Pore Structure of Nuclear Graphite Obtained via Synchrotron Computed Tomography

Abstract

Nuclear graphite that can be used in molten salt reactors, one of the main types of fourth-generation nuclear reactors, has the properties of high purity, high density, and radiation resistance, and it also has a special pore structure that prevents infiltration of the molten salt fuel. The nuclear graphites IG-110, NBG-18, and NG-CT-50 were characterised using X-ray computed tomography (CT) to understand their internal pore structures. The porosities, isolated pore volumes, and connected pore volumes were obtained. The average diameter of connected pores in IG-110, NBG-18, and NG-CT-50 were 9.9, 10.4 and 9.5 μm, respectively, but the nuclear graphite NBG-18 had the broadest diameter range for connected pores, with many of > 25 μm. To further analyse the connected pores, pore throat networks were established based on the connected porosities. The pore throats, channel lengths, and coordination numbers of the connected pores were herein quantitatively analysed. Among IG-110, NBG-18, and NG-CT-50, the average diameters of the throats were 4.5, 5.2, and 4.6 μm, and the peak throat diameters were 1.8, 2.6, and 1.2 μm, respectively, but the NBG-18 pore throat distribution also included multiple peaks at larger diameters (16–20 μm), which was consistent with findings from the mercury intrusion test. Our statistics based on the pore throat networks suggested that NBG-18 is the most easily infiltrated by molten salt among the three nuclear graphite samples, while NG-CT-50 is the most difficult. The quantitative information from the pore throat modelling could be a basis for future studies on liquid flow simulations inside graphite.