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What exactly is exploding in Japan's Fukushima Nuclear plants?

We've all seen on the news the frightening explosions at the Fukushima nuclear plants, but there hasn't been much information.

The first explosion to me looked like a steam explosion in that there was no fire and you can clearly see the blast wave propagate upwards, and the dust and debris laterally; the second explosion seemed different in that there was fire in it and it appeared to produce a lot of smoke that rose vertically.

What a mess.

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Jason210
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Jason210
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KendorCommented:
As far as I am informed about what happened at fukushima 1 it was an explosion of hydrogen (when mixed with air) that blasted away the outer shell of the reactor plant.

There's a nice article here explaining some of the basics:
http://news.in.msn.com/national/article.aspx?cp-documentid=5030106
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deightonCommented:
re that article above, it says that Fukishima has had a 'partial meltdown'.  I'd not heard any of the engineers admit to that!  I believe it has possibly melted down of course, but I';m pretty sure they aren't telling us that yet

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BigRatCommented:
It is not precisely known whether the hydrogen, which naturally forms in the coolant by neutron capture and which is normally allowed to leak out into the atmosphere comes from the coolant or from the core rods. The rods are encased in steel which can react with the water producing rust and hydrogen. In the latter case highly soluable fission products - most notably caesium - dissolve in the coolant, which then indicates a partial breakdown of the cores structure.
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CallandorCommented:
The problem is that the fuel rods are exposed, due to insufficient power for the coolant pumps and the backup diesels are offline due to the tsunami.  This causes a significant rise in reactor temperature, and the reactors were not designed to operate for an extended period with the rods exposed.  As the operating limits are exceeded, a pressure explosion is very likely.  The partial meltdown is probably the excessively high temperatures causing the rods to deform, at the very least, and perhaps turning to liquid, at the worst.  They are pumping in seawater as a last-ditch effort to contain it (seawater effectively ruins the core for any future use).

There is also the zirconium used in the fuel rod cladding, which normally does not react with water, but which can cause hydrogen release when heated: http://www.springerlink.com/content/pv7r731892558683/  This can lead to the hydrogen explosions hinted at.

I wonder what happened to the emergency scram system which is supposed to shut down the system when catastrophic events occur - perhaps insufficent time for the core to cool down?  But I read that they have battery backup to keep the coolant pumps running - this event must have exceeded all risk scenarios planned for.
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Jason210Author Commented:
Thanks for the links and info.

I was just surprised at the explosions, especially the first one which seemed like a steam explosion. The second explosion seemed characteristci of a hydrogen explosion but I had no idea there was so much hydrogen....

Something I have been wondering about...If they are pumping in sea water to the core to cool it down, what are they doing with the hot and radioactive sea water that comes out of the core?
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CallandorCommented:
The coolant for the core itself never goes out of the reactor containment - there is a heat exchanger to cool down the fluid in the core, and the heat exchanger can use coolant from outside without contaminating it.
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Jason210Author Commented:
That's how it should work, but as I understand it they are putting seawater directly on the core...
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CallandorCommented:
I think they are just replacing lost coolant that has evaporated; any water in contact with the core would become highly radioactive and could not be dumped out into the environment.  Another possibility is that it could be pumped into a holding pond that all reactor sites have for spent fuel rods.
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wslbCommented:
Good discussion at:
http://www.hy-ramp.eu/news/regional-news/hydrogen-in-nuclear-accidents-what-is-the-role-of-the-gas-in-fukushima

Oxidation of hot fuel cladding: the cladding metal reacts with water to form metal oxide, taking the oxygen from water and leaving the hydrogen behind.  In the reactor pressure vessel the hydrogen is not a problem because there is no free oxygen for it to react with.  But when the hydrogen is vented (to relieve excess pressure) to the containment building, then it builds up to cause a potential - then actual - problem.  I don't believe the hydrogen comes from the release of fission products by fuel melting, because in that case there would be a lot more radioactivity release, and not so much hydrogen.
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Jason210Author Commented:
Well, at least we know now where all the water ended up:

http://edition.cnn.com/2011/WORLD/asiapcf/04/05/japan.nuclear.reactors/index.html?hpt=T2
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Jason210Author Commented:
I don't think the world will suffer from an extra boost to it's background radiation, but a lot radioactive material doesn't go away, it merely disperses and therefore is a cummulitive problem.
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