The term is associated in the public mind with the two most notorious accidents in recent memory – Three Mile Island, in the US, in 1979, and Chernobyl, in Ukraine, seven years later.
You can think of the core of a Boiling Water Reactor (BWR), such as the ones at Fukushima Daiichi, as a massive version of the electrical element you may have in your kettle.
It sits there, immersed in water, getting very hot.
The water cools it, and also carries the heat away – usually as steam – so it can be used to turn turbines and generate electricity.
If the water stops flowing, there is a problem. The core overheats and more of the water turns to steam.
The steam generates huge pressures inside the reactor vessel – a big, sealed container – and if the largely metal core gets too hot, it will just melt, with some components perhaps catching fire.
In the worst-case scenario, the core melts through the bottom of the reactor vessel and falls onto the floor of the containment vessel – an outer sealed unit.
This is designed to prevent the molten reactor from penetrating any further. Local damage in this case will be serious, but in principle there should be no leakage of radioactive material into the outside world.
But the term “in principle” is the difficult one.
“The job of keeping dangerous materials sealed in falls to the containment vessel inside.”
Reactors are designed to have “multiply redundant” safety features: if one fails, another should contain the problem.
However, the fact that this does not always work is shown at Fukushima Daiichi.
The earthquake meant the three functioning reactors shut down. But it also removed the power that kept the vital water pumps running, sending cooling water around the hot core.
Diesel generators were installed to provide power in such a situation. They did cut in – but then they cut out again an hour later, for reasons that have not yet been revealed.
In this case, redundancy did not work. And the big fear within the anti-nuclear movement, as used in the film The China Syndrome, is that the multiple containment of a molten core might not work either, allowing highly radioactive and toxic metals to burrow into the ground, with serious and long-lasting environmental impacts – total meltdown.
However, the counter-argument from nuclear proponents is that the partial meltdown at Three Mile Island did not cause any serious effects.
Yes, the core melted, but the containment systems held.
And at Chernobyl – a reactor design regarded in the West as inherently unsafe, and which would not have been sanctioned in any non-Soviet bloc nation – the environmental impacts occurred through explosive release of material into the air, not from a melting reactor core.
To keep things in perspective, no nuclear accident has caused anything approaching the 1,000 fatalities stemming from Friday’s earthquake and tsunami.