Molecular Dynamics Studies On Structural Parameters And Performance Parameters Of Pyrochlore Oxides

With the rapid development of nuclear energy,more and more nuclear waste is produced during the last few decades.How to dispose the nuclear waste safely and effectively has become one of key issues of nuclear energy development.Currently,it is still the most effective way to deal with high-level radioactive wastes(HLW)for immobilization and then deep geological burial.Therefore,to seek for a proper immobilizing substrate becomes one of the urgent problems to be solved in the development of nuclear power.Pyrochlore oxides,A2B2O7,occurring in nature with more than 450 synthetic chemical compositions,are expected to be one of the most ideal candidate materials for the immobilization of high-level nuclear waste due to their superior radiation resistance,stable structural properties,good mechanical properties,high capacity for radionuclide incorporation and low leaching rate of incorporated species.Recently,the pyrochlore oxides have been attracted considerable interest in the scientific research.Based on the classical molecular dynamics method,the structural properties,mechanical properties,and thermal properties of pyrochlores were systematically studied.Besides,the radiation resistance of pyrochlores was also evaluated.The potential functions used in this paper are long-range Coulomb potential and short-range Buckingham potential.Firstly,in order to avoid unrealistic attraction between close ion pairs due to the dominance of the dispersion term at very short distances,we joined a repulsive Ziegler-Biersack-Littmark(ZBL)potential to an existing short-range Buckingham potential of pyrochlore using the exponential spline function.The potential function model for simulating pyrochlore system was established,especially for simulation of dynamic properties.Based on this potential model,the static properties including lattice parameter,bulk modulus,specific heat capacity and thermal expansion coefficient etc.were calculated by molecular dynamics simulation,the reasonable value was produced.It is demonstrated that the results calculated in this work are consistent with the experimental value,which suggests that these potentials are more accurate to describe the interaction between the ion pairs of pyrochlore oxides,and expect to be used in the study of the problem of interest,namely simulation of dynamic processes,the low-energy displacement recoils,radiation damage and defect migration in pyrochlores.Secondly,the effect of cation radius on the static properties of pyrochlores(A2B2O7)was investigated.The Structural,elastic and thermal properties of 25 different kinds of pyrochlore were systematically calculated and analyzed.It is suggested that the lattice parameter increased dramatically with increasing A and B cation radii,but the influence of B-site cations radius on lattice parameter is more significant than that of A-site cations radius.Bulk modulus slightly decreases with increasing cation radius,the influence of B-site cation on the bulk modulus is much larger than that of A-site cation.However,the specific heat capacities and thermal expansion coefficient remained almost the same with increasing A-site and B-site cations radii.For Er2Ti2O7,the isobaric specific heat capacity(CP)is abnormal,it is assumed that incorporated Er into the titanate pyrochlore A-site may have a significant impact on its partial thermal properties.It is of interest to note that,for Gd2Ce2O7 and Sm2Ce2O7,the specific heat capacity and thermal expansion coefficient is significantly decreased,i.e.if Ce at the B-site of pyrochlore,may have implications for annealing of radiation damage in cerate pyrochlores.Due to the similarity between Ce and Pu at ion radius and valence,it is common to use Ce instead of radioactive Pu for simulated immobilization of high-level radioactive waste.It can be concluded that immobilizing Pu occupy into B-site of pyrochlore may have a greater impact on its thermal properties,thus,the change of thermal properties as an important factor should be considered in the selection of immobilization substrate.Thirdly,low energy ion-solid interactions and chemistry effects in a series of pyrochlore was studied using molecular dynamics method.Contours plots of threshold displacement energy(Ed)for Gd,Zr,O48 f and O8 b in Gd2Zr2O7 pyrochlore have been produced along more than 80 nonequivalent crystal directions,and Ed values along more than 320 directions of were determined.It is concluded that the Ed surface for each type of atom in Gd2Zr2O7 is highly anisotropic,and Ed of Zr exhibits the largest degree of anisotropy,while that of O8 b exhibits the smallest.The recommended values of Ed in Gd2Zr2O7 based on the observed minima were 56 eV,94 eV and 25 eV for Gd,Zr and O,respectively.In addition,the effect of chemistry on low energy displacement recoils events was investigated at 10 K for each type of atom in pyrochlores.The influence of cationic radius on Ed for each type of atom in pyrochlore A2B2O7(with A3+ ranging from the Lu3+ to the La3+ and B4+ ranging from Ti4+ to Ce4+)was also investigated along the three main crystal directions [100],[110] and [111].It is revealed that the value of Ed strongly depended on the atom type,atom mass,knock-on direction and lattice position.The defects produced after low energy displacement events included cation antisite defects,cation Frenkel pairs,anion Frenkel pairs,various vacancies and interstitials.However,Ce doping in pyrochlores may affect the radiation response,because it resulted in drastic changes in cation and anion displacement energies and formation of an unusual type of anti-site defect.The Ed values determined provide important input parameters for multi-scale Monte Carlo simulations.In addition,this work also demonstrates links between the threshold displacement energy(Ed)and amorphization resistance: the lower the Ed of the pyrochlore,the easier it is for the order-disorder transition from pyrochlore to fluorite to occur,the higher the critical amorphization dose,the lower the critical amorphous temperature,and the greater is the amporphization resistance.Finally,both the displacement cascade and the radiation damage of pyrochlore were evaluated in this work.The effects of primary knock-on atom(PKA)with different incident directions and different initial kinetic energies in Gd2Zr2O7 pyrochlore was studied by using molecular dynamics simulation.It is found that the radiation damage caused by displacement cascade vaied with different incident directions of the PKA with certain energy,and the radiation damage resistance of the material exhibited highly anisotropic.If the PKA with different initial kinetic energy along specific direction,the radiation damage caused by cascade collision is also different.With the increasing of the incident energy of PKA,the number of defects increases,the radius of damage area increases,and defects recombination rate decreases significantly.The molecular dynamics simulation of the radiation damage of Gd2Zr2O7 pyrochlore with different proportions of Ce was studied.It is suggested that with the increase of Ce incorporation,the radiation resistance of the material varied significantly,and the effect of the radiation resistance of the material is different with the Ce element doped with A-site and B-site cation position.