Design of the steam generator should be considered the safety about the sodium-water reaction occurred by water leakage in heat transfer tube. Water leakage mainly occurred from welding defect at the process of tube connection, the vibration of heat transfer tube bundle in steam generating system, fretting, and pin hole in original tube manufacturing. The classification of water leak divided to two parts, roughly, in case of the water leak studies. One is small leakage phenomena analysis, and the other is it of large leakage which was mainly treated to the evaluation on pressure increasing from hydrogen gas formed by sodium-water reaction in sodium system. In this study, the corrosion phenomena on the target tube surface appeared by small water leakage was analyzed, and the temperature increasing profile phenomena formed by sodium-water reaction through the leak nozzle under water medium was observed.