Hopefully we do not start hearing about corium in the news coming out of Japan. Here is a little primer on the stuff in case mention of it does somehow start to creep into newscasts.
Corium is the lava-like material formed from the melted core of a nuclear reactor. What happens to this material determines the extent of environmental contamination a meltdown causes. Corium is not much of an environmental problem if it stays in the reactor containment vessel (assuming there is a containment vessel; there was not one in the Chernobyl plant). Corium can cause big problems if it escapes from the reactor vessel, and it is very difficult to contain molten corium.
Corium, since it is formed from a critical mass of nuclear fuel, can form a critical mass itself. It is possible in a pressurized water reactor (or boiling water reactor) for a “prompt critical” mass of fuel material to collect at the bottom of a reactor vessel. This would be a worst-case scenario, and highly unlikely.
A much more likely scenario is one where the nuclear fuel material mixes with all sorts of other material as the molten material drips out of the core. It is possible that, since molten corium should be rather viscous, there are unmixed zones in the corium blob that are still locally critical.
The reason criticality is a very important concern is that a critical corium blob continues to generate heat until it is dispersed. Because there is no mixing of the corium blob unless it moves the heat would likely build up until some critical failure moved the blob. In the case of a “prompt critical” blob the failure might even be vaporization of portions of the blob and containment structures. The resultant plume would cause a catastrophic environmental disaster. For the locally critical blob the movement would likely come in the form of a containment vessel breach; the blob would be dispersed as it splashed onto the containment building’s floor.
It does not require a locally critical mass to breach a containment vessel and create a corium splash. However, with a large enough locally critical nuclear fuel zone in the corium blob the containment vessel breach is almost inevitable. A blob without a locally critical zone is the most likely scenario.
In the most likely scenario, and the only scenario observed in any nuclear disaster to date, the blob cools as it dissipates the heat generated while it was critical. The number of neutrons the mass generates decreases, and the sub-critical mass begins to act like a lump of hot metal. The famous corium flow at Chernobyl called “the elephant’s foot” is now only slightly warmer than ambient temperatures, and it has only been 25 years since that was formed. It is important, however, to remember that the Chernobyl corium flow was many metric tons in size (the elephant’s foot alone was two metric tons) and so the fuel was diluted far beyond the concentration where any sub-critical nuclear reactions would contribute significantly to its heat.
The hot metal is very hot even for hot metal. When it comes into contact with normally non-volatile material, like concrete, the outgassing can cause explosive dispersal. The amount of dispersal is dependent on the amount of heat in the corium, and the outgassing potential of the material it comes into contact with. The dispersal energy would determine the size and scope of the environmental disaster.
The other contributing factor would be the concentration of radioactive material in the ejecta. Most of the material in the corium blob would either be highly radioactive before entering into the blob, or become highly radioactive because it adsorbed neutrons from fuel material fission within the blob. Other material would become radioactive if it were dispersed with splatters from the blob.
If corium is explosively dispersed it could become a very widespread problem. Corium is so hot that many materials interacting with it are melted into a glasslike or ceramic state. Much of this material is naturally friable. In addition to natural friability the highly radioactive material spontaneously degenerates, causing small-particle generating fractures. Even large chunks of ejecta can form small respiratable particles which easily disperse over enormous areas, or re-aerosolize.
Corium is scary stuff, but please turn off your nightlight. Conserving energy is much safer than building any type of reactor. And who needs nightlights these days anyway?