During hypothetical severe accidents in nuclear power plants, a large amount of hydrogen is generated rapidly as a result of zirconium-water reaction and released into local compartments. Deflagration or detonation risk is probably caused by the uneven hydrogen distribution. Generally, buoyancy drives light combustible gas up to establish gas stratification. On the other hand, stratification may be destroyed by convection flow from steam injection due to coolant's discharge from the primary loop. The mechanism of erosion of stratification can be divided into buoyancy dominated and momentum dominated regimes, the priority of which depends on a nondimensional parameter, named of interaction Froude number. However, jet velocity and the diameter on the interface between steam and stratification are difficult to measure in experiments. Therefore, CFD method is adopted to capture a detailed velocity profile to solve this problem. In this article, a CFD model of single local compartment facility is built and helium distribution behavior within the local compartment is fully investigated, as a substitute of hydrogen, which is usually used in experiments. Prior to determining the interaction Froude number, four flow models, including laminar flow model, algebraic model, k-epsilon turbulence model and shear stress transport (SST) k-omega model are evaluated under three typical experimental stages, including light gas injection, the formation of stratification, and the erosion of stratification. Results show that SST k-omega model can predict much better under these specific conditions in the small-scaled configuration, compared to the experimental results. Next, with the setup of a helium stratification structure with homogeneous layer and gradient layer, the interaction between steam and gradient layer is analyzed in detail. The mechanisms of erosion of stratification can be divided into complete penetration, destruction from the bottom level to the top and soft dissolution without penetration, corresponding to the interaction Froude numbers of being greater than 1, close to 1, and far less than 1. The simulation results are consistent with relevant literature findings. The present work can help studying the complex hydrogen distribution process in local compartment and provide data for evaluation of postinerting strategy in severe accident management.