some of the formatting was lost. some of the text (usually right before the "UPDATE:") was crossed out and there were a fair few links to sites with real information.
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Aside from the mass devastation yesterday's record 8.9 quake in Japan had yesterday, one of the major consequences has been in some damage to one of Japan's nuclear facilities (Fukushima Daiichi Unit 1). As is usual, whenever anything involving a large-scale disaster is mixed with the term "nuclear" and reported by those with limited technical backgrounds, much tends to be lost.
First, the most authoritative place for news would be the Tokyo Electric Power Company, which is releasing regular press releases on the situation. Likewise, ANS nuclear cafe is featuring constant media updates on the situation. Rod Adams at Atomic Insights also has a good summary of the situation.
However, for the benefit of folks here, I'm going to attempt to try and boil some of this down for a lay audience. If any of my NE colleagues want to jump in and offer insights or corrections, by all means.
What happened to the reactors during the earthquake?
When the earthquake struck, Japan's reactors were immediately shut down by a quick insertion of control rods, stopping the chain reaction responsible for producing fission (known as a "scram.") This was successful in all of the reactors. However, the reactor still remains "hot" for a short time after shutdown, because of very short-lived radioactive fission products which are in the fuel. As these fission products decay, they produce heat - this heat still must be removed from the reactor in order to keep the fuel cool. (Update: As Cyrus points out, this can be 6-7% of full power at shutdown - in a larger reactor like Fukishima, this can be 60-70 MW, and around 10 MW right now.)
Normally, when the power is cut off in an emergency such as this, a diesel generator serves as a backup system, which powers pumps in order to circulate coolant (much like in your car's radiator). However, it appears that these diesel systems were damaged during the earthquake - thus, the pumps had to operate on limited battery power until this was exhausted. Contrary to early reports, the USAF wasn't "flying in coolant" (as this just consists of ordinary water); however, a backup diesel generator was flown in and installed to get the pumps working again. (Update: Cyrus points out that the diesel generator was working for about an hour until the subsequent tsunami struck, which is what disabled the diesel backup system.)
What is the big concern?
The biggest concern in this case is keeping the fuel cool - even though the reactor is "off" (e.g., not producing any more fissions), heat is still being produced which needs to be "wicked" away. Without the water circulating, what will happen is similar to in your car's radiator if the car's water pump fails - i.e., the coolant will continue to get hotter and hotter until it boils. This increases pressure inside containment (or your radiator). In each case, the pressure build-up can eventually cause a blow-out, where the containment (or your radiator) is breached. This is obviously undesirable.
To prevent this, some of the steam is being vented, to "bleed off" the pressure. The downside of this of course is that letting out steam means there's less water available for cooling now. Likewise, a very small amount of radiation may be released in the process (carried with the steam). However, the amounts are generally incredibly small - the largest dose indicated has been in the control room reactor building, where one worker received 1000 microsieverts - which sounds like a lot, but is in fact only 1 mrem. (For reference, a typical chest x-ray is about 10 times the dose.) (Update: The worker, who was in the reactor building received 106.3 mSv - elevated beyond regulatory limits, but far from fatal. Current estimates of the control room put the dose around 70 microsieverts/h, or about 7 mrem/h - elevated, but quite small. One would have to be exposed continuously for nearly an entire work year for it to begin to hit regulatory limits, which are themselves conservative.)
The big concern about keeping the fuel cool is to keep the fuel intact. When fissions occur, almost all of the radioactive isotopes created are trapped in the ceramic fuel itself - this is a safety feature. Thus, the main concern about keeping the fuel cool isn't a "China Syndrome" type of situation (which itself is physically impossible), but rather a matter of keeping the radioactivity safely confined.
As water is boiled away from the reactor, there is a chance that the fuel can be "uncovered," which is where the risk of partial melting of the fuel exists. (i.e., nothing is left to wick away heat from the element itself). However, the fuel itself is only the first radiation containment barrier - the containment building itself is also designed to prevent the release of radiation to the environment, specifically under these types of circumstances.
Has there been a meltdown? (Is this like Three-Mile Island or Chernobyl?)
Basically, no. First, it's helpful to define the term "meltdown." Were the reactor completely devoid of coolant, eventually the entire core assembly would heat and melt - producing a large, very hot radioactive pool of metal on the floor of the containment building. (Rod Adams helpfully points out that it's unlikely it would even get this far - in Three Mile Island's case, a substantial portion of the core melted, however it cooled into a lump of metal - "corium" - at the bottom of the pressure vessel.) This is not what is happening, nor is it the danger. The risk is in "uncovering" fuel from coolant, where the top portion of the fuel may melt and release radioactive fission products.
