Status, as of this point on the reactors, is as follows:
- The second plant, containing four units, reached cold shutdown early in the week. All four units there are stable and being maintained by grid power. Their time for a restart (that is, on-site damage to things like switchgear, turbines, etc) is unknown at this time.
- At Fukishima we have the following that is reasonably presumed correct:
- Unit #1 is stable but remains hot. Containment is presumed good and seawater is being injected into it. Grid power may be restored to Unit 1 this weekend.
- Unit #2 is stable but remains hot. Containment is questionable - the official view is that it "may not" have been breached. There was a tourus explosion (presumed hydrogen) in the suppression system however, and as such I would consider containment status questionable. Grid power has been connected but is not yet turned on.
- Unit #3 is stable but remains hot. Containment is presumed good as pressure has been fluctuating while relief valves have been venting pressure (instrumentation is at least working at some level, while in the first two cases we're not sure.) In terms of potentially unstable reactors themselves, this is the one that is of most concern due to the continued venting, implying that cooling has not managed to stabilize the internal pressure builds from decay heat. The fuel storage pool has water in it (verified by direct observation); part of the external concrete wall has collapsed but the internal steel liner appears to be intact.
- Units 4-6 were shut down at the time of the earthquake. Previous speculation (including by our NRC Chairman!) that unit #4s spent fuel pool was void of water appears to be incorrect. #5 and #6 are intact in their entirety and there is one working generator at the site providing limited power to both for instrumentation and limited circulation.
Let's look at the two "bookend" scenarios, understanding that actual results could fall anywhere between these two. The first has a utility feed restored to the plant over the weekend. This immediately takes units #5 and #6 off the field in terms of potential risk of release. That is, in the most-severe ("run like hell") scenario you now have two plants that have a high degree of confidence of being ok with minimal or no manual intervention for extended periods of time. It also brings a high degree of stability to all the reactors except for #3, and makes very likely prevention of any material problem with the cooling pools in terms of radioactive release. This leaves us with the already-released materials and the mess to clean up at the plant - not a small problem by any means, and unit #3 could still prove very troublesome to fully stabilize. However, this "bookend" likely ends the risk of significant release of further radioactive material to the environment.
The second, or "unmitigated disaster" scenario likely begins with one of the fuel pools melting down and violating the inner liner of steel (the part that is watertight.) This will cause an immediate and massive spike in radiation levels throughout the plant area as molten fuel contacts other things in the plant infrastructure, including things that are wet or concrete, and disassociating it due to heat (a chemical blast-like dispersion event that sprays molten radioactive material all over the immediate area.) That will effectively terminate all attempts to deal with the problem as the only mitigation factor remaining is "run like hell."
Incidentally, as I said a couple of days before when people started running claims that as massive pool fire and/or breach had already happened, unless you can find the massive and pervasive spike in radiation levels plant-wide, including on the ground, the initiation of that worst-case scenario hasn't occurred. Yes, radiation levels are going to be high (very high in fact) directly over the pools, as the water level is down and in addition Units #1-3 have been releasing pressure from their cores into the atmosphere (that's "straight up" for those who have reading comprehension problems.) The pools are open on the top and as such you'd expect very high levels directly above them with water below normal levels. But a fuel pool void of water that melts down and either violates the water-tight liner (made of steel) or worse, ignites, is going to spray fission products everywhere, and they're going to be hotter (radiologically) than a pistol. There's no way you're going to mistake that for anything else nor can it be hidden.
The "good" scenario effectively ends the radiological emergency. The "aw crap" one has to be presumed to ultimately lead to release of all of the Curies in the spent fuel pools, although it will take quite some time for it to happen and the overwhelming majority of the release will be at the plant itself. That event would poison (permanently) a material diameter of land around the plant, perhaps out to a few miles, and the "thou shalt not enter" line would remain for a long period of time (decades or more.) I'd hazard a guess that a radius of 10-20 miles from the plant would be rendered uninhabitable for a very long time (10+ years) and agriculture would be impossible within perhaps another 20-30 miles due to uptake of deposited material into plants. The number of people killed outright would be relatively small (most of them workers at the plant and others attempting to prevent the occurrence) but cancer risks would also rise materially, likely resulting in a few thousands to tens of thousands of cancers over the next 30-50 years. Even basic cleanup and encasement operations would likely not be possible for months - and maybe longer, with the ultimate "mitigation" strategy being encasement in sand and concrete, much as it was Chernobyl.
To put this in perspective, however, there's almost-certainly far more than 20,000 people dead right now from the tsunami, as the official dead and missing numbers are, at last report, in excess of those numbers.