First, the latest news on the Fukushima reactors, via Nuclear Information and Resource Service:
NHK TV reports that there has been an explosion at Unit 2 at Fukushima Daiichi. There is speculation that this explosion has damaged the primary containment (inside the concrete containment building, which is the secondary containment. Tepco is evacuating some non- essential personnel from the reactor site. 2.5 meters of the core are currently uncovered by water—which means it is almost certainly melting. Winds from the site are currently blowing toward the North.
If true, this is arguably the worst case scenario for this disaster. The primary containment building is responsible for containing the nuclear fuel of the reactor. For those of us not directly working at the plant bravely sticking to their duties despite all the risk or within a hundred miles or so from the damaged reactors, there is no heath risk or radiation exposure risk.
Do not panic.
The prior explosions at reactor 1 and 3 involved the secondary containment building. If the primary containment is actually breached at reactor 2, this might make the site too radioactive for workers to stay at for any length of time.
(If you're curious to read some more details about what has, and is, happening at the plants, I can recommend this article by Dr Josef Oehmen from MIT.)
There are two perspectives to come at, when considering the significance of this event.
For those within a short distance of the plant, this is now a massive event—with significant local contamination possible. A release of radiation on the scale of the Chernobyl disaster is simply not possible; a release larger than that of Three Mile Island is increasingly likely—particularly as salvage efforts are hampered by the local radiation levels.
For the rest of us, even given the potential release of reactor contents to the environment, the amount of radioactive isotopes we'll be exposed to from this accident should be considered in comparison to that from other sources.
Coal burning power plants, worldwide, release approximately 8000 tons of Uranium and 20,000 tons of (radioactive) Thorium into the environment each year—as a part of routine operations. (Coal is contaminated with these elements, and there are no regulations requiring them to be stripped out before the coal is burned.)
Similarly, the mining of rare earth elements (used to manufacture touchscreens and windmill generators, among many other high tech items) results in vast waste pools of Thorium—also left exposed to the environment.
Despite the magnitude of this disaster—larger than many suspected could be possible with non-Soviet-style reactors—I firmly believe some perspective is needed when you consider if nuclear power is 'safe'.
Another point, that I fear people are confused by: A 'Meltdown' does not mean the same thing as a nuclear explosion. A meltdown means the nuclear fuel in the reactor—normally in pellet form, neatly loaded in metal rods—has melted and lost the orderly shape. A 'partial meltdown' means the fuel rods have partially melted. A 'full meltdown' indicates a complete loss of form to the fuel. The more deformed the fuel becomes, the harder the eventual cleanup.
There is zero, none, zilch, nada chance of a nuclear explosion at one of these plants. None. Not even the shittiest imaginable midnight scifi flick could that happen. (As the Iranians and North Koreans can attest—creating a successful nuclear detonation takes lots of careful forming, enrichment and orderly compression.) Again. A meltdown is a problem for the eventual cleanup of these plants, but otherwise doesn't mean much for anyone who isn't planning on hiring with the Tokyo Power Company.
From a (near) real-time Geiger Counter located in Tokyo: