Research regarding small- and medium-sized nuclear reactors (SMRs) has increased because of multipurpose applications and increased safety. According to this tendency, a new conceptual nuclear reactor, the Regional Energy rX-10MWt (REX-10), is being designed. REX-10 adopts a way to remove heat by natural circulation and integrates the primary systems within a reactor pressure vessel. To evaluate the steady-state and transient behavior of natural circulation in REX-10, a NAtural Circulation TEst Reactor (NACTER) is designed using the scaling law. The ratio of the height and core power are 1/3 and 1/500, respectively.

This research can be divided into three parts - a steady-state experiment, a transient experiment, and MARS (Multidimensional Analysis for Research Safety) code analysis. To investigate the natural circulation characteristics under the steady-state conditions, two parameters were chosen and various experiments were conducted. As a result of the steady-state experiment, we show that the most important parameter that affects the natural circulation behavior is the heater power. In addition, we carried out a transient experiment. The results of the transient experiment are that the NACTER facility is well controlled and guarantees safety in abrupt changes in experimental conditions. Finally, MARS code simulations were conducted. The MARS code results show good agreement with the experimental results.