[en] The experimental results have affirmed that in the primary circuit of a VVER-440 reactor a natural circulation can be established both in the one-phase and in the two-phase flow mode. A temporary stagnation of the circulation before the clearing of the hot leg loop seal does not lead to fuel rod overheating. Moreover, in a certain range of the coolant inventory the system tends to flow instabilities. The calculations have proved the ATHLET code to be suited for the calculation of natural circulation phenomena in VVER reactors. The agreement between experiment and calculation is comparable to the results of other thermohydraulic codes like RELAP or CATHARE. The detailed analysis of the flow rate oscillations is a special result of the Rossendorf contribution to the standard problem. The fact that ATHLET predicted these oscillations rather good, demonstrates the quality of the code. The water accumulation in the steam generator tubes was neither considered in the calculations with ATHLET nor with the other codes. This hints to a need of further development of the steam generator model. (orig.)