Effect Of Molten Salt Infiltration In Nuclear Graphite And Its Mechanical Behavior Revealed By In-situ Synchrotron Radiation X-ray Diffraction

Nuclear energy is one of the clean,efficient and potential green energy sources.Thorium Molten Salt Reactor(TMSR)is a typical representative of fission nuclear energy utilization in the fourth generation nuclear reactor.Its characteristic is that molten fluoride is used as the coolant and fuel carrier.In molten salt reactor,nuclear graphite is used as moderator,reflector and core main structural material.And nuclear graphite requires direct contact with molten salt in molten salt reactor.Nuclear graphite has complex porous structure.Molten salt is easily infiltrated into the porous structure of graphite under definite pressure.Owing to the complexity of interaction between nuclear graphite and molten salt,the problem molten salt infiltration will affect the mechanical properties of nuclear graphite material,resulting in damage to nuclear graphite,which also affects the service life of nuclear graphite and the safe operation of the molten salt reactor.However,the micro-mechanism of the effect of the mechanical properties of nuclear graphite and the damage of the micro-structure caused by molten salt infiltration are still unclear and lack of systematic and comprehensive understanding.Therefore,it is necessary to study the influence of molten salt infiltration on the microstructure of nuclear graphite under in-situ environment(such as mechanical loading and high temperature)by synchrotron-based X-ray diffraction(XRD).In view of that molten salt may infiltrated into the nuclear graphite pores in the molten salt reactors,the dissertation carefully studies the evolution process of the microstructure of nuclear graphite with molten salt infiltration under mechanical loading condition.This part study mainly includes two aspects.1.Microstructure evolution of IG-110 nuclear graphite infiltrated with molten salt was studied by in-situ tensile synchrotron-based X-ray diffraction technique under different external tensile load condition.The experimental results show that the fracture position of IG-110 nuclear graphite is below the center of notch during the process of applying external tensile load.The analysis of two-dimensional and one-dimensional XRD diffraction patterns showed that the interlayer spacing and the full width at half maximum of(002)diffraction peak of IG-110 nuclear graphite with FLiNaK molten salt infiltration was increases,the diffraction intensity of(002)diffraction peak was decreases and the diffraction intensity of FLiNaK salt was decreases,which indicates that a smaller grain size,poorer crystallinity of IG-110 nuclear graphite with FLiNaK molten salt infiltration and poorer crystallinity of FLiNaK molten salt appear in the fracture position during the process of applying external tensile load.The surface and fracture surface of IG-110 nuclear graphite with and without FLiNaK molten salt infiltration were characterized by scanning electron microscopy(SEM).At the same time,the element distribution of FLiNaK molten salt on the fracture surface of IG-110 nuclear graphite was detected by electron probe X-ray micro-analyzer(EPMA).The element analysis of EPMA showed that the distribution of FLiNaK molten salt in the IG-110 nuclear graphite infiltrated with molten salt was not uniform.2.The surface and fracture surface of NBG-18 nuclear graphite without and with FLiNaK molten salt infiltration were characterized by SEM and X-ray energy dispersive spectroscopy(EDS),and the fracture surface of NBG-18 nuclear graphite with molten salt infiltration was characterized by EPMA.The experimental results showed that the FLiNaK molten salt is unevenly distributed on the surface and inside of NBG-18 nuclear graphite.Owing to of the non-uniform dlistribution of FLiNaK molten salt,a mapping scanning experiments of in-situ synchrotron-based X-ray diffraction under different external tensile load condition was carried out to reveal the local microstructure evolution and their corresponding strain distribution.Notably,a stress concentration area in NBG-18 nuclear graphite with molten salt infiltration,that is,the main interaction area between NBG-18 nuclear graphite and FLiNaK molten salt,was found and then transformed from one region to another region because of the unbalanced squeeze interaction between NBG-18 nuclear graphite and FLiNaK molten salt with the increase of external tensile force.During the process of applying external tensile load,a smaller grain size,poorer crystallinity of NBG-18 nuclear graphite and a larger grain size,better crystallinity of FLiNaK molten salt appear in the stress concentration area;meanwhile,the changes of crystallographic preferred orientation of FLiNaK molten salt domains in a stress concentration area imply that the external tensile load force makes better the ordered stacking of the larger crystal grains of the FLiNaK molten salt infiltrated into NBG-18 nuclear graphite.Most importantly,we have found for the first time that the fracture position of NBG-18 nuclear graphite infiltrated with FLiNaK molten salt always occurs near the a stress concentration area rather than at a fixed region under the external stress load.Raman spectroscopy analysis showed that the disorder degree of NBG-18 nuclear graphite with FLiNaK molten salt infiltration was decreased,the defects became less and the crystallinity was improved under the external stress load.The present study in this thesis not only helps to reveal the interaction mechanism between nuclear graphite and molten salt under the external stress load and the micro-mechanism of fracture of nuclear graphite with molten salt infiltration,and explains the changes of mechanical properties of nuclear graphite with molten salt infiltration but also contributes to accurately predict and analyze the stress state of components,which will have an significant impact on the design and safe operation of molten salt reactor and provide important reference value for the preparation of high performance nuclear graphite.