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In the case of a meltdown, what exactly is the damage?

If a meltdown does occur, damage will depend on whether or not the reactor's containment structure holds. In the case that it does hold, the plant itself is FUBAR'd but, otherwise, there isn't significant cause for concern. This would be analogous to the Three Mile Island accident -- in which a partial meltdown occurred but was successfully contained.

If the containment structure is breached, the resulting fallout will be comparable to that of the Chernobyl disaster which, like most Soviet reactors, had no containment structure whatsoever. Wikipedia has a good article on the effects of the Chernobyl disaster: http://en.wikipedia.org/wiki/Chernobyl_disaster_effects

Chernobyl was worse than just a meltdown. The graphite moderator caught fire and helped carry fallout into the atmosphere. This would not likely happen at Fukushima, as it is water-moderated, nor would the uranium fuel be able to catch fire as happened at Windscale, because modern reactors use uranium oxide instead of metallic uranium. In the event that containment is lost, there will still be contamination, but it probably couldn't be spread anywhere near as widely as the fallout from Chernobyl.

Are the containment domes intact in Japan? Edit: found this on IAEA site: "Containment remains intact at Fukushima Daiichi Units 1, 2 and 3". It doesn't even mention the 4th, but does say the explosion occurred outside unit 1s primary containment dome. What does that mean? It's outer shield is wrecked?

There's no containment dome.

http://i.imgur.com/CckjP.jpg (X)

The reactor is the red cylinder in the middle, the containment is the pear-shaped cavity it sits in, closed on top with the yellow hood and connected to that torus in the lowest-level which in case of an accident is supposed to condense the steam back into water, thereby removing heat from the containment.

The part that's exploded is the flimsy metal "hut" on top of the concrete building.

(X) from another HN comment I can no longer find, sorry, no attribution for who posted it initially

Original was

NRC: "Reactor Concepts Manual | Boiling Water Reactor (BWR) Systems"

http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf

Here's one with labels:

http://i.imgur.com/Oj4kg.png

Magdi Ragheb, "Containment Structures"

https://netfiles.uiuc.edu/mragheb/www/NPRE%20457%20CSE%20462...

It doesn't even mention the 4th,

There are six, and units 4,5,6 were already in a cold state before the tsunami (for maintenance); they're not involved.

You could be thinking of Fukushima Daini, which has four reactors.

That comparison is not correct.

The chernobyl plant exploded because of a instantaneous power excursion to many times the rated maximum, and it contained a huge amount of Graphite (=Coal!) which caused a fire that dispersed a lot of fine-grained core material.

The activity in the reactor core(s) in Japan right now should have decayed to a heat-output that is a few % of their rated power and, to make up some vivid comparison with the reality as we know it: The reactor core should look like what you get if a blacksmith puts glowing-hot iron (which in the reactor-case magically heats itself up constantly) in water, the resulting steam bubbles are what creates the increase in containment pressure.

Even if there would be another hydrogen induced explosion there's no chance that even a tiny fraction of the amount of radioactive material set free in Chernobyl would be released to the environment.

The plant itself would not be FUBAR'd. As it stands, that reactor will never be brought back online, but the other units at the plant can still generate power. (This is assuming that containment at reactor 1 succeeds and that it would be safe for humans to be working at the plant.)

This is what happened at Three Mile Island. Reactor 2 was taken offline and retired, but reactor 1 has continued operation. [1]

[1] http://en.wikipedia.org/wiki/Three_Mile_Island_accident#Curr...

Everything?

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