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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.

Pressure suppression containment has been adopted extensively in boiling water reactor design due to its remarkable pressure suppression capacity. To study the pressure response characteristics of the containment in the initial stage of the loss-of-coolant accident, it is necessary to build a scaling facility to simulate the prototype. Analysis of the steam mass and energy that are released from the break as well as of the steam condensation that occurs in the suppression pool is conducted, respectively, for the prototype pressurized water reactor. The suppression pipe system resistance is also scaled with a certain ratio. The length and diameter of the main pipe and the branch pipe as well as the orifice diameter are calculated. At last, the prototype and the experimental facility are modeled by MELCOR to analyze the steam mass and energy release. The results show that the containment pressure evolution trends of the prototype and the scaled experimental facility are consistent. The pressure difference exists at some times, with a maximum relative error of 4%. [ABSTRACT FROM AUTHOR]

Steam condensation in the presence of air is an important thermal-hydraulic phenomenon in containment under loss of coolant accidents (LOCAs) or main steam line break accidents (MSLBs). Previous studies have focused on the effects of thermal parameters such as air mass fraction, gas pressure and sub-cooling on heat transfer characteristics, but less on the influence and action principle of geometric parameters such as tube length, tube diameter and inclination angle. Fully understanding the influence of geometric parameters on condensation heat transfer of steam containing air is of great significance for efficient heat transfer, so as to improve the safety and economy of related applications. The effects of tube diameter (4-60 mm), tube length (0.1-7 m) and inclination angle (0°-90°) on condensation heat transfer characteristics of air containing steam were studied based on the diffusion boundary layer condensation mechanism model by using three-dimensional CFD numerical simulation method. The results show that the tube diameter, tube length and inclination angle have significant effects on condensation heat transfer characteristics of air containing steam. However, when tube diameter exceeds 30 mm, tube length exceeds 1 m and inclination angle exceeds 60°, the average condensation heat transfer coefficient is no longer sensitive to the change of geometric parameters. The average condensation heat transfer coefficient decreases with the increase of tube diameter, and the descent speed decreases rapidly. When the tube diameter is 4 mm, the average condensation heat transfer coefficient is twice that of 60 mm. It first decreases and then increases with the increase of tube length, and reaches the minimum value at about 3 m. And it increases with the increase of inclination angle. The high concentration air layer near the wall is the main thermal resistance in the condensation process of steam-air, and its thickness makes a good negative correlation with condensation heat transfer coefficient. The high concentration air layer will first accumulate and thicken along the tube wall, and then produce a certain negative pressure due to the high density and high flow rate of mixed gas near the wall, so as to form a transverse velocity, so that the air layer will be thinned slowly after 1 m from the top of the pipe. Thus, the local condensation heat transfer coefficient decreases rapidly at first and then increases slowly along the direction of tube length. In addition, the tube circumferential local condensation heat transfer coefficient is uneven obviously under the inclined arrangement, the heat transfer on the upper surface is significantly enhanced due to the thinning of air layer by natural convection scouring and falling off, and gradually weakened to both sides, while the lower surface forms a stagnation zone due to the obstruction of the tube wall, which causes the accumulation and thickening of the air layer, resulting in a certain heat transfer inhibition. Compared with the vertical tube, about 75% of the circumferential area has different degrees of heat transfer enhancement, and only the dorsal flow surface has a certain heat transfer inhibition.