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The following is supposedly from a commentor on a CNN article. I grabbed it from here Japan Quake: Fukushima Reactor Must Be Shut Down, Core Cooled but couldn't actually find it at the CNN article but there were hundreds:

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I’ve worked in the US nuclear industry for 30+ years and for 18 years I was a licensed senior reactor operator/control room supervisor on a boiling water reactor (BWR) similar to the Fukushima plant. I helped write the emergency procedure guidelines that are used at US BWRs. There is a great deal of information flying around that just does not make sense. There just seems to be no detailed technical information getting out to the public on this. At the risk of over simplifying the system, a BWR is like a giant pot of boiling water. Regular light water, not heavy water, goes through the reactor, is heated by the splitting of uranium atoms, turns to steam and spins a turbine-generator to make electricity. The steam is condensed back to water and pumped back into the reactor to continue the cycle.

There are 3 basic barriers to the release of radiation: the metal clad that encases the uranium fuel, the reactor pressure vessel, and the containment. If 2 of these 3 are compromised, and the third is in jeopardy, US plants will advise shelter or evacuation of nearby residents.

The reactor operates at a normal pressure of about 1000 psig. During an earthquake of this magnitude, the reactor would be expected to automatically shut down (called a reactor scram). Control rods are hydraulically driven into the core in less than 7 seconds. I do not know if this took place but if it did not, we’d probably hear about it because it would be such a big deal. Even with rods inserted, the reactor continues to produce heat equivalent to about 3% of its full power level. This is not the same as taking a pot off the stove and letting it cool. There are still some atoms splitting and fission products decaying that produce heat. This drops off slowly and is why there needs to be layers of redundant cooling with backup power. During such an earthquake, power from outside the plant would not be expected to be available.

The plants have several back up diesel generators (locomotive style engines) that supply power to motor driven cooling systems that will supply high flow of water up to about 300 psig.. There are also steam driven systems to supply cooling water up to 1100 psig. There are also pressure relief systems that active at about 1100 psig. If reactor pressure gets too high, relief valves open and discharge steam to a water filled pool inside the containment.

Here are some information being reported that does not make sense. Reports that the pressure is 1.5 times normal; incorrect. There are at least 10 relief valves and any one can handle the energy after a plant shut down. CNN reports the US military has flown coolant to the site, but the coolant they use is regular water; I can’t imagine why the US would need to fly in coolant.

Right now I’d want to know a few things.

Are all rods fully inserted? What is the water level in the reactor? It’s normally about 12 feet above the top of the fuel. What injection systems are available? What is the reactor pressure? What is the status of containment?

Based on limited information, this is what I think might happen:

Earthquake hits, high vibration on the main turbine automatically trips the turbine by rapidly closing stop valves. Reactor automatically shuts down (scrams) all rods go in. Earthquake disrupts off site power to the plant and back up diesel generators should have started, maybe they did not. Main sources of water to the reactor are not available. If there is no pipe break off of the reactor, the pressure will slowly increase. After about an hour, a relief valve(1 of about 10) will open at about 1100 psig and drop pressure to about 1080. The steam is sent to a pool of water called a suppression pool in the containment that condenses the steam. This valve will cycle open and close every 5-10 minutes. Operators would use a small steam driven turbine (RCIC) to supply water at high pressure to the reactor under these circumstances for several hours. You can sit like this a long time, hot and at 1000 psig it’s no big deal as long as water covers the fuel in the reactor pressure vessel. If that turbine is not available, there is a larger steam driven turbine (HPCI) that supplies more water meant to provide make up if there was a pipe break.

If neither of these systems is available, the relief valve will continue to cycle and reactor water level will slowly drop. At some point before the water level lowers to the point of uncovering the fuel, all the relief valves would be open (ADS) and pressure would be reduced to below 300 psig to allow the low pressure but high flow systems (CS & LPCI) to restore water level and cooling. These pumps however, need electricity, like from the diesel generators, to run. If things get this far but there is no injection, in US plants there are things like diesel fire pumps that can be tied in to provide alternate sources of water. I’m not sure if they are set up to do this in Japan. Without cooling, eventually the fuel temperature will exceed 2200 deg F and the clad will melt. Fission products that are highly radioactive will get dispersed into the reactor vessel. If there is a pipe break or relief valve open, those fission fragments get dispersed through containment.

That's a VERY good summary! Thank you for finding it, forwarding it here to Free Republic.

Can they dump a bunch of graphite control rods into the reactor core and shut it down that way? Or have they already tried that by now? Just wondering.
From what I understand, it shut down immediately. But will take a LONG time to cool the core to below meltdown temps.

No, no, no, and again - no. You're thinking of a different kind of reactor (the Russian Plutonium-producing fast reactors like Chernobyl used graohite as a moderator, not a control of the reaction levels. In any case, the solid graphite could be pumped in since they wouldn't help and couldn't physically fit in this kind of control system.

The reactors are shutdown - that's NOT the problem. The problem is that fission reactors continue to produce a small, but significant amount of extra heat after you shut them. If you turn your car engine off, you can’t put your hand on the engine block immediately -> it's still hot. But you don't need to keep the oil pump running to cool it off, do you?

Now, imagine that you shut your engine off, but still need to pump a little bit of oil for 144 hours through the engine to keep it from breaking the bearings.

If you don't keep pumping this little bit of oil through the engine, the bearings break and it leaks engine oil all over your garage floor (into the containment building) but it won't get in your house, unless the garage gets completely full of oil, right? But the oil won't get into the yard and your neighbor's house unless it leaks all the way up to the window level in your house first.

Well, the engine is not running, so you need a backup oil pump to keep the oil flowing. You could use the house power, but that's out from the earthquake. You could use the car battery, but that is going to run down. You could use your gasoline generator, but that's broken because the garage fell down it. You could borrow your neighbor's gasoline generator, but his fuel tank got contaminated with salt water from the tsunami. You could use a manual pump, but that's too small.

So the Japanese engineers are trying to get that backup oil pump going before their battery runs out so they can prevent the oil from leaking out of the engine bearings. In the meantime, their garage floor is getting messy - and it will be an expensive cleanup!! - but the neighborhood is only threatened if the oil cannot be stopped AND a lot of other things go wrong.