A high temperature gas-cooled reactor (HTGR) is a helium gas-cooled and graphite-moderated thermal-neutron reactor with excellent inherent safety features. The HTGR meets various heat application requirements, including hydrogen production and power generation. The HTGR also meets the 3E+S policy targets in Japan's Strategic Energy Plan. JAEA's final target is the successful demonstration of coupling a nuclear reactor and heat application systems by connecting a gas turbine and hydrogen production facilities to Japan's first HTGR, that is, the high temperature engineering test reactor (HTTR)