Feasibility study on naturally-safe HTGR for air ingress accident

Ohashi, Hirofumi ; Sato, Hiroyuki  ; Tachibana, Yukio ; Kunitomi, Kazuhiko ; Ogawa, Masuro

The concept of the Naturally-safe HTGR is that the release of radioactive materials is kept at very low level and no harmful effect on people and the environment is ensured by only physical phenomena without any engineered safety features. At an air ingress accident, possible physical events to loss of the confinement function of the fuel coating layers are the crack of the coatings caused by the explosion of CO produced by the graphite oxidation and failure of the coatings by melting or sublimation caused by core heat up due to the reaction heat. The CO concentration and the heat generated by graphite oxidation inside the circular tube were numerically evaluated. It was confirmed that the CO concentration at the outlet of coolant channel can be maintained below the explosion limit due to the reaction with oxygen in the air, and the reaction heat can be removed by physical phenomena under certain conditions of the coolant channel geometry without any engineered safety features.