In case of hypothetic severe accident in a Pressurized Water Reactor (PWR), interaction of the melted core with the cooling water can generate large amounts of hydrogen. Hydrogen can also be generated by oxidation of metals present in the corium recovery pool or in the basemat during the molten corium-concrete interaction phase. Then hydrogen is dispersed into the containment by convection loops arising essentially from condensation of steam released via the Reactor Cooling System (RCS) break or during corium-concrete interaction. Distribution of hydrogen can be more or less homogeneous, depending on mixing in the containment atmosphere. If considerable hydrogen stratification exists, then local concentration of hydrogen may become substantial, and may exceed the lower flammability limit. In case of ignition, the subsequent overpressure may adversely affect the containment building, internal walls and equipments integrities.
Evaluation of such scenarios needs CFD codes. However, a thorough validation process is necessary before using such codes with a high level of confidence. The original vertical facility ENACCEF, which represents a scale down of a PWR Steam Generator casemate, has been built to fulfil this objective.
The present work aims to validate the commercial code FLACS on ENACCEF flame acceleration tests characterized by hydrogen gradients. Positive and negative gradients are considered and experimental data are compared to 3D calculations.