Hi all,
Thanks for the answers. A few more info, so as to share the little information I could read, and check if I actually understood the even smaller amount I am under the maybe delusional impression I did understand.
To Most Curious, #10285, re Chernobyl: again, if I have understood what I have read (a big if), a good deal of the nuclear fuel inventory was scattered around by the initial explosions; the total quantity of corium was anyway smaller than at Fukushima. Moreover the graphite control bars, which fed the fire in the core, also acted as impurities slowing neutron reactions within the corium. As a result the corium was less active than it could have been. Even then, according to the NEA report, the corium was turned off, so to speak, using 5000 tons of dry material (not water), including 40 tons of borate compounds, all of it dumped from helicopters. The corium was also fractioned using gravity and available ducts and spaces in the basement. Last but not least, this already significantly weakened corium, which had still managed to eat its way through the concrete floor, was countered with a specially constructed concrete mat built underneath its possible path, and which was to be cooled if there was need. In the end there was no need, which did indeed prove that a corium could stop -but that was through dry material attacks and fractioning, not water cooling from above. The thing is still pretty hot today, by the way, you wouldn't want to sit on it...
http://www.oecd-nea.org/rp/chernobyl/c01.html
By contrast, the papers I have read are not deliriously optimistic about dousing an active corium with water. Anyway, does not water get cracked way before getting at it? Water cracks at 700°C and the thing is well above 2000°C, so it generates oxygen and hydrogen rather than steam, no? Bottom line, I am not sure a corium turns off by itself once it becomes active after a criticality, the references I gave in my first post (#10281) make no mention of such an automatic switch-off process, and assume rather that the corium must be fractioned and cooled down to be containable.
And corium becomes active awfully fast, compared to Fuku1 timeline. Jim Hardy's doc (# 10284, thanks, and Hello Arkansas, do come over some time, Paris is as rainy as Seattle but at least it's not freezing, personal message) (and thanks for the papers) confirmed RPV failure at T0 + 7 hours or so in case of complete station blackout, but makes no mention of what happens next. The ablation rate of an active corium in the aforementioned references would make it able to eat through eight meters of solid concrete, including the embedded shell and the drywell wall, in 14 hours flat -that's the model from 1981. Also, I don't believe they added borate to their seawater at first, so it was just heat transfer, and eating a few neutrons here and there. A key point would be whether water got under the RPV before meltdown was effective, and that's not clear to me. If water had gotten under, they would not fret about RPVs being broken, would they? The effect of a corium falling into water would have been hard to miss even from far away...
If the RPVs are pierced as Tepco and the Japanese seem to say, these coriums were sure pretty active at some point, and went down without meeting water -we would have noticed otherwise, right? In this case, while a "China syndrom" is indeed not possible if I read what I read correctly, I was not referring to that, nor to any dramatic post-leak event, just to a plain old contamination of the ground and the water table and eventually ocean, all of which is supposed to be countered by this concrete wall they talk about. By the way even an inactive corium, if it has pierced all barriers, can contaminate its environment I suppose?
Last but not least, I know this is hearsay, but from French nuclear engineers, so it's a little less than pure specualtion, anyway it seems that the concrete basemat was hollowed out for seismic resistance reasons at Fukushima. So the drywell and its embedded shell sit atop a cavity which could indeed act as core-catcher, provided the corium is cool enough.
Don't get me wrong, but recapping your various posts I am under the distinct impression that nobody is very sure of where that corium is exactly, if it's cooled, if it's divided (some French models from what I heard forecast division of the corium, part of it seeping down, part of it remaining stuck in the RPV and cooling down), and what energy it generates exactly.
By the way, as a way to say thanks, here is what is UNDER the concrete
http://en.scientificcommons.org/49101133
The Tomioka formation (p. 4) is two layers, 60m + 140m; both fairly soft, more or less impermeable sedimentary formations, first layer argilite over fine then coarser sandstone, second layer solid sandstone intespersed with sandy argilite and coarse sandstone. I was unable to find anybody having ever thought of corium-argilite interactions -yeah I know, no surprise there.
So, here is what I have understood: Jim Hardy better be right and the RPV not broken through! A corium generates a lot of energy in a few cubic meters with very high density, not the kind of stuff you want to leave lying around. If anybody has more info or papers or sources, please bring 'em on, to quote a well-known poet.
PG
