Background information from Wikipedia (already updated with the events of yesterday and today) :
The Fukushima I Nuclear Power Plant,, often referred to as Fukushima Dai-ichi, is a nuclear power plant located in the town of Okuma in the Futaba District of Fukushima Prefecture, Japan. The plant consists of six boiling water reactors. These light water reactors have a combined power of 4.7 GW, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).
In March 2011, in the immediate wake of the Sendai earthquake and tsunami, the Japanese government declared an “atomic power emergency” and evacuated thousands of residents living close to Fukushima I. Ryohei Shiomi of Japan’s nuclear safety commission said that officials were concerned about the possibility of a partial meltdown at Unit 1. The following day, Chief Cabinet Secretary Yukio Edano said that a partial meltdown at Unit 3 is “highly possible.” [end of Wikipedia entry]
The World Nuclear Organization (WNA) publishes updates at http://www.world-nuclear.org/. As of today 5 of the 6 units have been successfully shut down. Three were running when the earthquake hit. The earthquake tripped the start of an automatic shutdown sequence per the design of the reactor, but under these conditions a nuclear plant requires power from another source to run all the pumps, motors, fans and instruments used in the shutdown process. If there is no off-site electricity from the grid available as was the case here, the plant gets this power from a large diesel generator/(s). But the tsunami destroyed the diesel generators, leaving the units with no power. There may have been other procedures, contingencies or redundancies for emergencies, but it appears the scale of the damage caused by the earthquake quake/tsunami combination was not in the plan. Let it be said that at 8.9 on the scale this was a very big earthquake that would have sorely tested any nuclear power station’s contingencies.
So for the remaining reactor that is still running : it is being cooled by spraying seawater with boron onto it. This is inside the containment shell. The water brings the temperature and pressure down and the boron helps to absorb radiation. The explosion shown by the media was the refueling floor which is still outside the primary and secondary containment area. But it should be clear by Monday if all of this worked and the core can be stabilized and cooled. Presumably all of the control rods have been dropped into place (the WMA does not mention that they are not).
The plant personnel are certainly in danger and the evacuation around the reactor is justified. This emergency at the Fukushima power station has brought comparisons to the 1979 disaster at Three Mile Island near Harrisburg PA, which resulted in a partial meltdown of the reactor core – but in the TMI incident the reactor was brought under control quickly (in a day) and no one was injured. Of course, then there is Chernobyl 1986, the only Level 7 (‘Major Accident’) event on the International Nuclear Event Scale recorded so far, which exposed large numbers of people to substantial amounts of radiation.
This cutaway diagram shows the central reactor vessel and thick concrete containment in a typical boiling water reactor of the same era as Fukushima Daiichi 1