability of vitrification,serving as the first safety barrier for ensuring the safe deep geological disposal of HLW.In this work,the radiation field induced by alpha decay and spontaneous fission of actinide nuclides was simulated by Xe ion irradiation.The influence of Xe ion radiation on mechanical properties and microstructure of borosilicate glasses were characterized with nano-indentation technology,Atomic Force Microscope（AFM）,Fourier Transform Infrared spectroscopy（FTIR）and Raman spectroscopy（Raman）,etc.Based on experimental results,the mechanism of the mechanical properties variations in irradiated glasses was analyzed.Furthermore,the Inductively Coupled Plasma Optical Emission Spectrometry（ICP-OES）and Scanning Electron Microscopy（SEM）were applied to investigate the changes in the elemental concentrations of the leachate and the corrosion layer of leached glasses.Combining with the microstructure changes of glasses before and after leaching,the microscopic mechanism of the changes in leaching behavior of borosilicate glasses under strong radiation field and water erosion coupling conditions was discussed in detail.The main research contents are described as follows:1.Influence of Xe ion radiation on mechanical properties and microstructure of borosilicate glassesIn this part,borosilicate glasses（NBS）were irradiated with 5 Me V Xe ions.After irradiation,the changes of mechanical properties and microstructures of borosilicate glasses were performed.The experimental results revealed the following conclusions:（1）Regardless the type,both hardness and Young’s modulus of irradiated NBS glasses dropped exponentially and then saturated at 0.1 dpa with the increase of irradiation dose.The maximum variation of irradiated NBS glasses’hardness and modulus are about-33%and-15%,respectively.（2）The changes of glass structure were caused by nuclear energy deposition in glass.After irradiation,the polymerization of glass network decreased,which led to the decrease of hardness and Young’s modulus.（3）After irradiation,the volume of NBS glasses decreased with dose and then saturated at 0.1 dpa.The saturated volume change is about+2.2%.Moreover,with irradiation of ions,the crack resistance of the borosilicate glasses increases substantially.（4）The[BO₃]units of NBS glasses gradually increased when the radiation dose increased,while the silicon network showed little changes.The decrease in hardness of irradiated NBS glasses is independent of the silicon structure,which is mainly caused by the transformation of[BO₄]to[BO₃].Furthermore,the decrease of coordinate number of boron would also lead to the increase of glass volume and crack resistance.2.Relationship between leaching behavior and microstructure changes of borosilicate glassesIn this part,with the MCC-1 static leaching method,we investigated the leaching behavior and the corresponding microstructures of the borosilicate glasses.The experimental results revealed the following conclusions:（1）The normalized mass losses of elements in samples increased and then saturated after leaching.As leaching time progressed,the normalized mass losses of elements began to increase again.（2）The glass composition plays an important role on the leaching stability of borosilicate glass.The leaching rates of NBS glasses increased with the increase of the proportion of Na₂O.（3）After leaching,the proportion of Si-O-Si structures in glass increased while the B-related structures disappeared.The microstructure in the layer near the surface was similar to fused silica.（4）With the leaching time prolongation,the cracks between the corrosion layer and glass substrate were observed in the surface of samples.The formation of the cracks led to the increase of glass surface area.Eventually,this led to the abnormal increase of the dissolution rates.The leaching rate is easily affected by the structure changes such as the stratification of corrosion layer,etc.There are limitations in evaluating the leaching stability of borosilicate glasses solely through the leaching rate variations.The leaching performance must be evaluated in conjunction with the changes in glass structure and the dissolution rate.3.Study on microscopic mechanism of the changes in leaching behavior of borosilicate glasses induced by Xe ion irradiationIn this part,the leaching behaviors of NBS glasses irradiated by Xe ions were investigated using the MCC-1 static leaching method.Based on experimental results,the changes in the structures and leaching properties of irradiated glasses were discussed.The following conclusions can be determined:（1）Irradiation could considerably increase the leaching rate of glass and the influence of irradiation on leaching rate of borosilicate glass was directly evaluated by the measurement of the initial dissolution rate.With the saturation dose,compared with pristine glass,the NBS5,NBS6 and NBS7 initial dissolution rates of Si increased by 77%,76%and 78%,respectively.（2）Irradiation of ions could lead to the transformation of[BO₄]and[BO₃]units and the strength of the glass network deteriorated.Irradiation of ions accelerated the ion exchange and network hydrolysis reactions,resulting into the rapid dissolution of glass network.（3）The dissolution rate change of Na is more obviously compared with that of Si after irradiation,which may be related to the different roles of sodium and silicon in glass network.Hence,the dissolution rates of Na in NBS glasses increased by139%at the saturation dose,while the dissolution rates of Si showed little changes and rose by 77%at the saturation dose.（4）The glass composition plays an important role on the leaching stability of irradiated glass.With the same R values,the initial dissolution rate variations of Si in the NBS5,NBS6 and NBS7 glasses ranged from 76%to 78%after irradiation,which are roughly the same.However,the dissolution rates of Na increased from 78%to139%with the increase of the proportion of Na₂O.The results are helpful to understand the influence of radiation effects on the leaching process of vitrification.Furthermore,this work is of great significance for evaluating the safety of HLW deep geological disposal.