This cutaway diagram shows the central reactor vessel and thick concrete containment in a typical boiling water reactor of the same era as Fukushima Daiichi 1 (image: www.world-nuclear-news.org)

First, thanks much to our great commentors — Masoninblue, earlofhuntingdon, Professor Foland and many others — who continue to provide updates and expert analysis on the Japanese nuclear crisis. And hat tip to commentor “lobster” for finding a better schematic that helps illustrate the conflicting stories about a “meltdown.” [Updated to reflect suggestions for clarity.]

Desperate efforts continue to contain the damage at at least six nuclear reactor units at Fukushima Stations I (Daiichi) and II (Daini), following the explosion early this a.m. (US Eastern) at Daiichi Unit 1.

There are confusing/conflicting reports of a “meltdown,” at Unit 1. CNN reported a Japanese nuclear official seeming to confirm that a “meltdown” was probably underway, but there has been no other official confirmation. As Bill Egnor’s comment thread highlights, there’s no official definition, and there are different ways the term “meltdown” is being used.

1. Using lobster’s diagram, we can see in the middle the metal core of the reactor, which contains the fuel and control rods. Let’s call it the reactor vessel. Emergency cooling systems are beneath that vessel.

2. But surrounding and containing that reactor vessel and other equipment is a fairly substantial containment structure, with various parts of the normal steam and emergency cooling water systems feeding into it to reach the reactor.

3. Then all of this structure and more is housed in a large building, which some might call a reactor building. In this vintage, it’s not a hardened structure designed to contain explosions or high pressure. The roof and outer walls of this building, we believe, are what was destroyed by the explosion.

Explained this way, we can then account for some of the apparently conflicting reports and claims.

Was the containment structure destroyed? Officials say no. Well, it’s possible it wasn’t destroyed; however, the walls and roof of the larger building were destroyed, leaving the internal structure exposed. We don’t know the extent of the damage to this internal structure.

Is the reactor core still viable? Officials say yes, but what they may mean is that there hasn’t been a major breach of the inner reactor vessel, even after the explosion. We can’t tell for sure, but other evidence suggests some damage inside.

Has their been a “meltdown,” as CNN quotes the official as confirming [and see LA Times Version]? It depends on what you call a “meltdown.” If you ask, did any of the fuel rods melt? the answer is “maybe”
or even “probably.” There are reports the fuel rods were expose somewhat, for some period, and some melting could have occurred then. That type of initial “melting” would account for the presence of hydrogen, which led to the explosion, and the release of cesium. [I'll edit this as I get confirmation/better explanation.] But . . .

It’s also possible this limited melting has not proceeded to the stage where it becomes so pervasive that it melts through the reactor vessel, and continues on through the rest of the containment structure and floor. If the reactor then became “critical” again, that might lead to a full “meltdown” — the so-called China Syndrone — but we don’t seem to have evidence that that has occurred. Hence the confusion over whether a “meltdown” has or hasn’t happened.

In the meantime, the plant emergency teams continue to pump sea water into the reactor, though there’s no evidence they’re also trying to fill the internal “containment structure.” Since the normal and emergency cooling water systems have failed, the point of the sea water is to continue carrying heat away from the reactor as it slowly cools. The hope is that the core reaches lower temperatures where further melting cannot occur.

There are also reports of sea water being injected into at least [one other?] reactor. Unit 3′s cooling water system failed earlier today, and there is a BBC report that seems to suggest the core has become partially uncovered.

Quoted by Kyodo, Tepco said the tops of the MOX fuel rods were 3m above water.

So there are still at least six units in varying stages of risk. We should note, however, that as time passes and matters don’t deteriorate further, the heating processes being driven by the core/fuel rod decay will slow and the danger lessen. For the first time, time may be on the peoples’ side. [Update: Prof Foland notes via e-mail that “on the other hand, if coolant is slowly boiling off in other units, then the cores are slowly heating up.” So they may need to take additional measures — sea water cooling? — to bring those temps down, provided they’re willing to lose those units by using sea water.

Japanese authorities are steadily increasing the number of people being evacuated in areas surrounding the two Fukushima nuclear stations. The last confirmed report I saw was 140,000 people being asked to leave, but later reports suggest the number at 180,000 (CNN) or exceeding 200,000 (BBC).

People are being given iodine treatment to help limit the effects of radiation exposure, as the number of folks possibly exposed climbs. That suggests they’re clearly worried about losing more of these plants and having more radiation releases, with associated health implications.

Again, we’ll update as events warrant. And thanks to our commentors for all their help.

John Chandley