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The explosions at the Fukushima nuclear reactors were caused by a build of hydrogen. I was wondering where this hydrogen came from.

This question Fukushima - Isn't the hydrogen presence a proof of meltdown? suggests that the hydrogen is caused by thermolysis of water.
But surely this can't be the case? If it were, wouldn't it require the hydrogen and the oxygen to be separated whilst they cooled - else they would recombine into water in a more gradual fashion as the gases cool once they are clear of the scorching hot fuel rods?

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up vote 8 down vote accepted

This was (one of the few) facts reported clearly and correct in German news.

The Zirconium metal of the fuel tubes reacts with water vapor at elevated temperatures (above 1200 °C if I remember correctly) The reaction is simply: Zr + 2 H2O => ZrO2 + 2 H2 This is proof, that parts (at least) of the fuel rods were not immersed in water, but it does not prove melting.

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This reaction is exothermic unfortunately. So, if we see it, it's very very unlikely that there is no meltdown, especially looking at 1800C Zirconium melting temperature.. – BarsMonster Mar 16 '11 at 4:50
@Barsmonster Ah, You know data about relation of the heat of this reaction compared to the heat of the radioactive decay process and the "doing" of the rest of water presumably at the bottom? Let us know! – Georg Mar 16 '11 at 11:00
No precise numbers, just common sence: We need 1200-1400C to see Zr+H2O reaction, and we need just 400C more to see meltdown. Reaction itself does not consume any heat, so there is nothing here to stop it from heating 400C more. We already did 85% of the way, and nothing stopping us at the last 15% ) – BarsMonster Mar 16 '11 at 13:41
PS, In case you're wondering, why use zirconium if it's so risky? The answer is, it doesn't absorb many neutrons. It's by far the most practical material that can contain the fuel pellets over their entire useful life, without disintegrating due to neutron embrittlement. – james large Nov 15 '15 at 14:09

If oxidation is the result of hydrogen gas, this means the Zirconium tubes holding the pellets of fissile are corroding. If the core temperature is 2500 C, thermolysis of water could be a significant contributer of hydrogen.

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ChrisF excluded thermolysis in his question already with good arguments! – Georg Mar 15 '11 at 18:15
O, O2, H, H2 and OH are the main products of thermolysis. H2 and O2 will not recombine after cooling. – user2566 Mar 16 '11 at 1:02
Also, at 2500C whole core and it's casing would be a nice blob of molten metal. – BarsMonster Mar 16 '11 at 4:52

Of course, it is not from neutrons going beta to Hydrogen, that should be a hell of neutrons.

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