Thermal stratification phenomena usually occur in the upper plenum after a scram of sodium-cooled fast reactors, which should be closed concerned in the fields of structural integrity assessment and residual heat removal capacity. A 2-dimensional analysis program under cylindrical coordinate was developed for predicting the in-vessel thermal hydraulics. Non-orthogonal block-structured grids were generated to resolve the problems with complex structures. A second-order discrete scheme based on midpoint rule was applied to the spatial discretization of convection and diffusion terms. Two sets of experiments characterized by distinct shapes of their apparatuses were used for the validation, mainly from the viewpoints of vertical temperature distribution and rising behaviors of the stratification interface. Results show that RANS-type turbulent models make the significant impacts on different flow regimes. In the calculation of a scaled model with plenty of stagnant sodium in the upper region, the realizable model (RKE) considering the mean deformation rate gives better outcomes than the standard model (SKE). For the plant-type upper plenum with considerable flow rate along the entire height, buoyancy modeling is the crucial issue to follow the upward movement of the interface and the relaxation process of the temperature gradient. In this case, employment of the turbulent Prandtl number reflecting the damping effect by incorporating the local Richardson number well reproduced the experimental results.