Pore structure evolution of A3–3 matrix graphite during heat treatment

Matrix graphite (MG), a key component of fuel elements for high-temperature gas-cooled reactors (HTRs), has a profound effect on the comprehensive performance and service safety of fuel elements. A3–3 MG was selected as the matrix material for the pebble fuel elements of the 10 MW experimental high-temperature gas-cooled reactor (HTR-10) and the high-temperature gas-cooled reactor pebble-bed module (HTR-PM) in China. During the preparation process of A3–3 MG, the green MG pebble must undergo two-stage heat treatment, namely carbonization and purification, to obtain excellent comprehensive properties for safe service. However, the porosity of A3–3 MG and its change during heat treatment remains unclear. Herein, the pore structure evolution through three different stages of A3–3 MG - the green, carbonized and purified samples- were tested using the gas adsorption method, mercury intrusion porosimetry and X-ray computed tomography (X-CT). The green sample had the smallest pore diameter and a uniform pore size distribution. The pore structure of the carbonized sample was the most developed, with the most micropores, mesopores and macropores. The molecular-sized micropores were produced due to the pyrogenic decomposition of the resin binder. Purification led to a decrease in pore diameter, together with a slight increase in closed pores and a decrease in pore connectivity due to pore merging and conversion. Two- and three-dimensional (2D and 3D) pore structure models were established by X-CT scan. The variation in pore size and shape, different types of pores as well as the pore conversion during the heat treatment process of A3–3 MG were observed. In this work, the porosity evolution of A3–3 MG was studied in detail, and references and strategies were provided for optimizing the preparation process and performance of pebble fuel elements.