Ion Irradiation Effects And Molten Salt Impregnation Property Of Nuclear Graphite

Nuclear graphite, an important neutron moderator and reflector material, has been widely used in nuclear reactors. Graphite is mainly used as the neutron moderator in an MSR. During the moderating process, carbon atoms would be displaced from their lattice sites several times, resulting in property changes of graphite, such as dimension, strength, electrical resistivity, thermal conductivity, and stress irradiation creep behavior. This will directly affect the safety and lifetime of graphite reactors. The performance of the graphite under irradiation is closely associated with the defects produced in the graphite by the irradiation itself such as vacancy clusters, interstitial clusters, and dislocations. Therefore, the characterization of the changes in the physical properties, arising from radiation-induced defects, and establishing the relationship between them will be helpful in understanding the irradiation mechanisms and predicting the property changes in graphites. The irradiation effect assessment is essential for the application of a new nuclear graphite. Ion irradiation can be used as a surrogate of neutron irradiation. The efficient characterization methods of ion irradiation (MeV) effects may be successful for the preliminary assessment on the newly developed fine-grained isotropic graphite materials, which have the potential to be applied in the MSR such as fine-grained isotropic graphite made from natural microcrystalline graphite and super fine-grained isotropic graphite prepared from mesocarbon microbeads.Thorfore, one of the focuses of this dissertation is the ion irradiation effects. Three parts work were included in this section.1. The irradiation-induced damage on the fine-grained isotropic nuclear graphite, IG-110, was investigated by 3-MeV proton irradiation. The irradiation effects were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM). Raman spectroscopy, X-ray diffraction (XRD), and nano-indentation. The surface morphology showed a fragmented shape after the irradiation, indicating that the surface microstructure of the graphite was damaged by the proton bombardment. The TEM images revealed clear and convincing evidence for the increase in defect clusters (probably interstitial clusters), basal plane bending, and basal plane dislocations, which might be the main reason for the property changes. The Raman studies indicated a rapid increase in the interstitial and vacancy defects, and decrease of in-plane "crystallite size". The XRD results indicated a slight increase in the interlayer spacing and decrease in crystallite size. The enhancement in the hardness and modulus can be attributed to the pinning of basal plane dislocations by lattice defects produced by proton irradiation.2. The irradiation damage on the nuclear graphite IG-110 was investigated by 1-MeV C+ irradiation. The changes of microstructure and mechanical properties were characterized using SEM, Raman spectroscopy, Dopplor broadening positron annihilation spectra (DBPAS), TEM and nano-indentation. The surface of IG-110 changes from flaky structure to spheroidal micro-particles, indicating an amorphization process. The Raman analysis results indicated that a process from microcrystalline graphite to nanocrystalline graphite and then gradually amorphization occurred under irradiation. The DBPAS and Raman studies indicated a continuous increase in the vacancy defects with a rapid increase in the initial stage of irradiation. The TEM images revealed an increase in doo2, basal plane bending, and a gradual disappearence of the micro-crack. The rapid increase in the modulus in the initial stage of irradiation is consistent with the rapid increase in defects, indicating that the initial rapid increase in modulus can be attributed to the pinning of basal plane dislocations by radiation induced lattice defects, after 0.72dpa the increasement can be attributed to the dislocation pinning and a closure of the micro-cracks.3. The structural difference of the filler and binder phases of nuclear graphite 1G-110 was given and the behavior of both the filler and binder phases under 70 keV 12C+ ion beam irradiation was studied by Raman spectroscopy simultaneously. Selected micro-region positions in the filler and binder phases were marked for comparison of the changes of Raman spectra due to the nonuniformity of the micro-scale structure in graphite. The first- and second-order Raman scattering spectra of the marked positions in the filler and binder phase were monitored and investigated. Although slight differences exist, the same variation trend of Raman spectra was found in both of the filler and binder