Research On Solid Phase Densification Of Matrix Graphite And Its Properties

Molten salt reactor is one of the reactor types of the fourth-generation reactor.Solid fuel molten salt reactor will use coated particle fuel elements.The elements are composed of Tristructural-isotropic coated particles and matrix graphite.The matrix graphite bears the function of slowing down neutrons,conducting fission heat,and protecting the coated particles.Under the molten salt reactor circumstance,the graphitic matrix will face the unfavorable environment including high-temperature fluoride salt soaking,erosion and corrosion,frictional collision during molten salt cyclic transportation and loading of self-weighted loads,and high flux neutron irradiation.One of the crucial problems need to be solved is how to effectively increase the density of the graphite matrix and reduce its average pore size on the basis of guaranteeing the resistence to irradiation.Solid phase densification technology,owing to its superiorities such as simple operation,high efficiency,and low cost,can therefore be possibly used for densification of the matrix graphite of the fuel elements.In the thesis,mesocarbon microbeads and quasi-isostatic pressing method were used to improve the density of matrix graphite for spherical fuel element.Mercury intrusion and high-temperature molten salt infiltration experiments were conducted to investigate the pore structure characteristics and resistance to molten salt infiltration of densified graphitic matrix.Besides,the irradiation experiment of the matrix graphite was performed by using the Ar~+ ion beam supplied by the 4 MV linear accelerator platform of the Shanghai Institute of Applied Physics.Then Raman spectroscopy and nanoindentation tests were performed to evaluate the graphite grain size and irradiated layer hardness on the samples before and after irradiation.The study found that the addition of mesocarbon microspheres has a significant effect on densification of matrix graphite.The density of the densified graphite(2800? heat treatment)increased from 1.73 g/cm~3 to 1.80 g/cm~3 and it's average pore size decreased from 570 nm to 300-100 nm.The density of the densified graphite(1950°C heat treatment)increased from 1.68 g/cm~3 to 1.73 g/cm~3 and it's average pore size decreased from 924 nm to 600-500 nm.It can be seen that,compared with the mesocarbon microbeads witth partiele sizes of 10 ?m and 16 ?m,the mesocarbon microbeads with particle sizes of 2 ?m showed more densification effect.The 700?FLiBe molten salt impregnation result showed that adding a certain proportion of mesocarbon microbeads could improve the resistance of molten salt infiltration of the matrix graphite.Furthermore,as the 2 ?m and 10 ?m mesocarbon microbeads had more sintered active area and could fill pores and cracks more effectively than the larger ones,the molten salt infiltration could be prevented more effectively.The nanoindenteation test showed that the MDG2-15 sample had a higher hardness before irradiation,consistent with the fact that the MDG2-15 sample is more densified.Under 1 MeV Ar~+ ions irradiation over the dose of 0.37 to 3.67 dpa,the hardness of MDG2-15 sample possessed the stable increase percent from 333%to 373%compared to the other graphite samples,indicating that this type of graphite is more resistant to radiation than the others.Raman spectroscopy measurements showed that the Ar~+ ions irradiation results in the accumulation of interstitial atoms and vacancies in the matrix graphite and therefore the increase in the degree of disorder.In comparison,MDG2-15 graphite is less sensitive to ion irradiation and exhibits better resistance to radiatian.