LAUSR

Abstract The condensation on containment shell drives the nearby gas downwards creating a natural convection in containments which plays a key role for long-term passive containment cooling. In late phase of accident where hydrogen and steam accumulate in containment, recovery of containment cooling can cause the loss of steam-inert status for the containment atmosphere leading to hydrogen risk in containment. This process is analyzed in this paper focusing on the natural convection driven by wall condensation, and its influence on hydrogen distribution. Firstly, both convective heat transfer model and condensation model are validated with two separate effect experiments. The analogy argument among monument, heat and mass transfer is adopted in GASFLOW-MPI to analysis monument, mass and energy transfer between structure surface and fluid. The simulation result shows good agreement with experiment data. Then a simplified containment model including two steam generator compartments and pressurizer compartment is built and analyzed with GASFLOW-MPI with a postulated accident condition. To avoid long time calculation, the initial condition is calculated with a methodology that is designed to estimate containment status during severe accident provided by EPRI. Two cases are simulated, one without steam injection, where the natural convection drives only by condensation. Other one considers the decay heat that is postulated as a constant steam injection to simulate the natural circulation in containments. Result shows that, during the containment cooling, a transient stratification will occur, leading to high concentration of steam in the dome while low concentration at bottom. This is because the condensed gas is driven downwards near containment shell, pushing steam-rich gas at bottom upwards. The stratification of steam results in a reverse stratification of hydrogen, with high concentration at bottom while low concentration in the dome. Combustibility cloud shows that there is still a stratification of combustibility in containment, though the hydrogen distribution is quite uniform at the end of computation. Therefore, the hydrogen risk should be concerned when implementing containment cooling, especially the local hydrogen concentrate happening at the bottom of containment.