coolant. Such explosions are seen to be fuel–coolant interactions (FCI). The severity of a steam explosion based on fuel-coolant interaction (FCI) depends strongly on the so-called premixing process, which describes the mixing of the melt with the surrounding water-steam mixture. In general, water-rich premixtures are considered more favorable than steam-rich environments in terms of steam explosion initiation and strength. The theoretical maximum for the strength of a steam explosion from a given mass of molten corium, which can never be achieved in practice, is due to its optimal distribution in the form of molten corium droplets of a certain size. These droplets are surrounded by a suitable volume of water, which in principle results from the max. possible mass of vaporized water at instantaneous heat exchange between the molten droplet fragmenting in a shock wave and the surrounding water. On the basis of this very conservative assumption, calculations for alpha containment failure were carried out by Theofanous. However, these optimal conditions used for conservative estimates do not occur in the real world. For one thing, the entire molten reactor core will never be in premixture, but only in the form of a part of it, e.g., as a jet of molten corium impinging a water pool in the lower plenum of the reactor, fragmenting there by ablation and allowing by this the formation of a premixture in the vicinity of the melt jet falling through the water pool. Alternatively, the melt may arrive as a thick jet at the bottom of the lower plenum, where it forms a pool of melt overlaid by a pool of water. In this case, a premixing zone can form at the interface between the melt pool and the water pool. In both cases, it is clear that by far not the entire molten reactor inventory is involved in premixing, but rather only a small percentage. Further limitations arise from the saturated nature of the water in the reactor, i.e., water with appreciable supercooling is not present there. In the case of penetration of a fragmenting melt jet there, this leads to increasing evaporation and an increasing steam content in the premixture, which, from a content > 70% in the water/steam mixture, prevents the explosion altogether or at least limits its strength. Another counter-effect is the solidification of the molten particles, which depends, among other things, on the diameter of the molten particles. That is,