DEVELOPMENT OF CONCRETE ABLATION MODELS FOR GAMMA ASSESSMENT OF TOTAL RCCS FAILURE WITH AN AIR-INGRESS ACCIDENT IN HTGRS

Abstract

We considered total reactor cavity cooling system (RCCS) failure with an air-ingress accident due to unthinkable natural disasters as the worst safety issue in a high temperature gas-cooled reactor(HTGR). For this worst accident scenario, we analyzed the integrity of a 600 MWth gas turbine-modular helium reactor (GT-MHR) using the GAs Multicomponent Mixture transient Analysis code (GAMMA). The concrete ablation models, which consist of the endothermic reactions, were developed and implemented into the GAMMA. Through the system assessment we predicted the maximum core temperature, the maximum reactor pressure vessel (RPV) temperature, the ablation thickness of the concrete containment, and the cavity pressure. The maximum fuel temperature was still lower than 1600°C during the 500-hour transient simulation. The concrete absorbed some fraction of decay heat below 40%.The thermally-driven chemical reactions of the concrete produced a considerable amount of gases which can be released through the confinement venting valve. The RPV temperature exceeded its safety limit at 30 hour from the beginning of the accident.