Analysis of natural circulation in VVER-440 geometry with CATHARE2 V1.3U

The aim of the work is to study the phenomena taking place in the primary circuit during the different phases of the natural circulation in the Loviisa VVER-440 geometry. The analysis in carried out using the CATHARE thermal-hydraulic system analysis code. The phenomena which are analyzed in more detail are; the primary inventories during the changes of circulation mode, the intermittent behaviour of the hot leg loop seals, interaction between the loops, primary coolant distribution during the different phases of natural circulation and the effect of the low secondary water inventory. The effect of the surge lines is also analyzed, since they may act as loop seal connecting lines. The different phases of the natural circulation is studied by decreasing the primary side water inventory in steps. This procedure has been chosen in order to simplify the situation and to minimize the disturbances to the natural circulation caused by the break. The calculated results are compared to the PACTEL experimental results to find similarities or differences, although the experiments are not analyzed. In addition to an overall quantification of the phenomenology, the following observations were made: intermittent two-phase flow goes predominantly through the loop(s) that have no loop seal connections, and only one- and two-phase natural circulation stages are affected by the secondary water level. (9 refs., 71 figs., 7 tabs.)