A small and fast reactor employing sodium-cooling system is expected and 4S (Super Safe, Small and Simple) Fast Reactor generating 50 or 10 Megawatts electric has been designed. This reactor pursues simple operation system, less maintenance, higher safety and improved economic features. The electric power is controlled by a neutron reflector, which is required to be elevated at a very low speed. In the present study, a reflector drive system by a magnetic impact type micro-stepping actuator has been proposed for the 4S reactor. The actuator is composed of electromagnets, air-core coils and internal inertia guided by linear slider. This internal inertia carries air-core coils, which are opposed to air-core coils arranged inside the actuator. The actuator adheres to a metal base wall and supports a payload by electromagnet adsorption force. The actuator moves by a magnetic repulsion force of internal inertia by adding momentary large electric current to opposed pair of air-core coils. A miniature model of "internal inertia" magnetic impact type actuator was made. Two different ways to support the internal inertia were proposed to restore the internal inertia to the original state. In this paper, experiment was performed to evaluate the basic characteristics of the two types of actuator. As a result, the actuator elevated about 4 micrometers per step in the case of the "electromagnet support type" and about 25 micrometers per step in the case of the "spring support type" with 10-kilogram payload. Also, it was found that the payload carries out minute quantity separation instantaneously with the actuator by the shock of a repulsion force. This separation decreases the magnetic force by which the payload pulls down the actuator and makes the actuator upward movement easy. Moreover, it became clear that the driving distance could be sorted by "effective magnetic impact impulse", which can be calculated from current waveform. Therefore, driving distance can be controlled by condenser capacity and charged voltage that generates current waveform of the air-core coils. In light of these results, improvements will be made for design of an enlarged engineering model of the actuator. Experiments with larger payload are planned.
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