A Theoretical Investigation For The Basic Physical Properties Of U₂Mo₃Si₄ And The Microscopic Plasticity Of ?-Fe In Accident Tolerant Fuel Systems

Nuclear energy is attracting widespread attention as a reliable,environmentally sustainable and cost-effective clean energy source.Recently,research on reactor safety has become priory in terms of nuclear energy development.The study and development of Accident-tolerant fuel systems?ATFs?is a major direction to improve the safety of commercial nuclear reactors.At present,the investigation of ATFs mainly focuses on two aspects:the fuel pellet and the fuel cladding.When it comes to ATF pellets,uranium silicide have become essential options due to their high thermal conductivity.Specifically,as the most common uranium silicide compound in research reactors,U₃Si₂ mainly has problems like poor mechanical properties,poor oxidation resistance and poor radiation stability.Yet alloying U₃Si₂ can be recognized as a feasible solution.Therefore,in this thesis,a U₂Mo₃Si₄ compound in U-Mo-Si ternary system has been selected for systematically theoretical study to explore the possibility of adding metal elements to U₃Si₂ to improve its mechanical properties;meanwhile,the exploration of other physical properties of U₂Mo₃Si₄ has been conducted.As for ATF cladding,Fe-Cr-Al alloy cladding has been paid more attention because of its various excellent performance.The study on the mechanical properties of Fe-Cr-Al alloy is of great significance to its application.Furthermore,the study on the deformation mechanism of?-Fe in Fe-Cr-Al alloy matrix material is of great value for predicting the mechanical properties of Fe-Cr-Al alloy.Therefore,in this thesis,the method of three-dimensional discrete dislocation dynamics has been chosen to simulate the?-Fe single crystal and double crystal,while the relationship between the strain rate sensitivity?SRS?and grain size of?-Fe under room temperature and dynamic loading conditions has been investigated,among which the internal mechanism of?-Fe has been analyzed based on dislocation-based structural evolution.The main results of our research can be listed as bellow:1.The ground state structure of U₂Mo₃Si₄ can be obtained by DFT+U method;the electronic structure analysis shows that U₂Mo₃Si₄ is a metal,and the bonding between U-Mo,U-Si and Mo-Si is mainly metal bond,ionic bond and valence bond;the calculation of mechanical properties illustrates that U₂Mo₃Si₄ is mechanically stable and is a ductile material,proving the possibility of improving its mechanical properties by adding a third component to the uranium silicide;the calculation of the phonon spectrum indicates U₂Mo₃Si₄ is dynamically stable,and contributes to its four thermodynamic parameters;the calculation of the defect formation energy of vacancies shows that Si vacancies are most likely to occur and all vacancies will cause volume shrinkage of the unit cell.2.A simplified grain boundary model has been built and the uniaxial tensile deformation simulation of?-Fe single crystal and double crystal has been carried out by utilizing three-dimensional discrete dislocation dynamics,which reveals that the yield strength of?-Fe single crystal and double crystal rises along with strain rate,indicating that?-Fe is sensitive to the strain rate during microscopic deformation;the existence of grain boundaries is found to hinder the movement of dislocations and improves the strength of the material.3.The simulation results on mechanical response show that the strength of?-Fe directly with strain rate in explored regime.The calculated dynamic SRS decreases with the reduction of sample size,which is consistent with the general observations for BCC metals.In this work,the evolution mechanisms of dislocations change from forest hardening to dislocation starvation when the size decreases,which is thought to be the dominate effect for the reduction of SRS.Specifically,the activation of dislocation sources can be more difficult because of the dislocation starvation in smaller samples.In addition,the results on the evolution of screw dislocations show a consistent trend with reported literatures.However,this trend of slight decrease does not seem to be the primary factor that affecting the SRS of?-Fe.