After the Fukushima NPP accident in 2011, it has become increasingly important for reactor safety design to consider design measures to prevent the occurrence of severe accidents. In this study, a new subassembly-type passive reactor shutdown device is proposed to enhance the diversity and robustness of prevention measures for core disruptive accidents in sodium-cooled fast reactors. The proposed device contains pins with a fuel material that is kept in the solid state during normal operation but melts into the liquid when its temperature exceeds a prescribed value under the accidents. When a ULOF (unprotected loss of flow) or UTOP (unprotected transient overpower) accident occurs, the device can provide large negative reactivity passively by the relocation of liquefied device fuel into the lower plenum region of the pins by gravitation alone in a short time. The reactor, in which part of usual fuel subassemblies is replaced with the proposed devices, becomes subcritical before driver fuels are damaged, even if ULOF or UTOP transient occurs. In the present study, candidate materials for device fuel such as metallic alloy and chloride, optimum device pin structure for liquefied fuel relocation, and nuclear and thermal-hydraulic characteristics of the device-loaded core under accident conditions will be mainly investigated to demonstrate engineering feasibility of the proposed device. This paper describes the project overview and discusses preliminary results on nuclear requirements for negative reactivity to be inserted for reactor shutdown under expected device conditions.