Microstructural modification and porosity evolution during carbonization of fuel element compact for HTGR

The fuel element for an HTGR is manufactured by mixing coated fuel particles with matrix graphite powder and forming either pebble type or cylindrical type compacts depending on their use in different HTGR cores. The basic steps for manufacturing a fuel element include the preparation of a graphite matrix powder, overcoating the fuel particles, mixing the fuel particles with a matrix powder, pressing the mixture into a green compact, carbonizing the green compact and the final high-temperature heat treatment of the carbonized fuel compact. During the carbonization of the green compacts at 800 °C in a flowing inert gas atmosphere, the microstructure and density are modified due to volatilization of the binder materials incorporated during the matrix graphite powder preparation step. In this work, changes of microstructure and density, as well as the evolution of the porosity were investigated during the carbonization of green compacts prepared using two different binder materials (phenol resin and poly-vinyl butyral) and under different carbonization temperatures (1073 and 1173 K).