Preparation And Irradiation Behavior Of Nano-pore Graphite For Molten Salt Reactor

Molten salt reactor(MSR),as one of the next generation reactor types,has shown good performance in safety and economy and it has become one of the main types of reactor that many countries are striving to develop.In liquid MSR,liquid molten salt is used as both coolant and fuel carrier,while graphite is used as moderator,reflector and structural material,with the inevitable problems of infiltration of liquid molten salt and diffusion of fission product,especially 135Xe called neutron poison.Therefore,the pore size of nuclear graphite must be reduced to less than 100 nm to inhibit infiltration of liquid molten salt and diffusion of fission product.However,in the traditional manufacturing process,commercially mature nuclear graphite will produce pores and cracks due to volatilization of volatile substances and thermal shrinkage during graphitization,which makes the produced graphite have larger median pore size and porosity,and cannot meet the requirements of MSR for graphite pore size.Therefore,for MSR,nano-pore graphite with pore size below 100 nm is prepared to avoid the damage with high rate in graphite structure caused by infiltration of liquid molten salt and the increase of neutron absorption cross section of graphite caused by diffusion of fission products.In addition,considering that the ability of slowing neutrons through a series of knock-on collisions between carbon atoms in graphite lattice and neutrons,it is inevitable that graphite will suffer a large dose of radiation damage.Moreover,radiation damage alters the lattice parameters of graphite,introduces dislocations and other defects in the crystal,and produces micro-strain,which not only influence many of material properties,but also influence the operation of the reactor and limits its lifetime.Therefore,the pyrolytic carbon coating and CGB graphite papered by the extrusion molding method adopted in Oak Ridge National Laboratory have unfavorable factors such as coating shedding,cracking and short lifetime in irradiation environment.Therefore,it is necessary to research and develop new sealing technology or preparation technology of nano-pore graphite.However,the newly developed nano-pore graphite has a different structure from the traditional nuclear graphite,and the new sealing technology will introduce a new structure into the matrix.Therefore,it is necessary to study the irradiation behavior and law of irradiation damage through irradiation experiments,and to guide its preparation technology or sealing technology.In this paper,several kinds of binder-free graphite,sealing technology and fine particle graphite with median pore diameter below 100 nm were prepared or developed.Ion irradiation is a common and good substitute for neutron irradiation because of its high irradiation dose,high irradiation damage efficiency and non-radioactivity of irradiated samples.Therefore,neutron irradiation damage with lower damage dose was simulated by 3.0 MeV He+ irradiation and the higher damage dose was achieved by 7.0 MeV Xe26+irradiation,scanning electron microscopy(SEM),atomic force microscopy(AFM),X-ray diffraction(XRD),grazing incidence XRD(GIXRD),Raman spectroscopy,transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS)and nano-indentation were used to characterize the change in morphology,pore structure,micro-structure,crystallite size,micro-strain,defect density and mechanical properties of nano-pore graphite irradiated by different ions,different damage doses and different temperatures,revealing the radiation damage law of nano-pore graphite under different conditions.The thermal annealing behavior of radiation-induced defects is studied by annealing.The main work of this paper is as follows:1.In order to realize the binderless preparation process and avoid the unfavorable factors caused by the anisotropy of graphite,nano-pore isotropic graphite(NPIG)was prepared by isostatic pressing method with mesocarbon microbeads containing ?-resin as raw material.The mercury intrusion showed that the pore size of NPIG was 69 nm,then NPIG was irradiated with 3.0 MeV He+ at room temperature and 650?(the operating temperature of MSR),and compared with IG-110.After irradiation at room temperature,NPIG with low graphitization degree appeared irradiation-induced catalytic-graphitization after irradiation with low ion implantation fluence,and its d002 and lattice parameters in c-axis decreased,and the defect density on the surface also decreased.However,IG-110 did not show similar behavior.After irradiation at higher dose,the change trend of IG-110 is similar to NPIG,that is,the defect density caused by irradiation damage increases and it alters the lattice parameter.As for the results of irradiation at high temperature,because the radiation damage dose is lower as a whole,after irradiation at high temperature,annealing effect slows down the damage caused by irradiation and dominates to reduce the defect density,and NPIG with lower graphitization degree is more sensitive to this.The change of lattice structure and defect density of graphite determines the change trend of its surface morphology.At the same time,after irradiation at room temperature,the graphite surface is hardened due to the pinning effect of defects,and with the further increase of defect density,the structure is destroyed,resulting in the decrease of hardness and Young's modulus.However,after irradiation at high temperature,the annealing effect is dominant,and the reduction of defect density reduces the pinning effect of defects,thus reducing their hardness and Young's modulus.Furthermore,the characterization of NPIG shows that cracks appear at the depth where the radiation damage dose is the maximum.It is worth noting that these irradiation instability factors demonstrated by NPIG will affect its use as graphite for MSR.2.Self-sintered nano-pore graphite(SSNG)was prepared by direct isostatic pressing with self-sintered coke powder with lower particle size,with