I'm not an engineer, but I thought the control rods to scram a nuclear reactor were held in place by a constant flow of electricity so if the power failed, the control rods would fall on their own.
The rods did fall or had the outer sleeves drop over them. But it is not like flipping a light switch. There is still some residual reaction and a crap-load of residual heat. Think about how long it takes your oven to cool down after to take out the previously frozen pizza. Yes, it is off with no power going to it. But there is still a lot of residual heat in there. Only with a nuclear reactor, you still have some heat still being produced.
Once the tsunami took out the back-up generators, there was no power at all. No cooling pumps, no control room, nothing. Yet the residual nuclear reaction (think 100% power down to maybe 6% when it scrams) and the residual heat is still there and still boiling water. No pumps to cool the steam in the condenser (after the steam turbines) so no cool water goes back into the reactor. What you have is a giant pot of boiling water.
Eventually, enough remaining water turns to steam and the tops of the fuel rods are exposed to steam. When that happens, the cooling of that section of the fuel rod gets massively worse (steam cools a lot less than water) so you have localized hot spots. This blisters the fuel cell and the outer shell.
Now several things happen. The steam starts reacting with the outer cladding of the fuel shell which has hydrogen as a by-product. The pressure in the reactor is also increasing because of the boiling so eventually some is released into the building to avoid a rupture. Along with the released steam is some of the hydrogen. It is this hydrogen that ignited and blew the roof off.
Now, with the fuel rod exposed and blistering, eventually it's integrity fails and fuel pellets can fall out. With the high heat, you can also get melting. The net effect is more heat released and a glob of nuclear goo on the bottom of the reactor vessel. More heat, more steam, less water produces more damage...
Hence the Japanese started to pump sea water directly into the reactor to try to cool it down and cover the fuel rods. The reactor uses dionized water normally to minimize corrosion, minimize damage to the turbines, and minimize the radioactive elements produced. But the addition of the sea water means this reactor will never operate again. So it shows they exhausted every possibility before committing the reactor to a permanent death. But it will take months to get the reactor cooled down and the residual reaction slowed down. It never stops, just is slowed.
Now those out there that have also had the training/education, please correct me if I have stated anything incorrect.
I really respect the guys that are staying and fighting this. It's hard, dangerous work. If they get a lethal dose of radiation, it is a very painful death. Not quick, but very painful.
