Study On Material Properties Of Nuclear Graphite And Stress Analysis Of Nuclear Graphite Component In Molten Salt Reactor

Nuclear graphite as an important neutron moderating material,is widely used in various reactors.During its service life,the dimensional and material properties of nuclear graphite components will be changed due to the effects of fast neutron irradiation and oxidation,,which will influence the stress level in nuclear graphite components.In order to make the design of reactors more reasonable and ensure the stability and integrity of the graphite components,it is necessary to evaluate the stress level of nuclear graphite components under irradiation.The premise of this work is to predict the material properties of nuclear graphite.Nuclear graphite is a kind of artificial porous brittle material.During its complex manufactural process,a large number of randomly distributed pores with different shapes and various sizes will be generated inside the nuclear graphite.Pores with different characteristics have different effects on the material properties of nuclear graphite.Young's modulus is an important material parameter for the stress-strain state in nuclear graphite and that the coefficient of thermal expansion determines the thermal stress.In this work,the influence of porous on young's modulus and the coefficient of thermal expansion have been studied.In addition,the nuclear graphite components are immersed in the molten salt within the reactor,its stress-strain state will be influenced by molten salt impregnation.The main contents are as following:(1)Taking the porous characteristics of nuclear graphite materials into consideration,a multi-scale modelling method have been developed.And then studied the influence of porosity,pore distribution,and pore shapes on young's modulus of nuclear graphite.Our numerical results suggest that as the increasing of porosity,Young's modulus satisfies the Knudsen's formula,where the decay coefficient is related to pore distribution and pore shape.(2)Using the multi-scale modelling method studied the relationship between the coefficient of thermal expansion and porosity.The result suggested that porosity and pore shape have insignificant influence on the coefficient of thermal expansion.The reason why the coefficient of thermal expansion is insignificant influence by porous has been proposed.(3)Using a user defined material subroutine analyzed the stress of nuclear graphite components under irradiation in molten salt reactor.These components have prefabricated crack with different width or have different molten salt penetration with different areas.The results suggest that the stress concentration of an intact component occurred at its center.If the component with prefabricated crack,its stress concentration occurred at the tip of the crack,and increasing the width of the crack,the stress concentration degree increased.The results of components with different penetration zone suggest that change the shape and area of penetration zone would modify their stress state.