phases. The results of the structural change and defect evolution in the irradiation process indicate that a process from microcrystalline to nanocrystalline graphite and gradually to amorphous carbon in both phases under irradiation was found. The Ramam results also show that the behavior in the nanocrystallization process (0.05-0.35dpa) is gradual and relatively stable compared to the initial rapid process of graphite under irradiation.Furthermore, graphite is a porous material and its pores can be easily impregnated with the molten fuel salt in a high pressure environment. A seepage of the fuel salt into the graphite leads to the formation of local hot spots, which significantly damage the graphite, thereby reducing the service life of the graphite components. The nuclear graphite, grade CGB, produced for previous molten salt reactor experiment (MSRE) was not suitable for modern reactors. Choose appropriate graphite for an MSR becomes a serious problem.Therefore, another focuses of this dissertation is the molten salt impregnation property of nuclear graphite. Three parts work were included in this section.1. The porosity properties of nuclear grades graphite IG-110, IG-430, NBG-17, and NBG-18 were investigated by optical microscopy(OM), mercury porosimetry, and helium gas pycnometry. The molten salt impregnation tests were performed by impregnating all four kinds of graphites with molten fluoride salt at a temperature of 650℃ and a pressure of 1 and 5 atm, respectively. The molten salt impregnation characteristics were studied by SEM and TEM. The porosity results indicated that the IG-110/IG-430 showed uniformly-distributed gas-evolved pores, small entrance pore diameter～4-2μm, and a high open porosity; NBG-17/NBG-18 showed a big gas-evolved pores, wide range entrance pore diameter, and a high closed porosity(numerous calcination cracks in filler). The impregnation results indicated that the impregnation mechanism of molten salt was similar to mercury. Only the applied pressure is greater than threshold pressure, the impregnation starts. With the increase of pressure, the small pores will also be filled with salt. The distribution of molten salt indicating that most of the cracks(calcination cracks and Mrozowski cracks) were closed. This part given an molten salt impregnation assessment method for MSR graphite.2. The molten salt impregnation characteristics and microstructure of the micro-fine grained isotropic graphite ZXF-5Q investigated. The conventional nuclear graphite 1G-110 were used for comparison. The molten salt impregnation tests were performed by impregnating both ZXF-5Q and IG-110 with molten fluoride salt at a temperature of 650℃ and a pressure of 1,3 and 5 atm, respectively. The impregnation characteristics were studied by SEM. The results indicate that ZXF-5Q could not be infiltrated by the molten fluoride salt even at a pressure of 10 atm due to its very small entrance pore diameter. The microstructural characteristics of the gas-evolved pores, calcination cracks and Mrozowski cracks in the filler particles, and the crystal structure were characterized by OM, mercury porosimetry, helium gas pycnometry, TEM, XRD, and Raman spectrometry. The ZXF-5Q showed uniformly-distributed gas-evolved pores, very small entrance pore diameters (i.e., neck of the open pore), and a lower degree of graphitization than IG-110. The low closed porosity can be attribute to the lack of calcination cracks. The size of lenticular Mrozowski cracks were smaller than that of IG-110, may indicating a short service life.3. The molten salt impregnation behavior of nuclear graphite NG-CT-10 under pressure environment similar to solid fuel molten salt reactor, such as reactor running and shutdown states, were studied. The weight gain of the samples under each condition were measured and compared. The distribution difference of molten salt was studied by SEM. The weight gain of the pre-gas-filled sample was less than that for degassed sample and the distribution of salt in the pre-gas-filled sample was in gradient, indicating that the gas inner the graphite played a part in preventing impregnation. A simplified model was given to make the impregnation process more cleraly and the impregnation depth of molten salt for the pre-gas-filled sample was calculated by the simplified model. The impregnation behavior balance soon, the influence of impregnation time could be ignored. The salt in unloaded sample was much less than that in the pre-gas-filler sample, however, the distribution of salt was also in gradient, similar to that in gas-filler sample. This indicates that though most of the intrusive molten salt was extrude, there was still many salt trapped in the pores of graphite.