[en] The amounts of gaseous radionuclides released to the environment through the reactor stack during the HANARO operation are reevaluated. Because the release amounts in the Safety Analysis Report (SAR) of HANARO were evaluated under very conservative assumptions, it was not easy to obtain reasonable results in the radiological dose assessment or the establishment of the derived release limit of radionuclide. The source distributions in the coolant during the reactor operation are analyzed by dividing the primary coolant and reactor pool into three regions - primary coolant, pool water below hot water layer and hot water layer. The measured activities of Na-24 and Ar-41 are utilized to determine the removal efficiency of filter and ion exchange, and the mixing rate of water at the hot water layer boundary. The evaporation rate of water and the exchange coefficient of noble gas at the pool top are determined from the reactor operation data and the SAR of OPAL reactor in Australia. For Ar-41, its production are examined and it is concluded that it is mainly generated from the activation of Ar included in the nitrogen gas in the pneumatic transfer system (PTS). Ar-41 is increased due to the inclusion of PTS effect. The annual release amounts of main nuclides are 2.18E+07 Bq/yr for I-131, 2.65E+11 Bq/yr for Ar-41 and 3.00E+10 Bq/yr for Xe-133. These are conservative estimates compared to the recent measurements