I cannot in good faith go without writing an update on this issue, given the new information made available to the public. Now, the worst sounding of these news stories (the first link) consists of unofficial information, leaked to the press. The basic summary, according that news story (and since reported in a great many places, including the PBS Newshour) was that the fuel in Reactor 1 melted very shortly after the loss of power, and may have violated the integrity of the pressure vessel (there has been some throwing around of the word 'containment' in an unclear way in the press, which is not helpful). There has also been a substantial uptick in the estimated amount of radiation released.
Now, a few points here: Firstly, the last news story linked above suggests that TEPCO does not believe the pressure vessel was seriously violated, which is in line with what happened at Three Mile Island as I discussed briefly before. Second, all of the information reported thus far has been based on estimates of what took place, the size of potential breaches, new information obtained by the recalibration of instrumentation on site, etc. What this means is that these new numbers are potentially not much better than the previous, and in keeping with much of the data presented by the Japanese, is something of a worst case.
Unfortunately, until radiation levels inside the containment buildings reaches low enough levels that the pressure vessels can be inspected, much this remains guess work, at best. Currently the robots being used to inspect the reactor buildings are not able to reach to the actual pressure vessels, probably due to design limitations (they have tank treads, and can't climb ladders, etc). However, given that TEPCO is able to circulate water through the reactors, the extent of damage to the vessels cannot be that severe (i.e. the fuel cannot have melted through and poured out into the containment structures), or water would not be able to circulate through the system. This is not to say the integrity of the pressure vessels is perfect: there may be smaller holes that are plugged with solidified fuel, which may not have been hot enough to flow well.
There has been some data that suggests that there may have been exposure of molten fuel to the air; the map linked shows a few data points where radiation present is coming from heavy, non-soluble metals that could only be dispersed by direct exposure of fuel to the air. It's not clear, however, how high the detected counts are above background.
The larger perspective here remains fairly straightforward, and not all that different than before: there have still been no fatalities related to the release of radiation. The area around the plant has not yet been surveyed, and the extent to which soils in the area have been contaminated with long-lived nucleotides is still unknown. The majority of the miles of plumbing and conduit that service the reactors have yet to be inspected (though some steam leaks have been found with robots, and it is these sorts of things that are preventing them from establishing a closed cooling water cycle at the plant, something that needs to happen rather badly). The backup generators were still badly sited and poorly protected. The reactors were still old, well past their design lifetime, and were subjected to disasters above what their safety systems were designed for, and the US NRC was still probably wrong to suggest that a larger area needed to be evacuated.
The best summations of what is known definitively so far will be found on the IAEA's website, which I linked to above, and an excellent summary of take-away messages thus far was assembled by some folks at MIT, and is well worth reading if you're interested in this stuff, and the future of nuclear power.
The things that continue to concern me are the fate of the contaminated water on site, much of which has leaked into the ocean, though that appears to be nearly under control, and the degree of contamination of soils and surfaces around the plant. Only time will show how bad those situations are, unfortunately.
I would also add that none of the news released thus far has changed my views on nuclear power, at least in part because much of it is simply estimates produced by computer models, using untested assumptions regarding the conditions in the reactors. Not that you'd take that away from the news, of course, where this information is being treated as fact. Which I confess irritates me greatly.
But we can't expect the media to learn things that might reduce their ratings, can we?