## 1 Introduction  
The molten-salt reactor (MSR), which uses liquid molten salt as not only the reactor fuel but also a coolant, was originally proposed by Oak Ridge National Laboratory (ORNL) in order to design nuclear-powered aircraft [1]. At ORNL, a 2.5 MW Aircraft Reactor Experiment was conducted to support the feasibility study for applying molten fluoride fuels [2]. The 8 MW Molten-Salt Reactor Experiment (MSRE) was conducted in the 1960s, which was maintained as critical for operations. Several more MSR concept designs, such as thermal and fast-spectrum reactors and single- and two-fluid loops, were proposed on the basis of the experience gained from the MSRE [3, 4]. MSR systems can operate with very low excess reactivity requirements since online fuel processing allows the continuous removal of fission products, unlike in conventional solid-fueled reactors. High-level radioactive waste can be separated and taken away from the liquid fuel in the main circuit through online chemical treatment. It is worth mentioning that in serious accidents, ultra-hot fuel automatically flows into a safety storage tank [5]. The operational pressure can be designed to be close to atmospheric pressure with a high boiling point of fuel salt. In addition, because of the higher heat capacity of the molten fluoride fuel compared to the coolant of conventional reactors, the core has the potential to reach a higher density, and a more compact design is thus feasible [6]. Considering its safety features and economic advantages, the MSR was selected as one of six most promising reactor options in the GIF nuclear energy systems [7]. Over the last few years, numerical and experimental investigations of the MSR have been growing across many countries. Researchers from the United States proposed a high-temperature reactor, a fluoride-salt-cooled reactor that used coated particle fuel and was cooled by the salt, and conceptually and experimentally investigated the core design [8; 9]. A liquid-salt microreactor (MARS) was designed in Russia for space exploration [10]. Feasibility and safety assessments of liquid-fuel fast-reactor systems (EVOL and SAMOFAR) have been performed in the Euratom framework project, and the fast-spectrum MSR concept (MSFR) was developed to investigate the feasibility of applying the MSR for fuel breeding and nuclear waste transmutation [11; 12; 13; 14]. The iMAGINE was proposed for both energy production and nuclear waste management in an integrated system [15]. The cycle of thorium-uranium fuel was investigated in the SPHINX project in the Czech Republic, with experiments on the reactor physics, chemistry reprocessing technology, and structural materials [16]. The Dual-Fluid Reactor (DFR) concept was presented at the IFK Institute by [PERSON] et al. [17], in which two fluids exist in the core: molten salt as the fuel and liquid lead as the coolant. The neutronic and thermal-hydraulic behaviors of the DFR concept were preliminarily investigated [18; 19], and the smaller modular concept (SMDFR) was proposed by [PERSON] et al. [20]. A hybrid MSR system was proposed by [PERSON] et al. [21] in Japan, in which the nuclide of \({}^{233}U\) is produced by the accelerator spallation target. The Chinese Academy of Science launched the Thorium Molten Salt Reactor (TMSR) project to utilize thorium resources and produce hydrogen [22]. For the development of MSR concepts all over the world, the safety of the operation and power control of the reactor is always an important concern in the technology roadmap and engineering implementation. The MSR has many differences from conventional solid-fuel reactors, since the delayed neutron precursor drift, caused by the flow of the liquid fuel in the primary circuit, plays an important role in its dynamic behavior. Considering the unique dynamic characteristics, a dedicated control strategy for the MSR has to be developed and applied for a high control performance.