The changes in both the crystallographic coefficient of thermal expansion (CCTE) and the coefficient of thermal expansion (CTE) of FLiNaK-infiltrated nuclear graphite were studied using in situ-heating X-ray diffraction analysis and horizontal push-rod dilatometry. It was found that, at temperatures lower than the melting point of FLiNaK, the CCTE of the d(002) spacing of graphite decreases with an increase in the weight of the graphite sample because of FLiNaK infiltration. On the other hand, the CTE of bulk graphite increases after molten salt infiltration. The CCTEs of the as-prepared FLiNaK salt, and the salt in the graphite pores were compared. The decrease in the CCTE of the FLiNaK salt after it had infiltrated into the graphite pores confirmed that interactions occur between the graphite and the salt. These interactions are probably induced by the difference in the CTEs of graphite and the solidified salt. Further, it is likely that the crystallization pressure also plays an important role here. Thus, both the causes need to be considered when using nuclear graphite in molten salt reactors.

•Effect of Hastelloy N alloy and corresponding corrosion products on the corrosion of SiC was studied.•Ni in alloy can react with Si in salt to drive the corrosion of SiC by the formation of Silicide (Ni31Si12, NiSi,) on alloy.•Corrosion product NiF2 can increase the corrosion of SiC. Raman spectrum indicates that Si in salt is in the form of [SiF6]2−.•Corrosion product CrF2 can drive the corrosion of SiC by the formation of carbide (Cr7C3, CrC) on SiC.Interaction between SiC and Hastelloy N alloy in LiF–NaF–KF salt was studied for the application of SiC to molten salt reactors. Results reveal that Hastelloy N alloy and its corrosion products can induce the corrosion of SiC in salt. Ni can react with Si in salt to form silicide (Ni31Si12, NiSi). Cr2+ can react with SiC to form carbide (Cr7C3, CrC). Ni2+ can cause SiC with a thickness of 50 μm almost disappear after 45 days; thus, the Si content in salt can increase to 0.5 wt%. Raman spectrum indicates that Si in salt is in the form of [SiF6]2−.

•Corrosion of SiC in molten FLiNaK salt was studied.•Corrosion resistance of SiC in molten FLiNaK salt can be improved by decreasing oxygen content in SiC.•High purity CVD SiC can also be corroded by molten FLiNaK salt, which should be attributed to oxygen impurity in salt.•Corrosion of SiC results in the formation of a carbon-rich surface.•Corrosion of SiC results in the dissolution of Si from SiC into salt.The corrosion of SiC in molten FLiNaK (46.5 mol% LiF, 11.5 mol% NaF and 42 mol% KF) salt was studied. Results reveal that oxygen impurities from SiC and salt can affect the corrosion. SiC with a large amount of oxygen impurity is corroded, whereas high purity SiC is only slightly corroded. SiC can react with oxygen impurity in salt to form oxide and then corroded by the cleavage of SiOSi bond. Corrosion decreases the Si content in SiC, resulting in formation of a carbon-rich surface. A portion of excess C reacts with F to form CF bonds.

Infiltration of molten FLiNaK salt into degassed nuclear graphite samples under inert gas pressure was studied. The weight gain of different grades (2020, 2114, IG-110, NBG-8, G1 and G2) of nuclear graphite during infiltration with different pressures was measured. Molten salt infiltration was compared with mercury intrusion porosimetry where it was found that mercury infiltration was a useful predictor of the threshold pressure and infiltration volume per gram graphite for molten salt infiltration. The distribution and morphology of salt in the graphite were observed by scanning electron microscopy, with very little difference between the molten salt content at the center and edge of samples for samples infiltrated at pressure higher than the threshold pressure. Increased molten salt infiltration with increased pressure resulted from the occupation of smaller pores and full occupation of the larger irregular pores. The similarity of weight gain between molten salt infiltration equilibrated at 20 and 100 h showed 20 h was adequate to obtain equilibrium.

SiCf/SiC composites reinforced with either two-dimensional woven or three-dimensional braided preforms were fabricated by the chemical vapor infiltration (CVI) process. The microstructure, mechanical and thermal properties of the composites were studied. In comparison, the 3D-braided out-performed 2D-woven composites based on the mechanical and thermal properties. The geometry and microstructure determinants of the properties of the composites were investigated. Additionally, we tested how resilient the SiCf/SiC composites were in contact with eutectic molten salt of LiF, NaF and KF to determine whether such materials could function in a molten salt reactor. The molten salt had minimal influence on mechanical properties of SiCf/SiC composites due to the SiC coating blocking the infiltration of molten salt. Our data demonstrated the feasibility of CVI produced SiCf/SiC composites with potential applications in molten salt reactor.

Isotropic pyrolytic carbon (IPyC) prepared at 1300 °C by chemical vapor deposition was implanted with 129Xe26+ ions to obtain a wide range of information and understanding about the coating materials in nuclear energy field. Microstructure of the pristine and ion-implanted IPyC on nuclear graphite substrate was firstly investigated using polarized light microscopy, scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, nanoindentation, and X-ray photoemission spectroscopy. It was demonstrated that the Xe ion irradiation resulted in concurrent changes in both physical and chemical structures of our standard polycrystalline sample. Influences of the thermal annealing temperature on the properties of the implanted IPyC at 500 and 1000 °C were also studied. Ion-irradiation gave rise to the formation of structural deterioration along a and c axis, accompanying with the appearance of widespread clastic morphology among the irradiated zone of IPyC. There was a dose window that could be used to tune the mechanical properties of IPyC: the nanohardness and Young’s modulus increased after an irradiation, but decreased as the amorphization was reached.

The irradiation-induced damage on the fine-grained isotropic nuclear graphite, IG-110, was investigated by 3-MeV proton irradiation at room temperature. The irradiation effects were characterized using scanning electron microscopy, transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and nano-indentation. The surface morphology showed a fragmented shape after irradiation, indicating that the surface microstructure of the graphite was damaged by 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 property changes. 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.

SiC coating is produced on a nuclear graphite (NG) substrate using chemical vapor deposition at 1150 °C to protect it from molten salt diffusion. Infiltration studies, performed in molten FLiNaK salt under an argon atmosphere at 5 atm, show that uncoated NG exhibits significantly higher weight gain than SiC-coated NG. The continuous and compact SiC coating exhibits excellent infiltration resistance in liquid fluoride salt as confirmed by synchrotron radiation X-ray microbeam fluorescence.

Infiltration studies were performed on uncoated nuclear graphite and isotropic pyrolytic carbon (PyC) coated graphite in molten FLiNaK salt at 650 °C under argon atmosphere at 1, 3 and 5 atm. Uncoated graphite shows weight gain more obviously than that of PyC coated graphite. Nuclear graphite with PyC coating exhibits excellent infiltration resistance in molten salt due to the small open porosity as conformed from scanning electron microscopy and mercury injection experiments.

12C+ ion implantation has been shown to be an efficient way to increase the magnetization of highly oriented pyrolytic graphite, and a saturation magnetization of 17.6 emu g−1 at room temperature has been produced. The implantation was performed in four steps with decreasing ion energy for each step. Increasing the magnetization with the number of implantation steps has shown that the magnetization is correlated to the defect density. The temperature dependence of spin susceptibility calculated according to the electron spin resonance spectra indicates that the magnetism in the sample is intrinsic. The evolution of Raman spectra along with the degree of implantation supplied further evidence to the correlation between in-plane defects and the magnetization of the sample, and explained the decreased magnetization resulting from the final implantation step.