Abstract: Based on the third generation nuclear plant, the radioactive noble gas from core fission and the model of the gas movement in the waste gas system are analyzed, focusing on the delay state analysis of the isotopes of Kr and Xe creeping through the activated carbon. This paper gives a calculation method to determine the carbon mass and the system holdup time by annual radioactive release. The dimensions are determined based on the flow velocity recommended in ANSI55.4. By comparing the delay bed dimensions parameters of a certain nuclear corporation, the results by this method are consistent with them, which proved that the calculation method and the assumption in this paper are reasonable.