Steam explosions by the interaction of molten corium with water have been studied extensively because they may have the potential to impact the integrity of the containment. Since breakup and fragmentation processes during premixing are important mechanisms that influence steam explosion behavior, the particle size distribution characteristics on fuel-coolant interaction (FCI) have been investigated in the TROI (Test for Real cOrium Interaction with water) test facility.

The data characteristics indicate that FCI characteristics depend upon the composition of the prototypic corium material, and the particle size of the debris is related to the intensity of the dynamic pressure produced by an explosion. The mass mean diameters of the debris produced by explosive compositions were less than that of the nonexplosive compositions. A mass mean diameter of 2 mm was found to be a boundary size produced by a steam explosion of corium. The particle sizes of the molten corium involving a steam explosion were shown to be mainly 3 to 6 mm depending on the material and composition, but the size distribution shifted to smaller sizes if a steam explosion occurred. Small corium droplets of less than ∼3 mm did not seem to contribute to a steam explosion owing to solidification at an early stage before the explosion, but large droplets contributed due to their liquid state.

Zirconia, with the largest fusion heat, has almost always exploded, and the explosions have been energetic, while the eutectic composition (UO2/ZrO2 = 70/30 at weight percentage) frequently exploded. On the other hand, noneutectic compositions rarely exploded, even though the heat of the fusion was very similar to the eutectic composition that frequently exploded. The main reason why noneutectic corium compositions do not explode seemed to be that they undergo solidification by forming a “mushy zone” with a small freezing temperature range. To determine whether noneutectic corium melts cooled down through the mushy zone, particles of this composition were analyzed from the surface inward using a scanning electron microscope, an electron probe microanalyzer, and X-ray diffraction. However, all particles were found to have a homogeneous solid solution. The large particles showed the typical solidification shapes of a general molten material. The small particles generally had only a few small pores and small cracks. The morphologies of the large and small particles were found to be similar.