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The IAEA's latest update has this:
http://www.iaea.org/newscenter/news/tsunamiupdate01.html
The indicated temperature at the feed water nozzle of the RPV of Unit 1 has decreased from 323°C to 281°C and at the bottom of RPV remained stable at 134°C. There is a corresponding decrease in Drywell pressure.
At Unit 2 the indicated temperature at the feed water nozzle of the RPV has increased from 154°C to 177°C and at the bottom of RPV has increased from 78°C to 88°C. Indicated Drywell pressure remains at atmospheric pressure.

So Unit 2 drywell pressure is at atmospheric pressure (whereas Unit 1 is not).

What's that all about then? Does that mean reactor 2 isn't airtight now?

by LondonAnalytics (Andrew Smith) on Wed Mar 30th, 2011 at 04:56:39 PM EST
[ Parent ]
Reactor 2 isn't airtight since the wetwell explosion two weeks ago.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Wed Mar 30th, 2011 at 06:46:48 PM EST
[ Parent ]
Ta for this. I'm trying to understand how the loss of containment / protection differs between reactor 1 and reactor 2.  There was an explosion at reactor 1 too, wasn't there? So did that explosion not breach the drywell, whereas reactor 2's drywell is breached? Do the differences in pressure readings tell us anything about the intactness of the innermost reactor vessels?
by LondonAnalytics (Andrew Smith) on Thu Mar 31st, 2011 at 02:57:53 AM EST
[ Parent ]
There was a hydrogen explosion in the building of reactor 1, but outside the reactor. The explosion was from hydrogen released form the containment vessel when venting steam to reduce the pressure. The explosion blew off the top of the building only. The similar explosion around reactor 3 looks like the building is reduced to rubble. The explosion around reactor 4 which also affecter the entire building took place because of hydrogen released from the spent fuel pool, not from the reactor which had had all its fuel moved to the pool for scheduled maintenance.

The explosion in reactor 2 was in the "torus" wetwell below the reactor. But the outer shell of the building is relatively intact.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11

by Carrie (migeru at eurotrib dot com) on Thu Mar 31st, 2011 at 03:06:16 AM EST
[ Parent ]
Thanks Migeru. So reactor 2 has had an explosion below it, and breached the wetwell. The drywell at reactor 2 is depressurised, and so at atmospheric pressure.

And Richard Lahey thinks that the core may have "melted through the bottom of the pressure vessel in unit two, and at least some of it is down on the floor of the drywell"

Have I got all those bits right?
And we have a Mark 1:
and this: (click each to enlarge)

So are the contents of both the wetwell and drywell now exposed to the air inside the outermost reactor building, and there may be corium in the drywell - is that right? (or have I completely misunderstood?)

by LondonAnalytics (Andrew Smith) on Thu Mar 31st, 2011 at 04:09:24 AM EST
[ Parent ]
The drywell is flooded with water.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Thu Mar 31st, 2011 at 04:59:12 AM EST
[ Parent ]
So, if Corium formed it would solidify on contact with the water, with a possible steam explosion.

There's plenty of reason to believe meltdowns did occur, but are in the past.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11

by Carrie (migeru at eurotrib dot com) on Thu Mar 31st, 2011 at 06:19:04 AM EST
[ Parent ]
Drops in temperature and pressure indicated an end to meltdown. However, the latest analyses of info readily released from over a week ago and discussed in the newer Fukushima threads indicate that limited meltdown probably still goes on: the tops of the rods are still out of the water, and radioactivity in the containment occasionally rises.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Thu Mar 31st, 2011 at 08:07:42 AM EST
[ Parent ]
I fear that flooding the dry well with water might not solve the problem. It will certainly produce copious amounts of steam, but if the corium retains critical mass it can either re-melt or simply not be melted in the first place.

 A week ago, when I first heard the term "core catcher" I thought of a surface divided into diverging channels so that, should corium fall on it, the coruim would be divided by gravity into smaller and smaller portions down to a level at which re-criticality would be impossible. Perhaps I should say that I dreamed this was the case. Doing this would have involved only design time and the construction of a form to shape the surface of the dry well. But it turned out that the term "core catcher" was post hoc from attempts to stabilize Chernobyl.

Tom Burnett responding in comments noted that corium could not be diluted, but could be broken into smaller packets. I would note that this would only be a stable solution were it to be performed on a stable surface that had adequate heat capacity or heat removal capacity. If the corium melts through the (apparently flat) floor of the drywell and onto grade the next development would depend on the moisture content of the grade into which it melts and on whether geologic conditions happen to further disperse the corium or to re-concentrate it.

"It is not necessary to have hope in order to persevere."

by ARGeezer (ARGeezer a in a circle eurotrib daught com) on Thu Mar 31st, 2011 at 08:17:14 AM EST
[ Parent ]
ARGeezer:
Tom Burnett responding in comments noted that corium could not be diluted, but could be broken into smaller packets.
Corium also loses structural integrity as it decays radioactively.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11
by Carrie (migeru at eurotrib dot com) on Thu Mar 31st, 2011 at 10:16:17 AM EST
[ Parent ]
Core catchers are pretty new inventions, and I don't know if any nuclear plants have reactors with them yet. The EPR has a core catcher, which works by spreading the corium out over a large concrete surface which cools the corium. The concrete is itself cooled by waterpipes inside the concrete.

Peak oil is not an energy crisis. It is a liquid fuel crisis.
by Starvid on Thu Mar 31st, 2011 at 03:40:32 PM EST
[ Parent ]

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