[en] Model fuel assemblies of the Advanced Thermal Reactor (2nd design) and the JPDR-II have been tested in an air-water test loop (FAT-I) to study two-phase flow characteristics at the system pressure of 3.5 kg/cm² g, the void fraction of 10 - 50% and the water flow rate up to 60 t/h. Average void fractions and pressure drops due to spacers, base plate, tie plate as well as rod bundle were measured. The ratio of the average void fraction to the volumetric flow fraction of air was 0.95 for the ATR fuel assembly and 0.7 for the JPDR-II. The frictional pressure drop for the rod bundle was expressed as a function of the volumetric flow fraction. An estimation method of the total pressure drop at normal reactor operation pressure has been derived in the light of two-phase flow in pipes. Nearly one half of the total pressure drop was the pressure drop other than for the rod bundle friction. Observations through a window showed that bubble flow regime predominated throughout the experiments. (author)