Counter current flow modeling and Hydrogen generation influence on a PWR severe accident

This paper describes the main effects of the presence of noncondensable gases in the primary loop and the interface drag modeling in a counter-current flow during a severe accident in PWR reactor. The activities carried out by ENEA-DISP and University of Rome, in the framework of TMI-2 International Exercise organized by OECD/CSNI, stressed the importance, on the transient evolution, of noncondensable gases (mainly Hydrogen), and of the interface drag modeling in Counter-Current Flow Limiting (CCFL) conditions. The effects of the Hydrogen coming drag in the surge line are the likely causes of the anticipated pressurizer drainage at PORV closure timing. The pressurizer draining into the hot leg may be prevented by two mechanisms: the first is the counter-current limitation in the surge line; the second mechanism is the primary pressure increase due to reduction of primary natural circulation flow and steam generator heat transfer, caused by the presence of noncondensable gases in the primary loop. Calculation results show a partial pressurizer drainage after primary pumps trip and a faster drainage, after PORV closure with consequent core quenching. Such results differ from available experimental data on primary pressure and on pressurizer level. An underprediction of Hydrogen inventory and of interface drag in surge line flow condintions are likely to cause this mismatch