Abstract: The transport and retention of aerosols in the containment gap are closely related to the gas flow in the cracks. In order to evaluate the aerosol retention efficiency in the cracks after an accident by the existing aerosol penetration coefficient relationships, taking the capillary as the representative crack, a methodology for pressure difference driven gas flow calculation was proposed based on the one-dimensional compressible adiabatic flow equation, and the calculation program was compiled based on FORTRAN language. The input parameters of the program were capillary upstream pressure and temperature, downstream pressure, capillary length, inner diameter and roughness. When calculating the resistance, the classical laminar resistance relationship and turbulent Colebrook relationship were adopted, and the form resistance coefficients at the inlet and outlet of the capillary were 0.5 and 1.0 respectively. The program can calculate the subsonic speed, critical speed and choking state of air flow. The one-dimensional calculation method was validated by the capillary flow characteristic experiment carried out by Kagoshima University in Japan. The capillary length is 12 cm, the inner diameter is 397 μm and the roughness was 0.02%. The downstream pressure of the test was 100 kPa and the upstream pressure was 150-700 kPa. In addition, in order to obtain the detailed flow information in the capillary, the computational fluid dynamics (CFD) software ANSYS Fluent 19.2 was also used to model and calculate the experimental device. The structured grid scheme was adopted in the CFD modeling, and the number of grids was about 1.7 million. The numerical results show that the flow change trend is consistent, and most of the deviation between the gas flow and the experimental value is less than 10%, which proves the accuracy and rationality of the one-dimensional calculation method, and the efficiency of one-dimensional calculation method is better than that of CFD method. On this basis, the influences of pressure difference and inner diameter on gas flow were analyzed by one-dimensional calculation method. The results show that the leakage flow of small inner diameter capillary increases quadratic with the increase of pressure difference, and when the inner diameter is greater than 100 μm, the growth trend of mass flow transits to linearization if the pressure difference exceeds a certain value. When the pressure difference makes the flow in the transition zone between laminar flow and turbulence, the leakage flow through the capillary tube will appear a platform, affected by the change trend of resistance coefficient. The larger the inner diameter, the earlier the platform appears. The results of this paper can provide a technical basis for the retention evaluation of aerosols in the cracks. In the follow-up, the numerical methodology will be extended to the analysis of pressure-driven gas flow with multi-components through capillary tubes.