A Simulation On The Structure And Properties Of Pyrochlore And Spinel Ceramics

Nuclear energy gets extensive development worldwide as a clean energy,which can solve the ecological environment problems caused by the over exploitation of fossil fuels,but it also creates challenges.The most serious one is dealing with high-level nuclear waste in reasonable ways.Pyrochlores and spinels have been proposed as a potential superior host phase for the long-lived radionuclide in nuclear waste,because it possesses high chemical durability,good thermal stability,low leach rate and the enhanced radiation tolerance.Helium accumulation due to?-decay processes from plutonium and other minor actinides for hundreds of thousands of years can result in structural changes and then alter the chemical properties of immobilization matrix.It is necessary to study the behavior of helium atom in immobilization matrix.First principle calculations based on density functional theory have been employed to study structural effects of trapping helium in La₂Zr₂O₇ pyrochlore.Besides,we have analyzed the electronic structures and chemical bonding of He-La₂Zr₂O₇ systems.The formation energies have been calculated to assess the relative stability of various helium interstitial configurations.The cation antisite defect formation energies and the x positional parameter for 48f-site oxygen are calculated to predict the radiation resistance of He-La₂Zr₂O₇ systems with low concentration of He interstitials.The study found that there is the“bimodal effect”in rare-earth titanate pyrochlore,RE₂Ti₂O₇?RE=La,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu?.In order to verify whether this phenomenon exists in rare-earth stannate pyrochlores,a systematic density functional theory study is performed to investigate the lattice parameters,the internal positional parameter x_O48f and bond length of RE₂Sn₂O₇?RE=La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb and Lu?pyrochlores.To analyze the structural stability in extreme conditions and verify whether bond strength varies inversely to bond length in rare-earth stannate pyrochlores,the structural parameter and bonding strength under hydrostatic pressure are studied.We calculated the bond length and the bulk modulus of RE₂Sn₂O₇ and found that the<RE-O_48f>bond plays a predominant role in determining the bulk modulus.Meanwhile,the present calculations suggest that the“bimodal effect"also exists in rare-earth stannate pyrochlores.Spinel has broad application in the field of nuclear energy due to its excellent radiation resistance.The phase transformation in MgAl₂O₄ spinel has been found in radiation experiments,However,the inert mechanism of radiation tolerance and structure evolution are still unknown till now.In order to explain the inert mechanism of phase transformation and the process of structure evolution at the atomic level,the kinetics of phase transformation under irradiation of MgAl₂O₄ spinel has been explored using molecular dynamics simulations.The irradiation damage has been simulated by Frenkel defects accumulation in the crystal.The results show that the phase transformation includes three stage:the first is from ordered spinel to disordered spinel structure,the second is from disordered spinel structure to disordered rock-salt structure,the last one is to maintain the disordered rock-salt structure.Meanwhile,oxygen Frenkel pairs enhance the critical dose for phase transformation and the ability to preserve spinel structure under irradiation.