Abstract: To solve the offline leak detection problem for the irradiated low burnup fuel assembly and micro-rupture which can not bedewatered, the research on technology of fuel assembly leak detection, pumping gas technology research and process checking research are carried out in this paper. Low temperature adsorption method and gas pumping and adsorption method are applied to solve the problem of radioactive gas detection in water. Based on the results of above research, we have developed leak detection device which has been validated by experiment. The detection limit of the leakage detecting device for ¹³⁷ Cs and ⁸⁵ Kr are 0.5Bq/L and 19Bq/L.