What has happened is that the fuel in Unit 1 may have been exposed for some time due to loss of coolant, which may have resulted in some loss of radioactivity.
Three-Mile Island was a partial fuel melt due in part to operator errors - operators incorrectly believed the reactor was being flooded with coolant (when in fact a pump was stuck closed), turning off coolant to the reactor. While the core itself was rendered unusable and the unit shut down, the actual dose received by the public was extremely minimal.
Chernobyl was a reactor different than the kind operated in Japan or the United States (and in fact would be illegal to build in the U.S. for technical reasons). Chernobyl operators were conducting tests with poor communication and had bypassed several safety devices. This was a full "meltdown" in the true sense, resulting in an explosion in the containment building and a release of radioactivity. However, it should be noted that the death toll was relatively small, and most of the dose received (and subsequent casualties) were in the first responders to the accident.
In the case of Japan, the operators have been doing things correctly - the fuel has been kept as cool as possible to prevent any possible overheating of the fuel. Everything they've done so far has been to minimize the risk of damage to the core or accidental release of radiation to the public.
Wasn't there a radiation leak at one of the reactors?
Fukushima Daiichi Unit 1 appears to have released a small amount of radioactivity when the containment was vented in order to relieve pressue (due to the buildup of boiling water). However, this release appears to have been very small and of no real danger to the public. Update: The measured radiological levels near the plant have been reported to have been elevated from 0.007 rem/hr to 0.67 rem/hr. While this is elevated beyond normal acceptable limits, this is far below the levels of Three Mile Island (itself quite small) or Chernobyl (much larger). The IAEA has given the incident a 4 on its International Nuclear and Radiological Event Scale (on a scale of 1-7; TMI was a "5", and Chernobyl was a "7").
Why are they evacuating the area?
This is done as a preventative precaution to protect the public. While so far there has been no known escape of radioactivity (save for what may have been released when the containment was vented), the evacuation is to ensure that this can be verified without putting anyone at risk.
What caused the explosion? (Is this a meltown?)
There was an explosion in the reactor building (not the reactor or the containment building). Official sources speculate that this was due to an ignition of hydrogen gas. Hydrogen gas can build up due to the extreme heating of the water and dissolution into hydrogen and oxygen. As the pressure built up, the hydrogen may have ignited as containment was vented, causing an explosion. However, this is not a meltdown - so far, there has been no good indication that any kind of catastrophic fuel failure (melt) has occurred.
Why are they flooding the containment building with seawater?
Basically, they need to keep the temperature down in the reactor. Because they've been losing water to boiling, they need to quickly cool the reactor. In order to do this, the operators have made the decision to flood the containment building with seawater containing boron (boron is used to "quench" nuclear reactions by absorbing neutrons - the point here is as an added safety precaution). What this ultimately means is that Unit 1 is likely a total loss - i.e., it will never operate again. However, this appear to be the only reactor which was so significantly impacted. Again, the three other reactors on the same site (along with four other reactors in the general area) have shut down and been cooled normally.
Doesn't this prove nuclear power is fundamentally unsafe?
No no no. A thousand times no. First, bear in mind that earthquakes are part of the design basis for every single reactor built today. Second, after an magnitude 8.9 earthquake - the largest in Japan's recorded history, and the fifth largest earthquake in human history - and a subsequent tsunami, the integrity of the reactor (and all of the other 3 units at this same site) are intact. While breaking reports indicate that this reactor may have been "ruined" by the catastrophe, the danger to the public has been extremely minimal - namely, because engineered safety systems worked as planned.
Let me re-emphasize: nuclear reactors are often over-designed for the point of safety. The very first safety system to kick in was to turn off the reactor. This worked for every single unit. The second safety system, a diesel generator, worked in most cases - it would appear that the severity of the quake / tsnunami damaged the diesel backup in the case of Unit 1. However, battery backup systems gave operators time to provide a contingency. Second, the physical containment itself has operated as designed in providing a means of containing potential releases. (Update: As Cyrus points out, nuclear systems are designed with a "defense in depth" - with the fuel, clad, pressure vessel, and containment building providing multiple layers against a radioactive release. At the moment, the main concern is at the level of the fuel / clad - not beyond this.)
What this proves is that in the very worst scenario - a once-in-a-lifetime earthquake beyond the design basis - that the systems can safely contain the integrity of the reactor, particularly with well-trained personnel.
To put a further point to it, this is what is going on right now at a liquified natural gas facility in the same area. (More images of the devastation here.) Basically, no system out there is going to stand up favorably to a disaster like this, but nuclear systems are specifically engineered against situations like this - again, unlike natural gas.
