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We don’t know “for a fact” that the cores “had to” melt out through the bottoms of the RPV. There’s speculation that some of the core(s) might have gotten out of the RPV due to the radiation levels in the containment wells, but that can also happen as a result of material being flushed out of broken piping (when the fuel rods break, the fuel is now loose pellets), or excursion of molten fuel through existing penetrations of the RPV. The bottom of the RPV contains penetrations for control rods, sensors, etc. TMI-2 had some small excursion of corium through a few of these types of points, but that isn’t the same as a melt-through of the RPV floor.

Here’s a picture of the “final state” of the RPV at TMI-2:

http://www.americanhistory.si.edu/tmi/10-01.htm

Here’s the analysis of what happened to the RPV at TMI-2:

http://www.gl.iit.edu/govdocs/resources/NUREGCR6197part01.pdf

and the sequence of events at TMI:

http://www.gl.iit.edu/govdocs/resources/NUREGCR6197part01.pdf

TMI’s RPV wasn’t designed to hold back a molten core either... but it did.

TEPCO first thought that the RPV’s for 2&3 were compromised, then reversed themselves. Their assessment of reactor 1 is that they think the core might be extending out the control rod shafts at the bottom of the RPV. They still don’t know for certain, they’re still making estimations based on instrumentation.

On the flip side, at TMI, they didn’t think that the core had melted so extensively and the models of what happens in a LCA had to be adjusted as a result of TMI’s results. We heard all manner of “burn through the RPV” alarmism during TMI as well.

But let’s play devil’s advocate here: Let’s assume that the cores have dropped out the bottom of the RPV’s. What do the temperatures in the PCV’s tell us? They’ve achieved temps < 100C, which is “cold shutdown,” and there is no more self-sustaining reaction therein. The only way to get rid of the decay heat it to dump water across it. You can either carry the heat away, or the heat is absorbed by the water as it changed from liquid to gas phase. Overall, we should prefer a water cooling method that doesn’t involve the creation of steam.

The reality is that we won’t know exactly what happened until or unless they are able to section the bottom of the RPV, as they did with TMI years after the melt-down. The directly applicable experience we have (TMI-2) showed us that the core could melt down inside the RPV but not go through the bottom of the vessel.

For all the hysteria being offered by anti-nuke advocates and luddites the world ‘round, the physics of what goes on is pretty straightforward: When a core slags down, it takes in a whole lot of non-reactive stuff with it. The boron they’ve been dumping into the situation is a heavy neutron absorber, well known to poison fission reactions. There isn’t going to be some nice, uniform ball of fissile material in a runaway reaction, ready to head for the center of the earth. There’s going to be an ugly, molten mess of composite material that flows, solidifies and then cools. Even at Chernobyl, the core melt-down was contained within the reactor building as it slumped out of the reactor, combining with sand, steel, concrete and then cooling in the floors below the reactor. The most damaging thing about Chernobyl was the fire which spread isotopes over such a wide area.

On the subject of alarmism, here’s something useful to read:

http://fukushima.ans.org/report/Fukushima_report.pdf

If you read nothing else, start on p. 31 and continue to p. 33. NB the following passage on p. 32:

“A popular theoretical physicist and media personality made dozens of television appearances ridiculing the Japanese effort to cool the reactors and predicting the loss of the entirety of northern Japan unless the reactors were immediately entombed. There was no sustained counter view or strong challenge to these
claims—and perhaps, none was wanted.”

Seems as tho said TV personality is still offering observations of this sort today... despite his predictions not coming true to date.

When I’ve checked into the credentials of those being interviewed by the press, or offering these alarmist claims, very, very few of them have any science or engineering background. One guy who is well known for his alarmism from a DC “think tank” who is a supposed “spent fuel pool expert,” is, in fact, a music studies drop-out. Let’s get this understood: He wasn’t a music major... he dropped out of a music program. I’m not saying that being a music major is easy - I have friends who were graduates of serious music schools and they hardly had a cake-walk. But then again, none of my friends who were music majors a) dropped out, b) are trying to hold forth technical opinion on nuclear power, or c) were ever arrested for growing dope in their basement while holding a government job with a clearance.

The least the press should try to do is find some people who have passed a few physics and math classes. Just a few. Let’s say, three semesters of calc, three semesters of physics. They don’t even have to finish a degree. I’ll accept someone who dropped out of a BS program by the end of their sophomore year over some of the “experts” the press is parading around.

The ANS report also alludes on p. 32 to the hysteria disseminated by the chair of the NRC concerning the fuel pool at reactor 4. This is where the hysteria about the fuel pool(s) started, with his comments to Congress. Well, while he has some credentials in physics, let’s see what his tenure at the NRC has been:

http://www.politico.com/news/stories/1211/70427.html

And for other amusing stories:

http://www.politico.com/tag/GregoryJaczko

Not what I’d call a calm, cool individual.