Japan Does Not Face Another Chernobyl

Actually, about 1/3 to 1/2 of the Three Mile Island core melted completely (i.e. far worse than mere zirconium cladding damage).

This was because the Three Mile Island core was not continuously cooled. There were a couple of elements to this:

1. In normal operation, the primary system is kept highly pressurized to ensure that the primary coolant does not boil. The way this is done, however, is to use something called a pressurizer, which does have a steam bubble, for much the same reason cars use shock absorbers. When the steam flow was cut off to the turbine generators, that acted as a loss of the heat sink that had been cooling the reactor, which caused pressure to go up.

When pressure got too high, a relief valve in the pressurizer automatically lifted to relieve the pressure increase and maintain primary piping integrity. The valve was supposed to automatically shut once the pressure dropped enough, but did not. The indication that the operators had showed that the valve had been commanded to shut, but did not show that the valve had not actually shut. The location of the leak tended to artificially increase water level in the pressurizer by essentially "sucking" it from the reactor vessel (the operators had no indication of reactor vessel water level, since it was normally completely full of liquid).

This loss of water from the reactor meant that it uncovered the fuel much quicker than might otherwise be expected. The operators had other ways of telling something was wrong (such as the sharp pressure drop from a stuck open relief valve) but did not put the puzzle together. They even did worse: Automatic safety systems kicked on to force-fill the reactor with cooling water, but the operators were afraid the system would overflow the entire primary (remember, they thought there was still water in the reactor vessel), so they dampened the flow, then shut off the system completely.

2. As steam started forming in the reactor vessel, the coolant loop piping connecting the reactor vessel to the reactor cooling pumps started to fill with steam, which caused the reactor cooling pumps to cavitate (which is certainly not good for the pumps). The operators had indication of this, but assumed the cavitation was due to the pressure drop and, again thinking the core was still covered with water, they shut down the reactor cooling pumps. This represented the loss of the last line of defense against reactor meltdown.

It was not until some hours later when a different shift relieved that it was realized that a relief valve was stuck open and that there was no water over the fuel. They started re-established cooling at this point, but the damage was already done.

There is only one worst-case scenario, so although I very much agree that the information coming out of Tokyo Electric Power Co. and the Japanese national government is inconsistent, that doesn't mean new "worst-case" scenarios become possible.

IMHO the worst-case scenario is fuel melting through the reactor pressure vessel, through the thick steel containment which was not damaged in the hydrogen explosion, and through the bottom of the concrete containment (the top of which was damaged), and then from there into the environment.

The worst-case plausible scenario from what I can tell (which again, has been true from the moment they lost cooling systems for decay heat removal) is fuel melting to the bottom of the reactor vessel, possibly breaching the primary boundary but getting trapped by the steel containment, with possible further release of gaseous radioactivity.

I say this because cooling the fuel if it is outside the primary but inside the steel containment is much easier than cooling the fuel inside the primary: you must merely submerge the steel containment in seawater (which would boil away as it heats up), but this time you don't have to worry about pumping to high pressures. The fuel was transfer its heat across the steel containment to the seawater.

As long as the steel containment is intact (as was true last I heard, but refer to my earlier point about TEPCO) then there would be no additional release of radioactive contamination from this. If there are air leaks in the containment that would be worse, but a portable radiation monitoring device would quickly identify such areas so they can be quickly patched or covered to at least keep contamination from spreading.

So, I also disagree with "unimaginably bad". Unimaginably bad might be something like coal slurry breaking through its retention walls and killing all aquatic life in a nearby river for hundreds of miles (as actually happened in Kentucky in 2000 IIRC). At my last check all three reactors that had been operating at Fukushima-Daiichi have probably had fuel damage, which means that any steam they vent from the plant will contain radioactive contamination. So, definitely bad.

Unfortunately I can't seem to find to many numbers for activity concentration (how frequently the radionuclides are actually decaying) or for dose rate (a measure of biological damage per unit time). A Guardian article mentions 680 microsievert per hour to the northwest of the site, which would be 68 millirem/hour, which is a value that is actually fairly high, but far below levels that would lead to immediate health risk.

For a point of comparison, there are (inhabited ;) spots in the world (Ramsar, Iran) where you would receive 70 millirem/day, every day, which is far above what a trained radiation worker would be permitted to receive and yet still appears to have little (if any) effect on the local population.

Of course any unnecessary exposure is unacceptable! But I also wouldn't come close to calling it "unimaginably bad", except in regards to the economic cost to TEPCO and the substantial loss of generating capacity (which as you mention, can have a substantial economic impact).

You can't assume an r-squared dependency, as the radiation they were measuring came from the radioactive steam that had drifted out to sea (i.e. the "airborne radioactivity"). It was not from the Fukushima-Daiichi site directly (after all, there's many different radiation monitoring stations nearby that have much better indications of general area radiation levels).

In fact you proved yourself wrong! If being a mile away from the reactor gave you 2 Sv/hour, and no one lives past 7 Sv, that's only 3.5 hours anyone could survive, and there have been workers at the site for days now.

There is only one worst-case scenario, so although I very much agree that the information coming out of Tokyo Electric Power Co. and the Japanese national government is inconsistent, that doesn't mean new "worst-case" scenarios become possible.

This is an interesting question.

Repeatedly we've seen commentators and outside observers saying "this can be no Chernobyl because...". But the situation at Fukushima-Daiichi is still very dynamic and many questions remain. These cores still have a lot of energy left to dissipate and thus each still seems to have a life of its own.

As much as we would like to know the current state of the reactor cores and all the possible paths they could take, the picture emerges that those officials in charge of the mitigation efforts don't have a perfect picture either.

Whether there are multiple worst case scenarios, or there is only one that we can't see clearly yet seems to be a philosophical question about a highly physical phenomenon.