Estimation of graphite density and mechanical strength variation of VHTR during air-ingress accident

This study focuses on predicting the changes in graphite density and mechanical strength in VHTR during the air-ingress accident via thermal hydraulic system analysis code. A simple graphite burn-off model was developed based on the similarities between a parallel electrical circuit and graphite oxidation. The developed model along with other comprehensive graphite oxidation models were integrated into the VHTR system analysis code, GAMMA. GT-MHR 600 MW t reactor was selected as a reference reactor. Based on the calculation, the main oxidation process was observed 5.5 days after the accident when followed by natural convection. The core maximum temperature reached 1430 °C, but never exceeded the maximum temperature criteria, 1600 °C. However, the oxidation process did significantly decrease the density of bottom reflector, making it vulnerable to mechanical stress. The stress on the bottom reflector is greatly increased because of the reduction of loaded surface area with graphite oxidation. The calculation proceeded until 11 days after the accident, resulting in an observed 4.5% decrease in density and a 25% reduction of mechanical strength.