median pore size of 40 nm and graphitization degree higher than NPIG.The neutron irradiation was simulated by 7.0 MeV Xe26+ irradiation and its irradiation behavior and mechanism were studied in the dose range of 1-5.0 dpa.SEM and AFM show that after irradiation,the filler particles expand,micropores and the micro-gap between filler shrink,and the surface becomes more dense.With the increase of radiation damage dose to about 0.55 dpa,the absorption of swelling is saturated due to the closing of micropores and micro-gaps,which makes the swelling increase rapidly in the direction perpendicular to the surface and causes the growth of "Ridge-like" structure.GIXRD shows that the expansion of the filler particles is the result of the expansion of lattice constant along the c-axis,and Raman spectroscopy shows that the defect density in the plane increases and graphite structure is transformed into amorphous structure after irradiation.Through HRTEM before irradiation and HRTEM with different depths of SNNG irradiatied at 0.5 and 5.0 dpa,the nucleation of vacancy rings,the nucleation and growth of dislocation rings into interstitial dislocations and the generation of a large number of incomplete planes and dislocation dipoles by forward climbing of dislocations are demonstrated from the atomic scale.Moreover,the generation and further evolution of such defects cause the change in lattice parameter and defect density.The surface densification caused by the closure of micro-gap and micropore the increase of defect density caused by irradiation all increase the hardness and Young's modulus of SSNG after irradiation.Compared with NPIG and pyrolytic carbon coating,SNNG has a more complete surface and stable irradiation performance.3.In order to have more experience as reference,densified IG-110(D-IG-110)was prepared by polyimide impregnation process with commercial nuclear graphite IG-110 as matrix,and its median pore diameter was reduced to 68 nm.Irradiation behavior of polyimide-carbon(PI-C)after polyimide impregnation and carbonization was studied by using 7.0 MeV Xe26+irradiation.SEM shows that the filling effect of PI-C in IG-110 pores is the reason why the pore diameter and open porosity of D-IG-110 decrease significantly,and the initial graphitization degree of D-IG-110 also decreases slightly.After irradiation at about 0.1 dpa(surface damage dose is 0.02 dpa),the shrinkage rate of PI-C is greater than that of IG-110 pores,which leads to cracks at the interface.GIXRD and Raman spectra show that the change rate of Lc of D-IG-110 is lower than that of IG-110,and the defect density on the surface near PI-C increases slowly and the shrinkage rate of La is lower,all of which indicate that the irradiation-induced graphitization appears in PI-C.This irradiation-induced graphitization makes the volume of PI-C shrink,which is the main reason for cracks at the interface between PI-C and IG-1 10.With the increase of dose,the damage effect of irradiation increases,and the cracks at the interface disappear,even some small cracks before irradiation disappear.According to the statistics of the pore diameter shrinkage of IG-110 as a reference and the comparison with the pore diameter variation of D-IG-110,it is shown that this crack shrinkage and tight contact is due to the volume expansion of PI-C caused by irradiation damage effect.Through Raman spectrum analysis,in the region of PI-C the surface residual stress caused by irradiation also increase slowly.4.Considering the large-sized graphite blocks and the compactness of graphite,fine-grained graphite FG was prepared by liquid-phase mixing process with natural graphite sheets with excellent lubricating properties and highly graphitized structure as filler.According to different calcination temperatures of 400? and 450?,two FGs were prepared:G-400 and G-450.Mercury injection curves show that the median pore diameters of G-400 and G-450 are 23 nm and 18 nm,respectively,but the former has large cracks of about 10 ?m,which will affect their molten salt barrier properties.In addition,due to the higher calcination temperature,the binder carbon layer on the surface of natural graphite sheet is more compact,and the graphitization degree of G-450 is slightly higher.Therefore,compared with G-450,G-400 shows lower irradiation stability and less retention of graphite structure after irradiation.Furthermore,the evolution mechanism of microstructure and morphology was clarified by 7.0 MeV Xe26+irradiation experiment:after irradiation,the interlayer spacing d002 between planes increased,the buckling/wrinkling or new basal plane of graphite plane formed,and the stacking structure became disordered;Before the layered structure in graphite was completely destroyed,the micro-crystalline coherence length Lc in the c-axis direction increased due to the increase of the number of layers and the closure of micro-cracks between the micro-coherent regions.With the increase of dose,Lc decreases due to the serious destruction of graphite layered structure or the expansion of disordered structure;In the basal plane,the defect density increases,and the transverse dimension La of microcrystals decreases rapidly due to Poisson effect caused by the expansion in c-axis,and sp2 C-C bonds transfer to sp3 C-C bonds.The evolution of the microcrystals expanding along the c-axis and shrinking along the a axis leads to fine cracks and "Ridge-like" structure on the surface.In addition,annealing experiments at 650? also show that the annihilation of point defects increases the order of the structure.Compared with the surface dose of 0.11 dpa,the defect density of G-450 irradiated by 1.25 dpa after annealing at 1000? did not decrease significantly.This is because vacancies are easy to migrate at this temperature,while too many defects remain and it is easy to form more complex vacancy clusters with stable structure.The formation of vacancy clusters also prevents the annihilation of interstitial atoms and vacancies,which makes the defect density fail to decrease further.