| Molten salt is used as a carrier of nuclear fuel and coolant in molten salt reactor?MSR?,which have several advantages in reactor safety,economy and can be online refueling,but MSR also brings a challenge to structural materials which are exposed to high operating temperature and intense neutron irradiation.The main structural materials are nuclear graphite and high-temperature nickel-based alloy,both are in direct contact with molten salt.Therefore,it is necessary to analyze the mechanical properties of the two main structural materials,the nuclear graphite for the reactor core and the nickel-based alloy for the reactor'loop system,which provides the circulation of the molten salt.For unclear graphite,the seismic analysis is an important part to evaluate the safety performance of graphite core.In MSR the molten salt is denser than graphite,so the effect of fluid-structure interaction between molten salt and graphite components cannot be ignored in dynamic analysis of graphite core.A 4:1 scale experimental model has been determined by a similarity study.The nonlinear seismic motion of a graphite brick can be modelled as the viscous damping with single degree of freedom and the fluid-solid interaction effect of graphite and molten salt can be modelled as added mass and damping acting on the graphite brick.The experimental data was fitted according to the analytical solution of steady-state harmonic vibration equation.It was found that the added mass and the added damping were inversely proportional to the gap distance between the graphite bricks.A single layer graphite model has been established by using finite element method.The maximum principal stress and displacement of graphite brick increase with the increase of excitation amplitude and frequency.Studying the influence of dowel's diameter on the dynamic response of core,it was found that the maximum principal stress and displacement of the graphite brick increase with the diameter of dowel.The maximum principal stress and displacement of the graphite brick are affected by the size of key and the gap distance between graphite bricks.Considering the fluid-structure interaction effect of molten salt,the presence of molten salt will reduce the peak displacement and maximum principal stress within the graphite brick and will protect the integrity of the graphite core.For the nickel-based alloy,the loop tube is exposed to high temperature and load.The damage caused by thermal creep becomes a focus in mechanical property evaluation.Moreover,the stress state in tubular structure is more complicated than the traditional uniaxial sample.Research on the creep damage mechanism under biaxial stress is an important part for loop failure assessment and safety analysis.In order to meet a biaxial stress state in experimental samples,an in-situ creep experiment system was built to study thermal biaxial creep with various sample inner pressures 18MPa,23MPa,28 MPa and 32 MPa at 950°C.The creep curve of alloy 230 is mainly composed by the first stage and the third stage,and the third stage accounts for a large proportion of the entire creep curve.Microscopic characterization results show that the creep rupture model under different pressure conditions is intergranular fracture,and the carbide participate M??C?can be found near the grain boundary.Due to the influence of oxidation,a crack is formed on the outer tube wall,which is propagated to the inside of tube wall in perpendicular direction of hoop stress.The biaxial creep constitutive equation is derived based on the experimental creep data and written into the ABAQUS via a user-defined subroutine CREEP to establish a tubular structure creep model.Through the results of the model,the most part of tubular sample was found to be in uniform expansion and the stress and strain peaks appeared at both ends of the tube.The position of the crack in the sample was completely consistent with the position of the stress peak from the finite element analysis,which further verified the correctness of the model. |
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