Roger and Chris,

I am not working for any government agencies and can afford to speculate. However, as stated elsewhere, I am not an expert on this.

When highly radioactive materials melt together relatively slowly into a blob of molten metal etc. such as in a power plant core melt, the neutron radiation that keeps the nuclear reactions going is usually not strong enough in a given area to cause an uncontrolled nuclear chain reaction such as with a nuclear weapon.

Instead, the material just fizzles, melts through things, burns, generates large amounts of dangerous radiation, and causes major problems for everyone.

The radiation and contamination can easily claim lives, and ruin crops, land, and drinking water.

One of the most serious problems results when the extremely hot core material reacts with water producing hydrogen that can then explode. When that happens as it reportedly did in Japan the explosion can damage or destroy buildings and the power plant systems that are supposed to be keeping the reactor cool and under control.

The Chernobyl power plants from years ago had a different core design. Their cores contained large amounts of graphite, a form or carbon. And the graphite caught fire and caused massive amounts of radioactive materials to be released into the atmosphere.

I believe that it was reported that scores of people died from radiation exposure trying to deal with those graphite fires.

As far as I am aware, no present power plants are using graphite based nuclear reactors. However, graphite rods are I believe being widely used to control the number of high energy neutrons that are circulating within many present day reactors. Heavy water is also being used in some research reactors to keep the neutrons under control.