Study of nuclear graphite and molten salt interaction using X-ray computed tomography and image-based pore network modelling

The aluminium fluoride-sodium fluoride eutectic molten salt infiltration behaviour into two medium-grained nuclear-grade graphite: Gilsocarbon and NBG-18 was investigated. The salt in the infiltrated graphite samples was analysed using 3D X-ray micro-computed tomography (μ-CT). The μ-CT micrographs show that the salt has mainly occupied the larger, more spherical pores in the binder phase while the fine, lenticular-shaped cracks have remained salt-free under the current infiltration condition. The salt infiltration was more severe and more uniform in space for Gilsocarbon graphite despite it has comparable density (therefore overall porosity) with NBG-18 graphite. Mercury intrusion porosimetry (MIP) tests also showed that the Gilsocarbon graphite is more permeable than the NBG-18 graphite under an intrusion pressure of 3.45 bar (344.7kPa). The transport properties of the porous graphite structure were investigated using pore network models (PNM) derived directly from the 3D μ-CT images. The results suggest that the different molten salt infiltration and mercury intrusion behaviours for these two grades of graphite under the current test conditions is mainly due to the different connectivity among the larger pores in the binder phase.