First-principles Study Of Pu Incorporation In Gd₂Zr₂O₇

Over the past half century,the nuclear fuel cycle has produced large quantities of Pu and minor actinides such as Np,Am and Cm.Successful treatment of these actinides is of great importance to the development of advanced nuclear fuel cycle,the control of nuclear weapon proliferation and the immobilization of high-level radioactive nuclear waste.Zirconate pyrochlores such as Gd₂Zr₂O₇ have been proposed to be good candidates for immobilizing actinides and lanthanides because of their chemical resistance,high melting point,good thermal diffusivity,low thermal conductivity,excellent ionic conductivity and good stability under irradiation.Thermodynamic properties of zirconate pyrochlore after immobilization of nuclear waste such as Pu are still unclear.In order to dispose of nuclear waste safely,it is necessary to understand the thermodynamic properties of materials,such as elastic constants,elastic moduli,Debye temperature and so on.The main work of this paper is as follows:1.The thermodynamic properties of Pu-containing Gd₂Zr₂O₇ were systematically investigated using density functional theory plus Hubbard U?DFT+U?method.The Pu incorporation into Gd-site and Zr-site,i.e.Gd_2-yPu_yZr₂O₇ and Gd₂Zr_2-yPu_yO₇,have been considered.For these two types of compounds,we optimize their structural parameters.Their elastic constants,elastic moduli,Pugh's indicator,Poisson's ratio,elastic isotropy and Debye temperature are calculated.We also analyze their electronic structures.The calculated results show that when Pu is immobilized into the Gd-site of Gd₂Zr₂O₇,it has slight effects on the thermodynamic properties.However,when Pu is immobilized into the Zr-site,it has remarkable influences on the structural parameters,elastic moduli,elastic isotropy,Debye temperature and electronic properties.For Gd₂Zr_2-yPu_yO₇,as Pu content increases,the lattice constant increases and the oxygen position parameter x _O48f48f varies greatly.Besides,the elastic constants and elastic moduli change sharply with the increasing Pu content,and the Gd₂Zr_2-yPu_yO₇ exhibits elastic anisotropy.At the same time,with the increasing Pu content,the Debye temperature of Gd₂Zr_2-yPu_yO₇ decreases rapidly and the band gap decreases considerably.The differences in thermophysical properties between Gd_2-yPu_yZr₂O₇ and Gd₂Zr_2-yPu_yO₇ may be due to their different structural and electronic properties.2.The stabilities of various point defects in Gd₂Zr₂O₇,Pu₂Zr₂O₇ and Gd₂Pu₂O₇,including vacancy,antisite and interstitial defects,are systematically studied using density functional theory plus Hubbard U?DFT+U?method.The thermodynamic properties of the defective systems,including elastic constants,elastic moduli,Pugh's indicator,Poisson's ratio and Debye temperature are analyzed.We find that vacancy,antisite and oxygen interstitials affect the thermodynamic properties of the system slightly,while A interstitials and B interstitials?A=Gd or Pu,B=Zr or Pu?have significant influences on the thermodynamic properties of the system.As the A and B interstitials are introduced,the C₄₄ of Gd₂Zr₂O₇ and Pu₂Zr₂O₇ decrease sharply,and the C₁₁,C₁₂ and C₄₄of Gd₂Pu₂O₇ are reduced significantly.Besides,the presence of A and B interstitials results in significant reduction in the shear modulus and Young's modulus of Gd₂Zr₂O₇and Pu₂Zr₂O₇,and considerable decrease in the bulk modulus,shear modulus and Young's modulus of Gd₂Pu₂O₇.It is noticeable that the Debye temperature of Gd₂Zr₂O₇,Pu₂Zr₂O₇and Gd₂Pu₂O₇ are all decreased by the existence of cation interstitials.In summary,the thermodynamic properties of Pu-containing Gd₂Zr₂O₇ and the defective Gd₂Zr₂O₇,Pu₂Zr₂O₇ and Gd₂Pu₂O₇ have been systematically studied.We find that the Pu content affects the thermodynamic properties of Gd_2-yPu_yZr₂O₇ and Gd₂Zr_2-yPu_yO₇ slightly and remarkably,respectively.Different types of defects have different effects on the thermodynamic properties of Gd₂Zr₂O₇,Pu₂Zr₂O₇ and Gd₂Pu₂O₇.The presented results will advance the understanding of the nuclear waste immobilization by pyrochlores.