A new subassembly-type passive reactor shutdown device is proposed to expand the diversity and robustness of core disruptive accident prevention measures for sodium-cooled fast reactors (SFRs). The device contains pins with a fuel material that is in a solid state during normal operation but melts and fluidizes during an unprotected loss of flow (ULOF) or unprotected transient overpower (UTOP) accident. By rapidly transferring the liquefied device fuel into the lower plenum region of the pins via gravitation alone, the device can passively provide a large negative reactivity to the core. In this study, the nuclear and thermal properties of the device subassembly with metallic fuel were evaluated to determine the basic device specifications for proper device operation during ULOF and UTOP accidents. The results of the transient analysis of the ULOF initiating phase in a 750-MWe-class MOX-fueled SFR core showed that a conventional homogeneous core can maintain stable cooling of the core prior to coolant boiling in the driver fuel subassemblies. On the other hand, the negative reactivity required to terminate the event by device operation was found to be slightly larger in the low sodium void reactivity core than in the conventional homogeneous core.