Editing Steam explosion (section)

===Nuclear reactor meltdown===
 {{overly detailed|section|date=June 2024}}
Events of this general type are also possible if the fuel and fuel elements of a water-cooled nuclear reactor gradually melt. The mixture of molten core structures and fuel is often referred to as "Corium". If such corium comes into contact with water, vapour explosions may occur from the violent interaction between molten fuel (corium) and water as coolant. Such explosions are seen to be '''fuel–coolant interactions''' (FCI).{{Citation needed|reason=Water is used to cool any water-moderated nuclear reactor (PWR, BWR, etc.), and thus fuel-coolant interactions are required.|date=August 2010}}
<ref>{{cite journal |last1=Theofanous |first1=T.G. |last2=Najafi |first2=B. |last3=Rumble |first3=E. |title=An Assessment of Steam-Explosion-Induced Containment Failure. Part I: Probabilistic Aspects |journal=Nuclear Science and Engineering |date=1987 |volume=97 |issue=4 |pages=259–281 |doi=10.13182/NSE87-A23512|bibcode=1987NSE....97..259T }}</ref>
<ref>{{cite journal |last1=Magallon |first1=D. |title=Status and Prospects of Resolution of the Vapour Explosion Issue in Light Water Reactors |journal=Nuclear Engineering and Technology |date=2009 |volume=41 |issue=5 |pages=603–616|doi=10.5516/NET.2009.41.5.603 |doi-access=free }}</ref>
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.<ref>{{cite journal |last1=Theofanous |first1=T.G. |last2=Yuen |first2=W.W. |title=The probability of alpha-mode containment failure |journal=Nuclear Engineering and Design |date=2 April 1995 |volume=155 |issue=1–2 |pages=459–473 |doi=10.1016/0029-5493(94)00889-7|bibcode=1995NuEnD.155..459T }}</ref>
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, small particles solidify faster than larger ones. Furthermore, the models for instability growth at interfaces between flowing media (e.g. Kelvin-Helmholtz, Rayleigh-Taylor, Conte-Miles, ...) show a correlation between particle size after fragmentation and the ratio of the density of the fragmenting medium (water-vapor mixture) to the density of the fragmented medium, which can also be demonstrated experimentally. In the case of corium (density of ~ 8000 kg/m³), much smaller droplets (~ 3 - 4 mm) result than when alumina (Al2O3) is used as a corium simulant with a density of just under half that of corium with droplet sizes in the range of 1 - 2 cm. Jet fragmentation experiments conducted at JRC ISPRA under typical reactor conditions with masses of molten corium up to 200 kg and melt jet diameters of 5 - 10 cm in diameter in pools of saturated water up to 2 m deep resulted in success with respect to steam explosions only when Al2O3 was used as the corium simulant. Despite various efforts on the part of the experimenters, it was never possible to trigger a steam explosion in the corium experiments in FARO.(Will be continued ...)

If a steam explosion occurs in a confined tank of water due to rapid heating of the water, the pressure wave and rapidly expanding steam can cause severe [[water hammer]]. This was the mechanism that, in Idaho, USA, in 1961, caused the [[SL-1]] nuclear reactor vessel to jump over {{convert|9|ft}} in the air when it was destroyed by a [[criticality accident]]. In the case of SL-1, the fuel and fuel elements vaporized from instantaneous overheating.

In January 1961, operator error caused the [[SL-1]] reactor to instantly destroy itself in a steam explosion. The 1986 [[Chernobyl nuclear disaster]] in the Soviet Union was feared to cause major steam explosion (and resulting [[Europe]]-wide [[nuclear fallout]]) upon melting the [[lava]]-like [[nuclear fuel]] through the [[nuclear reactor|reactor]]'s basement towards contact with residue fire-fighting water and [[groundwater]]. The threat was averted by frantic [[tunnel]]ing underneath the reactor in order to pump out water and reinforce underlying soil with [[concrete]].

In a [[nuclear meltdown]], the most severe outcome of a steam explosion is early [[containment building]] failure. Two possibilities are the ejection at high pressure of molten fuel into the containment, causing rapid heating; or an in-vessel steam explosion causing ejection of a missile (such as the [[upper head]]) into, and through, the containment. Less dramatic but still significant is that the molten mass of fuel and reactor core melts through the floor of the reactor building and reaches [[ground water]]; a steam explosion might occur, but the debris would probably be contained, and would in fact, being dispersed, probably be more easily cooled. See [[WASH-1400]] for details.