Achievement of safety demonstration tests using HTTR; Loss of forced cooling test at 100% reactor power (30 MW)

Nagasumi, Satoru; Hasegawa, Toshinari  ; Nakagawa, Shigeaki  ; Kubo, Shinji  ; Iigaki, Kazuhiko ; Shinohara, Masanori ; Saikusa, Akio ; Nojiri, Naoki ; Saito, Kenji ; Furusawa, Takayuki ; Motegi, Toshihiro; Sawahata, Hiroaki ; Homma, Fumitaka; Kondo, Makoto; Sekita, Kenji; Kawamoto, Taiki; Nishihara, Tetsuo ; Hori, Naohiko ; Shinozaki, Masayuki

A safety demonstration test under abnormal operating conditions using the HTTR (High Temperature Engineering Test Reactor) was conducted to demonstrate safety features of the HTGRs (High Temperature Gas-cooled Reactors). Under a simulation of a control rod shutdown failure, all primary helium gas circulators were intentionally stopped during a steady-state operation at 100% reactor thermal power (30 MW), temporal changes of the reactor power and temperatures around the reactor pressure vessel (RPV) were obtained after the complete loss of forced heat removal from the reactor core. After the event (primary coolant flow stopped), the reactor power quickly decreased due to the negative reactivity feedback associated with the core temperature rise, and then the reactor power spontaneously shifted to a stable state of low power (about 1.2%) even after a recriticality. Heat dissipation from RPV surface to a surrounding vessel cooling system (water-cooled panels) ensured the amount of heat removal required to maintain the reactor temperature constant in the low power state. In this way, the transition from the event occurrence to the stable and safety state, i.e., inherent safety features of HTGRs, were demonstrated in the case of core forced cooling loss without active shutdown operations.