Study on the carbonization process in the fabrication of pebble fuel elements

The decomposition process of phenol resin which is a binder of spherical fuel elements matrix material for high temperature gas-cooled reactor (HTGR) was studied by means of TG-IR and DSC at the temperature of 20–800 °C. The thermal expansion characters of graphite ball samples were investigated through thermal mechanical analysis (TMA). The results showed that the phase transition of the phenol resin occurred at the temperature of 50–70 °C and the decomposition had two continuous exothermic stages. Accordingly, the graphite ball samples were expanding and shrinking successively during carbonization. The properties of the matrix material made through four-stages heating process completely satisfied the design requirements and the production efficiency of carbonization was increased by ∼70%. Studies showed that the heating process established according to the sample's size varied with carbonization temperature under dynamic conditions is more reasonable and the volume change caused by condensation is the critical factor to heating rate.