Japan Atomic Energy Agency has conducted a conceptual design of a 50？MWt small-sized high temperature gas cooled reactor (HTGR) for multiple heat applications, named HTR50S, with the reactor outlet coolant temperature of 750°C and 900°C. It is first-of-a-kind of the commercial plant or a demonstration plant of a small-sized HTGR system to be deployed in developing countries in the 2020s. The design concept of HTR50S is to satisfy the user requirements for multipurpose heat applications such as the district heating and process heat supply based on the steam turbine system and the demonstration of the power generation by helium gas turbine and the hydrogen production using the water splitting iodine-sulfur process, to upgrade its performance compared to that of HTTR without significant R&D utilizing the knowledge obtained by the HTTR design and operation, and to fulfill the high level of safety by utilizing the inherent features of HTGR and a passive decay heat removal system. The evaluation of technical feasibility shows that all design targets were satisfied by the design of each system and the preliminary safety analysis. This paper describes the conceptual design and the preliminary safety analysis of HTR50S. 1. Introduction Nuclear energy is one of the most promising energy sources to satisfy energy security, environmental protection, and efficient supply. Many developing countries have expressed their interest in deploying nuclear power plants in their own country. After the accident at the Fukushima Daiichi Nuclear Power Station, some developed countries which already installed the nuclear power plant have changed their policy for the nuclear energy. However, many developing countries still show their interest in the nuclear power plants. Since the small- and medium-sized reactors [1] can reduce capital cost and can provide electric power away from large grid systems, they are suitable for the developing countries. The high temperature gas cooled reactor (HTGR) [2] is one of the small modular reactors and has attractive inherent safety features. It is a helium cooled graphite moderated reactor employing a ceramic coated fuel particle with high temperature capability. It can operate at reactor outlet temperature of about 1,000°C, much higher than conventional light water reactor (LWR). Accordingly, HTGR can be applied to many kinds of heat applications such as hydrogen production, electricity generation by gas turbine and steam turbine, process heat supply, district heating, and sea water desalination [3–6]. HTGR has a superior safety potential as
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