The challenge of modeling fuel–coolant interaction: Part II – Steam explosion

• We present the status modeling of steam explosion in the computer code MC3D (a first paper is devoted to premixing stage of FCI).
• We also propose a general state of the art, highlighting recent improvements in understanding and modeling, remaining difficulties, controversies and needs.
• We highlight the need for improving the understanding of the melt fragmentation and oxidation.
• The verification basis is presented.

In the course of a severe accident in a nuclear power plant cooled or moderated by water, the core might melt and flow down into the water. Under certain circumstances, a steam explosion might develop during the mixing of the melt and the water. Such an explosion, if occurring in the reactor pit of a PWR or BWR, might challenge the containment integrity and is thus an important issue for nuclear safety. This paper aims at presenting both a status of research and understanding of the phenomenon and the main characteristics of the models developed in the 3-dimensional computer code MC3D. We make a particular emphasis on the underlying difficulties, uncertainties and needs for further improvements. We discuss more particularly the two major phenomena that are the fine fragmentation and the pressurization process. We also give insights on the impact of melt solidification on the fragmentation and on the issue of oxidation. The verification basis of the models is discussed and finally, an example of 3D calculation is presented to highlight the current code capabilities.