Graphite foil-incorporated PAN/Pitch/Phenolic-derived carbon/carbon composite and preliminary hermetic sealing test in molten fluoride salt

C/C composite has been recommended for use in molten fluoride salt-cooled reactors as structural containment for the highly corrosive salts in heat exchangers, piping, pumps, etc. Despite their excellent high-Temperature mechanical properties and chemical compatibility with molten fluoride salts, the inherently high porosity level of C/C composites is one major challenge to the material for such applications. The numerous inherent and/or high temperature heat treatment-induced macrocracks and microcracks in C/C composites are difficult to seal completely using conventional methods such as CVD or CVI. To overcome the porosity-related molten salt permeation problem, a graphite foil-incorporated PAN fiber, a pitch/phenolic resin matrix-derived C/C composite, was developed in the authors' laboratory. Experimental results showed that the C/C composite with the graphite foil had lower density value, higher porosity value, and lower bending strength value than the C/C composite without the graphite foil. A preliminary result of the hermetic sealing test in a molten mixture of LiF, NaF and KF salts ("Flinak" salt) showed that the graphite foil could effectively seal the underlying C/C composite against permeation from high temperature molten salts. Furthermore, given appropriate process parameters, the graphite foil could remain tightly bonded to the C/C structure during the entire fabrication process and the subsequent bending and hermetic sealing tests.