The 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 those related to hydrogen production and power generation. The HTGR also meets the 3E+S policy targets of Japan’s Strategic Energy Plan. JAEA’s final target is the successful demonstration of the coupling of a nuclear reactor and heat application system by connecting a hydrogen production plant to Japan’s first HTGR, that is, the HTTR.