I need to make this a quick update due to approaching deadlines for projects I've been neglecting as this emergency situation has developed. I'm going to skip the unit by unit rundown and other background stuff and just discuss recent developments and their implications.
Check these links for background info on nuclear plant containments and recent updates on each unit.
From information at the Japanese Atomic Industrial Forum, it appears that they are considering flooding the primary containment at units 1-3. Related to the topic "Water Injection to Containment Vessel" they list 1 and 3 as "To be confirmed", and unit 2 as "to be decided (Seawater)" Over the past two days they have cut back the rate of water addition to the reactors to slow the rate of water being leaked to the environment. This has caused the temperatures to begin to rise again at all three units. Flooding the drywell up to the level of the reactor vessel would provide an alternate way of removing heat. It would also not be subject to the same leak problems as have been showing up in the reactor's pressure vessel containment system.
This would also be done in the event of some breach in the reactor vessel itself, but it wouldn't be something they would contemplate for long if this were the circumstance. The leaks appear to be related to valves and piping connected to the pressure vessels, not the vessels themselves. There was a highly unreliable source supposedly quoting a press conference with TEPCO in which they talked about "a hole in the reactor". There are no other mentions of anything like this from Japanese news sources. The reference to zerohedge took you to a link for "natural news" who have already reported bogus rumors as fact. Their reference for the story was to a Japanese site with no English translation available. If you have taken that "news conference statement" as fact then I'd suggest you consider the sourcing.
The biggest obstacle now is getting the turbine buildings pumped out to allow necessary work to continue. Once they can get plant pumping systems to work they can begin to gool the plants more effectively and also filter the reactor water and gradually remove the dissolved fission products and saltwater impurities. I would presume that there is a parallel path being pursued by a team of plant engineers and maintenance workers to determine the sources of the largest leaks and see if there are any ways to isolate the system(s) involved.
The highly radioactive water in the service trenches is also an issue that needs to be addressed soon, but it is not as high a priority as getting the turbine buildings back. As per various conversations yesterday, it sounds like the most practical approach for that will be to use a staged filtration and demineralization system to remove the majority of the radioactive contaminants and then release the much less toxic result to the ocean - simply for lack of large enough storage areas to allow evaporation of this much water. It's not a good solution, but better than available alternatives at this point. Perhaps the civil engineers on site will have some other tricks up their sleeve, but that is on the hopeful side of likeliness.
Given the much larger amount of water at unit 2, it is likely that the folks who pointed to the popped torus as the source were correct. TEPCO eventually stated that there was damage in the bottom of the torus. This would leave the entire contents free to flow out into the reactor building and eventually drain through various paths - cable conduits, pipe runs, etc. until it reached the trench.
And considering that there is also water at unit 4, it seems likely that drainage from the fuel pool filling activities is also making its way to the turbine building and service trench. I've not yet seen reports on isotopic analysis of this water. If there is an overabundance of short-lived fission products in that water it could indicate that there has been disruption of the bundles in the fuel pool enabling them to have a higher than expected reactivity. The same could be the case at unit 3 and could help explain the volume of water found there.
Unit 1 has neither of these circumstances. It was, however, the unit that had the biggest temperature jump last week. Of the three units, this is the one that is more confounding. It is lower power than the others and did not have the same fuel pool problem as unit 3 or torus problem of unit 2. When I come back to this in the evening I'm going to dig up NISA data for an extended stretch over the past two weeks and see if there is anything in the trending that provides more clues as to what may have happened at unit 1 and when. This may also uncover another potential source for the large volume of water.
I'll need to drive-by on this for now, but will get back for a mid-afternoon break and then again in the evening to get back up to date on new developments. As stated in a comment the other day, the upside possibilities at this point are quite limited, any good news will only be good in a relative sense - the downside risks far outweigh them. I believe we are still in a very grave situation there. I don't see any of the individual issues having risen to the level of desperation at this point, but they are now closer to that than in the past. The next 24 hours will be interesting, seeing whether they choose to flood the drywells, and if so, seeing how the plants respond.