> Conflicting information keeps coming out about possible worst case scenarios (and their relative likelihoods) with the nuclear reactors.

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).