A liquid-metal-cooled fast reactor Proliferation-resistant Environment-friendly Accident-tolerant Continuable-energy Economical Reactor (PEACER) is being developed at Seoul National University with metallic-type U-Pu-Th fuel and lead-bismuth (Pb-Bi) coolant, employing the steam-generating Rankine cycle without an intermediate loop. The natural circulation potential is a key ingredient of the liquid-metal reactor design. PEACER is designed as a loop system, which increases the natural circulation capacity and reduces the maintenance cost. This work is concerned with design of a large-scale test loop Heavy Eutectic Loop Integrated Operation System (HELIOS) to simulate thermal-hydraulic behavior of Pb-Bi in the PEACER system following a single-phase one-dimensional flow loop model. The full-height loop is being constructed for Pb-Bi natural-circulation testing and operating procedure development. Electrically heated rods are used in the heating section, and a water-cooling method is adopted in the cooling section. HELIOS has the same elevation difference between thermal centers of the heat source and heat sink but reduced piping diameters.