Infiltration of graphite by molten 2LiF–BeF₂ salt

With the aim of developing neutron moderator and neutron reflector materials for fluoride-salt-cooled high-temperature reactors (FHRs), static infiltration tests were performed on graphite materials in molten 2LiF–BeF₂ (FLiBe) salt, which is a potential primary coolant, at 700 °C and various pressures. The weight gain ratios of four grades (NBG-18, IG-110, NG-CT-10, and NG-CT-50) of graphite after infiltration were measured to determine their infiltration curves. The threshold pressure for FLiBe salt infiltration for the ultrafine-grained graphite (NG-CT-50) was greater than 600 kPa and much higher than those of the other three grades (medium-grained/fine-grained graphites), indicating that this graphite grade more probably resists salt infiltration in FHRs than other grades of graphite. However, if the threshold pressure is exceeded, it has the highest potential capacity for infiltrated salt over the pressures tested. The four grades were also characterized using mercury intrusion porosimetry. It was found that the infiltration curves of these two unwetting liquids were very similar. Scanning electron microscopy characterization showed that the FLiBe salt was distributed relatively uniformly in all four grades, indicating the presence of interconnected networks of open pores throughout the samples. X-ray diffraction patterns showed that infiltration test at high pressure led to an improved structural order and a decreased d-spacing in graphite.