Composition-dependent Mechanical Property Changes In Irradiated Sodium Borosilicate Glasses

How to properly dispose of the radioactive waste generated in the nuclear fuel cycle or other nuclear activities like national defense,medical treatment and scientific research has become a key problem restricting the development of nuclear energy.Vitrification followed by geological disposal is the most mature technology and the most acceptable one for the High-level radioactive nuclear waste（HLW）among the world.Sodium borosilicate glass is considered as an ideal waste form for vitrification because of its low coefficient of thermal expansion,good chemical stability and simple processing.In order to achieve better solidification,the composition of sodium borosilicate glass should be adjusted according to the composition of HLW.However,most of the studies only focused on the irradiation-induced changes of mechanical properties,microstructure and leaching rates of the glass matrix.The detailed role of the composition in ion-irradiation induced mechanical property changes still remains unclear,detailed and in-depth studies are demanded to get further understanding on the micromechanics of irradiation effect.In this thesis,three kinds of sodium borosilicate glass which belong to KSiO2B2O3-RNa2O system were prepared by controlled their two composition-related parameters,i.e.,the molar ratio K（defined as[SiO2]/[B2O3]）and R（defined as[Na2O]/[B2O3]）.For these sodium borosilicate glasses,their R is varied（1.34,0.75,and 0.40 for NBS1,NBS2,and NBS3,respectively）,while their molar ratio K=[SiO2]/[B2O3]is fixed at 4.04.The strong radiation field of geological disposal was simulated by Au3+,Si2+ions and electrons.The irradiation-induced mechanical properties and microstructure changes of the glasses were studied by means of nanoindentation,molecular vibration spectroscopy,Scanning electron microscopy（SEM）and solid-state nuclear magnetic resonance spectroscopy（ssNMR）.The composition-dependent mechanical property changes in irradiated sodium borosilicate glasses and their micromechanics were also discussed.The main contents are described as follows.1.Composition-dependent mechanical property changes in electron-irradiated NBS glassesThe composition-dependent mechanical property changes and micromechanics in NBS glasses induced by 1.2 MeV electron were studied by nano-indentation,Fourier transform attenuated total reflection infrared spectrometry（ATR-FTIR）,as well as scanning electron microscope（SEM）.The results indicated that（1）The hardness,H and reduced Young’s modulus,M can be significantly affected by sole electron irradiation,and the variations were significantly related with the glass compositions（NBS1:ΔH=-7%,ΔM=-3%;NBS2:ΔH=-15%,ΔM=-7%;NBS3:ΔH=-27%,ΔM=-7%）.The sodium borosilicate glass with smaller R value tends to be more affected by electron irradiation.（2）The point defects induced by electron should dominate the mechanical property changes under this scenario.The formation of defects will then induce the transformation of[BO4]-to[BO3],the increase of polymerization degree of glass network and the decrease of average ring size.2.Composition-dependent mechanical property changes in ion-irradiated NBS glassesThe composition-dependent mechanical property changes and micromechanics in NBS glasses induced by 8.0 MeV Au-ions were studied by nano-indentation,molecular vibration spectroscopy and theoretical model.The results indicated that（1）Both the hardness and modulus were significantly decreased after irradiation（NBS1:ΔH=-24%,ΔM=-10%;NBS2:ΔH=-27%,ΔM=-9%;NBS3:ΔH=-19%,ΔM=-8%）,and the variations have no strong composition-dependence in these three NBS glasses.（2）The concentration of[BO3]and small-size ring structure in the Au3+-ion irradiated NBS glasses were increased in varying degrees.The degree of polymerization of silicon atom network in NBS1 and NBS2 glasses is also decreased by irradiation,but it was increased in NBS3 glass.（3）The mechanism of mechanical property changes in Au3+-ion-irradiated NBS glasses was studied by theoretical structure model and the modification of network structure.The results show that the reedmergnerite groups containing Si-O-B4 bond may be the structural origin of Au-ion-irradiated glasses with R>0.5;The modification of the silica network,as well as phase separation,may dominate the glasses with R≤0.5.（4）By comparing the mechanical properties variations induced by electron and Au3"ion,the deposited nuclear energy was demonstrated to be dominant the ionirradiation induced mechanical property changes in NBS glasses.At the same time,it also negates the view that "deposited electronic energy has little effect on mechanical property changes".In addition,when the deposited electronic energy reached approximately 9×1021 keV/cm3,the electron energy deposition can restore the hardness of sodium borosilicate glass.Based on the above facts,we propose that "the nuclear energy deposition of ions plays a important role in the decrease of mechanical properties of sodium borosilicate glass,but the deposited electronic energy can also partially restore the mechanical properties of sodium borosilicate glass".3.Composition Optimizing glass structure,and improve irradiation resistanceIn this part,we propose for the first time to improve the radiation resistance of the glass by optimizing the composition of the glass without changing the structure of the glass.Based on NBS2 glass,a series of 4.04SiO2-B2O3-O.75（Na2O-XiO）glass was prepared by replacing 5O%Na2O in the original glass with alkali metal and alkaline earth metal of different cation field strength（CFS）.The new glass sample is named XNBS,where X represents Li,Na,K,RB of alkali metals and Mg,Ca,Sr,Ba of alkaline earth metals respectively.The composition-dependence between structure and properties of XNBS glass was established by ssNMR and nanoindentation,and the composition-dependence and microscopic mechanism of mechanical changes of XNBS glass induced by 6.0 MeV Si2+ion irradiation were studied by ATR-FTIR.The results show that:（1）The results show that the density of sodium borosilicate glass is positively related to the average molecular weight of glass;the reduced Young’s modulus is positively related to the dissociation energy per volume and bulk density of oxide forming glass;the hardness is related to the bulk density and network polymerization degree.（2）The size of CFS is negatively related to the content of tetracoordinated boron atoms in the glass.Because the change of mechanical properties caused by ion irradiation decreases with the decrease of Si-O-B4 mixed bond content in the glass network structure,increasing the CFS of the system can reduce the content of SiO-B4 mixed bond,and then reduce the change of mechanical properties caused by ion irradiation,which provides a new idea for optimizing the radiation resistance of sodium borosilicate glass.（3）By comparing the changes of mechanical properties of soda alkali borosilicate glass and mixed alkali borosilicate glass caused by Si2+ion irradiation,it is proved that mixed alkali effect can inhibit the changes of mechanical properties caused by ion irradiation to a certain extent.