Numerical analysis and air-water test were performed in order to decrease pressure loss in a BWR steam separator. A current swirler and two low pressure-loss swirlers with a small swirl angle and with a small hub diameter were evaluated. Calculated results showed that the swirler with a small hub diameter had better gas-liquid separation than the swirler with a small swirl angle because of larger circumferential velocity and centrifugal force. It was also confirmed by the air-water simulation test. In case of the combination of the swirler with a small hub diameter and the current upper structure, the target of carryover was satisfied and its pressure loss was about 45% of the current separator. In case of a 3rd stage high-swirl type, which had the same barrel diameter as the 2nd stage pick-off ring diameter, carryover was as low as the current separator and pressure loss was about 60% of the current.
