Research On The Evolution Behavior Of The Microstructure Of Uranium-based Metal Fuels Under Irradiation

There will be a large number of defects reserved in the materials after irradiation,and interactions among the matrix and different types of defects will eventually lead to changes in the microstructures and properties of the materials.The grain boundaries?GBs?are effective point defects sink,interactions between GBs and point defects are closely related to the materials swelling dynamics under irradiation.In addition,GB has a significant effect on improving the radiation resistance of materials.However,there are still a lot of problems about interactions between GBs and point defects need to be explored.This thesis will focus on the problems related to the irradiation effects of metallic nuclear fuel,studying the microstructures evolution of the materials under irradiation and the interactions between GBs and point defects through experimental researches and theoretical calculation methods.In this work,the experimental research of irradiating U-Zr alloy with high energy Xe ions was earried out.The microstructures of U-10%wtZr alloy were characterized before and after irradiation,and the microstructures characteristics of the materials caused by irradiation were explained.For U-Mo fuels,UMo₂Al₂₀ is a typical ternary intennetallic compound easily formed at the interface between U-Mo alloy and aluminum matrix.It is easy to form amorphous under irradiation and will seriously affect the U-Mo fuel application.In this paper,the local atomic structure analysis combined with the first principle calculation offers a relevant explanation for the reason of the intermetallic compounds are easily transformed into amorphous.Some basic physical properties of UT₂Al₂₀?T=Mo,Nb,Zr,Ti?are obtained,and they could be used for inputs to higher scale calculations.Microstructures evolutions of materials involve large time and spatial scale spans,the atomic scale calculation is limited in this respect.Therefore,the phase field method was adopted in this paper to carry out the continuous scale research work.Firstly,the phase field model was established.Model was well understood and the correctness of the model was verified by simulating the basic problems such as spinodal decomposition.Then,constructed a grain boundary model to study the problems of GBs-point defects interactions,by coupling the grain boundary dislocations climb,the behavior of grain boundary absorbing point defects is systematically studied from the aspects of grain boundary structure,GB-point defect reaction kinetics,grain size,and GB-point defect elastic interaction.The width of the void denuded zone?VDZ?near the grain boundary is another important reference to characterize the ability of absorbing point defects for the grain boundary.In this paper,the relationship between the sink strength and the width of VDZ was studied under the same framework,and established the initially relationship between them.Through carrying out the above work,the understanding of the microstructure evolution behavior of materials under irradiation is deepened,and the cognition of the physical processes and influencing factors of GB-point defects interactions is promoted.Following results are obtained:1.Bubbles of several nanometers to tens of nanometers in size will form in U-10wt%Zr alloy irradiated by 5Mev Xe ions,the distribution of bubbles along the depth direction is comparable with the result calculated by SRIM?the stop and range of ions in the material?.And the densest region of the bubble appears in the interval of about 650 nm from the surface of the sample.Shape of bubbles is mostly square or polygonal,which is considered to be related to the anisotropy of the energy of interface between the matrix and bubble.Large-sized bubbles appear mostly at the grain boundaries,which may be mainly caused by elastic attraction due to the structural characteristics of the grain boundaries.2.The interfacial product UMo₂Al₂₀ between U-Mo and Al is spatially composed of two main clusters,[U-Al₁₆]and[Mo-Al₁₂].The icosahedral structure of[Mo-Al₁₂]is considered to be the possible reason for the compound easily transforming to amorphous.In addition,some mechanical properties and electronic structures of UT₂Al₂₀?T=Mo,Nb,Zr,Ti?are obtained,these results can be coupled with larger-scale calculations in the future,such as doing as basic thermodynamic and kinetic parameter inputs for mesoscale calculations.3.The geometric configuration of the low angle GB was analyzed,and the phase field dislocation model for describing low angle tilt GB was constructed.The low angle symmetric tilt GB with stable structure was obtained by simulation,and the grain boundary structure agrees well with the theoretical results.On this basis,the absorbing behavior of point defects at grain boundaries was studied by coupling the dislocation climbing process,and the effects of grain boundary structure characteristics,short-range reaction kinetics and long-range elastic interaction on the defect sinking behavior of grain boundary were considered.By comparing with the results of planar sink model and the ideal sink model,the similarities and differences of these three models are recognized.The importance of the grain boundary evolution is revealed.4.The effects of grain size and point defects generation rate on the point defects sink strength and the width of the VDZ at the grain boundary were studied.Results show that the point defect sink strength at the GB decreases?increases?with the grain size increases?decreases?,and is not affected by the point defects generation rate;and the width of VDZ is narrowed?widened?as the point defects generation rate increases?decreases?,and is not affected by the grain size.Both of the point defect sink strength and the VDZ width increase with the increasing of the GB angle,and they exhibit a proportional change relationship with the increasing of the GB angle.