[en] In a fast reactor, the thermal stratification phenomena occur in the reactor vessel when a scram shutdown is imposed. The thermal stratification phenomena can induce thermal fatigue in structural components in the reactor vessel. It is necessary for designs of real reactors to quantify an occurrence condition, a position, a rising velocity and a temperature gradient of the stratification interface. To study the thermal stratification phenomena, sodium experiments were performed using PLANDTL-DHX facility. Also, two dimensional analyses were carried out using a multi-dimensional thermal-hydraulic code AQUA with an algebraic stress turbulence model (ASM) and compared to the sodium experiments. Influence of Richardson (Ri) and Reynolds (Re) numbers were examined by experiments and calculations. In the cases of high Ri, the calculated temperature distributions were in good agreements with the experimental results. In the cases of low Ri, on the other hand, the analyses had milder temperature gradients in the stratification interface, compared to the experiments. The analyses, however, could simulate the trend; the level of the interface rises with decreasing Ri. The analysis using k-ε two equation turbulence model was also carried out. The ASM could simulate the experimental temperature distributions and thus the level of the interface better than the k-ε two equation turbulence model. (author)