Investigation of automatic shutdown of HTTR on May 21st, 2003

Hirato, Yoji; Saito, Kenji ; Kondo, Makoto; Sawahata, Hiroaki ; Motegi, Toshihiro; Tsuchiyama, Masaru*; Ando, Toshio*; Mizushima, Toshihiko; Nakazawa, Toshio

HTTR (High Temperature Engineering Test Reactor) was operated from May 6th, 2003 to June 18th, 2003 to obtain operation data in parallel loaded operation mode and in safety demonstration tests. Operated with the reactor power at 60% of the rated power on May 21st, HTTR was automatically scrammed by a signalof "Primary coolant flow rate of the Primary Pressurized Water Cooler (PPWC): Low". The cause of the shutdown was the primary gas circulator (A) automatically stopped. The primary coolant flow rate of the PPWC decresed and reached the scram set value due to the gas circulator stop. As a result of investigation, it became clear that the cause of the gas circulator stop was malfunction of an auxiliary relay which monitored electric power of a circuit breaker in power line of the gas circulator. The cause of malfunction was deterioration of the relay under high temperature condition because the relay was installed beside an electric part which was heated up by electricity.