A nuclear power reactor is an extraordinary machine. All other machines generate heat while they are running, but stop generating heat when they are turned off. A reactor continues to generate heat, even when it is turned off. That is the root of the problem in Fukushima, Japan.
It is not just that it takes a while to cool down, it continues to actively "burn" in the sense that nuclear radiation continues. Once the fuel is loaded, one does not add anything to start or sustain the nuclear reaction. Actually, one removes some of the "moderator" material to increase the rate of reaction.
Recalling high school physics, fission occurs when a neutron hits the nucleus of an element such as uranium (U235, in particular). The nucleus breaks, or "fissions" into two smaller nuclei and two more neutrons. The neutrons can hit other U235 nuclei, causing them to fission in turn, in a "chain reaction." If, on average, each fissioning nucleus triggers more than one other nucleus to fission, the chain reaction has "gone critical": this is how an atomic bomb works.
Nuclear power reactors are designed so that there is not enough fissionable material close enough together to permit the chain reaction from going critical. They simply cannot explode in the sense of becoming atomic bombs.
Let's get back to the two nuclei that resulted from the uranium nucleus splitting. Uranium-235 has 92 protons and 143 neutrons. In general, the heavier the nucleus, the more "excess" neutrons (that is, more than the number of protons) it can have and be stable, or at least only slightly radioactive. (Uranium-238, the most common isotope, takes 4.47 billion years for half of the nuclei to decay. That's one way we know the age of the Earth.)
But the lighter nuclei that result from the uranium nucleus splitting have way too many neutrons. They are very unstable, and hence very prone to decay, and hence very radioactive. The radiation from these fission products is absorbed by the material in the reactor and turned into heat.
When a reactor is running, its purpose is to generate heat. The heat boils water. The steam drives turbines, which in turn drive electric generators, which produce the electric power that people use. However, only a fraction of the reactor's heat becomes electricity: the rest is wasted and must be put into the environment.
When a reactor is idle, the fission products continue to decay. To shut down a reactor, operators insert "control rods" of material that absorbs neutrons. With fewer neutrons reaching uranium nuclei, fewer fission reactions occur. But even an idle reactor is still doing some fission.
The control rods do not, however, stop the fission products from decaying and radiating. Some fission products break down in seconds, but others continue to radiate for days, months, years, or centuries.
So in a sense, an idle nuclear reactor is just as dangerous as an operating reactor. Perhaps more so. Why? An ordinary machine stops generating heat when it is shut off. Leave it alone, and it will cool down. A reactor, on the other hand, keeps making heat. Lots of heat. Enough heat to melt itself if it is not continuously cooled.
Whether operating or idle, a nuclear reactor must be cooled constantly. No material known to man holds more heat in less volume or less mass than water, so water is the preferred way to cool reactors. It takes so much water to cool a reactor that reactors need to be located near large bodies of water: rivers, lakes, and oceans.
Unfortunately, people like to live near large bodies of water. Agriculture and transportation depended on water long before people discovered nuclear fission. Most of earth's people live close to water, which is why floods, tsunamis and nuclear power plant accidents are so dangerous.
If the cooling system of your car fails, the engine will overheat. The metal parts will warp, not melt. But they will warp enough that the engine can't run: the parts won't move past one another. Once the engine "seizes", it stops burning fuel, so it stops making heat.
But the problem with a nuclear reactor is that since it keeps making more heat, the damage does not stop at warped parts. In the worst case, enough heat can crack the containment vessel that surrounds the reactor, and let some of the highly radioactive fission products into the environment. To keep from having the worst case, the operators vent some of the radioactive steam from the containment area to reduce the pressure and hopefully keep it from cracking.
Then the problem becomes how much stuff gets out, and how far it travels. Chernobyl was a particularly bad event because the moderator material, graphite (a form of carbon), caught fire. The fire pumped large amounts of radioactive material high into the atmosphere, where it disbursed over a large area.
Even if the Fukushima reactors have full melt downs, they are not likely to put as much material into the air, nor put it as high in the air. That's the good news.
The bad news is that the Fukushima plant operators are running out of ways to cool their reactors. Normally, they would use power from the reactor itself to run the water pumps. Or power from another reactor on site. Or power from the electric grid. Or power from back-up diesel generators on site. Unfortunately, the earthquake and tsunami knocked out all of these means for powering the pumps. The plant operators fell back to battery power, which is limited, and to pumps in firefighting equipment.
But all this means people have to be there, literally to man the pumps. But as more radioactive material has been released, it is more an more dangerous for the plant personnel to stay. So the plant has been reduced to a skeleton crew: bad pun intended because the 50 or so men staying do so at great personal risk. But with fewer men to deal with more crises, it seems that things are likely to get even worse.
A bitter outcome for a place whose name means "fortunate island."