Failure to remove residual decay heat in the Fukushima Daiichi Nuclear Power Plant in 2011 has definitely influenced design attitudes toward nuclear reactors with passive-decay heat-removal features. Generally, modular high temperature gas-cooler reactors (HTGRs) have the ability to remove decay heat apart from their active and passive cooling systems. However, the feasibility of passive-decay heat removal in HTGRs is subject to design parameters. A systematic methodology for designing HTGR for passive-decay heat removal is introduced. As a part of this study, the conditions of several design parameters is used obtained in previous research for both underground and aboveground HTGRs capable of removing decay-heat successfully and satisfying the safety limit of temperatures of fuel and reactor buildings (RB) . The neutronic performance of a small HTGR whose design parameters were obtained using the conditions was investigated also.
