The capability to validate integral transient response models is a key issue for licensing new reactor designs. The Compact Integral Effects Test (CIET 1.0) facility reproduces the thermal-hydraulic response of fluoride salt–cooled high-temperature reactors (FHRs) under forced- and natural-circulation operation. CIET 1.0 provides validating data to confirm the predicted performance of the direct reactor auxiliary cooling system, used for natural-circulation–driven decay heat removal in FHRs, under a set of reference licensing basis events. CIET 1.0 uses a simulant fluid, DOWTHERM A oil, which, at relatively low temperatures (50°C to 120°C), matches the Prandtl, Reynolds, and Grashof numbers of the major liquid salts simultaneously, at 50% geometric scale and heater power under 2% of prototypical conditions. CIET 1.0 has been designed, fabricated, filled with DOWTHERM A oil, and operated. Isothermal pressure drop tests were completed, with extensive pressure data collection to determine friction losses in the system. The project then entered a phase of heated tests, from parasitic heat loss tests to more complex feedback control tests and natural-circulation experiments, with the ultimate goal of validating best-estimate FHR models using RELAP5-3D and the novel one-dimensional FHR Advanced Natural Circulation Analysis (FANCY) code. This paper introduces the scaling strategy, design, and fabrication aspects, and start-up testing results from CIET 1.0. The CIET 1.0 model in RELAP5-3D and FANCY is detailed, and verification and validation efforts are presented. For various heat input levels and temperature boundary conditions, mass flow rates are compared between RELAP5-3D and FANCY results, analytical solutions when available, and experimental data, for both single and coupled natural-circulation loops. The study shows that both RELAP5-3D and FANCY provide excellent predictions of steady-state natural circulation in CIET 1.0, with mass flow rates within 13% of experimental data, suggesting that both codes are good candidates for design and licensing of FHR technology.