The Study Of Nuclear Graphite Based On The Environment Of Thorium-based Molten Salt Reactor

Molten salt reactor is one of the six types of Generation IV Nuclear Reactors.It has significant improved in sustainability,safety,economy and performance.According to the form of fission fuel,molten salt reactors can be classified into liquid fuel molten salt reactor and solid fuel molten salt reactor.In liquid fuel molten salt reactor,molten salt is coolant as well as fission fuel.Liquid fuel molten salt reactor has two remarkable advantages due to liquid fuel.On the one hand,the liquid molten salt reactor does not require preparation of fuel rods and can realize the online processing and addition of nuclear fuel.On the other hand,the mixed molten salt can maintain a low vapor pressure under high temperature.And this reduces mechanical stress and improves the safety of nuclear reactors.In a solid molten salt reactor,molten salt is also selected as a coolant.Liquid molten salt reactor and solid molten salt reactor are going to use nuclear graphite as the moderator,and the material of molten salt channel is also the nuclear graphite.As a result,the nuclear graphite of the molten salt reactor is in direct contact with the molten salt.It is well known that nuclear graphite is a porous and brittle material.So it can be infiltrated while contact with liquid molten salt.According to the experimental date from the Oak Ridge National Laboratory,the infiltrated molten salt will form a hot spot when the molten salt infiltrate into the nuclear graphite.When the temperature reaches 1100?,the damage rate will rise sharply,it can be twice as much as that at 700?.Therefore,the molten salt infiltration seriously threatens the stability of properties of nuclear graphite and the safety of the nuclear reactors.In view of that molten salts may infiltrate into the graphite pores in the thorium-based molten salt nuclear reactors,the dissertation carefully studies the effect of molten-salt infiltration on the graphite microstructure and thermal properties.It hopes that this part can provide a method for screening nuclear graphite for thorium-based molten salt nuclear reactors.Therefore,one of the focuses of this dissertation is the effect of molten-salt infiltration on the microstructure of graphite and the thermal properties.This part study mainly includes three aspects.1.Studying the pore structure by using optical microscope(OM),scanning electron microscope(SEM),micro-CT,etc.The results of OM and SEM show that there are a large number of irregular pores,and the distribution of pores is extensive.The micro-CT of graphite shows that most of the pores are dumbbells type pores.And they connect to each other by narrow mouths and form a pore network.The infiltration of molten-salt in the nuclear graphite was compared with that of mercury infiltration and showed that the weight gains of the graphite samples increases with the increasing of the infiltration pressure.It is obvious that different graphite possess different threshold pressure by comparing the SEM figures of molten-salt infiltrated graphite at different pressure.And the weight gains of molten salt infiltration have nothing to do with the time of the infiltration process.The cross-sectional SEM images of virgin and molten-salt infiltrated graphite show that the interaction between molten salt and graphite will change the shape of molten-salt inside the pores.And pores occupied by molten salt are usually open pores in the infiltrated graphite.2.The effects of heating and molten-salt infiltration on the microstructure of graphite have beed studied by using the synchronous radiation X-ray small angle scattering.The result shows that the graphite pores have the sharp solid-gas interfaces,and there are some distributions of micro-inhomogeneous regions inside the microstructure of graphite.The number of pores of the graphite at the nanoscale gradually increases with the temperature increasing,and the fractal dimension gradually decreases(the solid-gas interfaces of graphite becomes smooth)with the temperature increasing.The effects of the molten-salt infiltration on the microporous of graphite were investigated and showed that there are interaction between graphite and infiltrated salt.More over it will affect the smoothness of the solid-gas interfaces of graphite.Compared the effect of molten-salt infiltration on graphite micro structure with that of temperature,it is obvious that the changes of the microstructure of graphite at the nanoscale may be more sensitive to temperature.3.Taking into account molten salt infiltration,the effect of molten-salt infiltration on the microstructure of graphite and the thermal expansion behavior of graphite are studied by using high temperature XRD to clarify the interaction between infiltrated salt and graphite.The result shows that the coefficient of lattice thermal expansion(CLTE)of the d(002)spacing of graphite increases with the temperature increasing.And the effect of temperature on the spacing of graphite layers is reversible.The effect molten-salt infiltration on microstructure of graphite were investigated and showed that the CCTE of the d(002)spacing of graphite decreases with an increase in the weight of the graphite sample because of molten-salt infiltration.The CCTEs of the FLiNa Ksalt in the graphite pores was smaller than that of bulk FLiNaK,indicating that interactions occur between the graphite and the salt in the graphite pores.The study of the thermal expansion behavior of graphite after molten-salt infiltration showed that the length increasing rates of the infiltrated graphite are higher than that of the corresponding virgin graphite at temperature from 25 to 450°C.And the form of the molten-salt is the key factor.For now,the research team of thorium-based molten salt reactor has not screened out the most appropriate graphite.Thereby,It is important to develop nuclear graphite for the the thorium base molten salt reactor.Taking into account molten-salt infiltration,the thorium-based molten salt reactor needs one kind of nuclear graphite,which would not be impregnated by molten-salt.The data from the Oak Ridge National Laboratory indicates that if the pore size of graphite used for molten salt reactor is less than 1?m,then it can effectively prevent molten-salt from permeating into the graphite.Because the permeability of ultrafine grained graphite,which has a smaller pore size,is much lower than that of fine grain graphite,ultrafine grained graphite is selected as candidate of neutron reflector and moderator by Shanghai Institute of Applied Physics.So the graphite in the molten salt reactor,not only solves the problem of direct contact with molten-salt,but also faces the challenge of high dose neutron irradiation environment.In order to confirm the safety of nuclear graphite in the molten-salt reactor,a new type of nuclear graphite needs a full assessment of potential risks before applying to a nuclear reactor.The reaseach team must collect enough reliable neutron irradiation data and theoretical result to examine stability of its performance.Then it is possible to estimate the service life of nuclear graphite and provide a scientifice valuation of the operation safety of the reactor.But it is difficult to accomplish the neutron irradiation experiment of nuclear graphite due to long-period,high-cost and low-repeatability of experiment.In the process of screening new graphite for the nuclear reactor,it is more diffucult to assess all candidate nuclear graphite materials.Thereby,studying the ion irradiation effects of nuclear graphite is another important focus of this dissertation.This dissertation assesses ion irradiation effects of ultra-fine grain graphite by ion irradiation and characterization methods.Then it is possible to accumulate knowledge of ion irradiation effects of nuclear graphite for thorium-based molten salt nuclear reactors.The behavior difference between ultra-fine grain graphite grade HPG-510 and fine grain graphite grade IG-110 under 7MeV Xenon ions irradiation at doses from 0.10 to 5.23 dpa were investigated at room temperature(RT).SEM,transmission electron microscopy(TEM),Raman spectroscopy and Nano-indentation were used to study the changes.According to SEM analysis,IG-110 shows more fragment on the surface than HPG-510 with the increase of irradiation dose.It indicates HPG-510 is more resistant to irradiation.The Microcrack density observed by TEM,the evolution of Raman spectra,and the hardness and modulus characterized by Nano-indentation also support the conclusion.