Influence of the wet-well nodalization of a BWR3 Mark I on the containment thermal-hydraulic response during an SBO accident

Abstract

In the field of severe accidents simulation one of the most challenging issues is nodalization. This paper explores the effect of the wet well modeling on significant variables describing the sequence evolution. The code used for the study has been MELCOR 2.1 and the scenario chosen has been a prolonged SBO occurring in a BWR3 Mark I. The results indicate that some significant magnitudes show a moderate scatter depending on WW nodalization (i.e., core uncovery, RPV failure, hydrogen production), whereas the SP thermal state might display outstanding deviations, which sometimes affect significantly key variables like containment pressure. The difficulties and uncertainties around defining a suitable WW nodalization have been highlighted and the need to properly balance the entire plant meshing has been stressed.

Even though a number of noding schemes has been explored, the discussion of the results underlines the importance of having a deep understanding of the potential phenomena governing the scenario and of mastering the code facilities to better model it. Some insights into WW nodalization in MELCOR 2.1 have been gained for the specific scenario (i.e., a prolonged SBO in a BWR3 Mark I) explored: a single node assumption might underestimate PCV pressurization; a loose coupling of water and gas exchanges in the WW nodalization would be preferred if the drift flux model is chosen for momentum exchange in the flow pathways between WW nodes; the potential of some axial thermal stratification in the pool should be taken into account when noding the WW; sensitivity analyses on physically supported WW nodalization schemes should be conducted and focused on key magnitudes describing containment evolution (i.e., pressure). Finally, it is stated that some uncertainties in WW nodalization might be reduced and/or moved if a properly scaled-down experimental project was launched on potential stratification in BWR3 Mark I SP during SBO accidents.