Rapid Fabrication And Lattice Immobilization Of Gd₂Zr₂O₇ Matrix For Nuclear Waste Form By SPS

Gadolinium zirconate?Gd₂Zr₂O₇?has been considered to be one of the ideal materials to tolerate actinides(²³9Pu,²³⁷Np,²³⁸U,²³⁸Np)and nuclear fission(¹²⁹I,⁹⁰Sr,and ¹³⁷Cs),which has a long half-life.This is also a hotspot and challenging problem of nuclear waste disposal.However,pyrochlore is difficult to synthesis.At present,it is mainly prepared by high temperature solid state method.Traditional method consumes a substantial amount of time?⁷2 h?,and the preparation process will inevitably produce effluent as by-products during the sintering process and cause new environmental problems.Spark plasma sintering?SPS?is a new kind of sintering technology developed in recent years,which combines of the plasma activation,hot pressing,resistance heating,and has advantages over high heating rate,short sintering time,high density,uniform particle size.Therefore,SPS technology is applied to disposal high level radioactive waste?HLW?for the first time.Gd₂O₃ and ZrO₂powders were adopted as starting materials to prepare Gd₂Zr₂O₇,and Nd³⁺and Ce⁴⁺were selected as a substitution of An³⁺and An⁴⁺in Gd₂Zr₂O₇.A series of gadolinium zirconate based simulated nuclide waste forms(Gd_2-xNdZr₂O₇?0?x?2.0?,Gd₂Zr_2-yCe_yO₇?0?y?2.0?and Gd_2-xNd_xZr_2-yCe_yO₇?0?x,y?2.0?)were prepared.The phase structure,vibration mode,microtopography and microstructure were investigated with help of XRD,Raman spectrum,SEM and TEM to study the isomorphism of substitutes in the waste forms,the occurrence state?A,B occupying,competition mechanism,oxidation valence and solid solution,etc.?and the crystal lattice immobilization mechanism.The research results are shown as following:?1?Taking Gd₂O₃ and ZrO₂ powders as raw materials,Gd₂Zr₂O₇ ceramic with a single fluorite phase has been successfully produced by SPS method at 1700oC for 3min under 80 MPa pressure.In comparison to the compound sintered by conventional method at 1500oC for 72 h,the SPS processed samples have compact micromorphology and higher density.?2?Through the formula design,a series of simulated nuclear waste forms Gd_2-xNd_xZr₂O₇?0.0?x?2.0?have been successfully fabricated using SPS method.It can be found that Gd_2-xNd_xZr₂O₇ ceramics with x?0.7 show a fluorite structure,whereas the solid solutions adopt a pyrochlore structure lies in 0.8?x?2.0.In addition,the Nd doping also leads to increase intensity of Raman vibration,a gradual density decrease and an increasing grain size.?3?Through the formula design,Gd₂Zr_2-xCe_xO₇?0.0?x?2.0?were rapidly prepared by SPS.The Gd₂Zr_2-xCe_xO₇ ceramics with a single phase can be obtained in?0.0?x?1.0?,while the sample with x?1.5 shows multiphase.The samples are cubic-shaping with a dense microstructure and have clear grain boundary.Besides,the density of samples enhances with increasing Ce content.?4?A series of Nd and Ce co-doped Gd_2-xNd_xZr_2-yCe_yO₇?0.0?x,y?2.0?ceramics have been rapidly prepared by SPS.The compositions with 0.0?y?1.0 show a single phase and beyond 1.0 exhibit multiphase.The lattice parameters and cell volume increase with elevated Nd or Ce content.The grains are densely packed on each other with cube-like shape.And the sample exhibits orderly arranged atomic layer and the elements are almost homogeneously distributed in the compound.