The buildup of Hydrogen in Fukushima Dai-Ichi?

Is it correct that for hydrogen to be present inside a nuclear reactor, the zircaloy has to melt off the fuel rods and come into contact with the water, thus, oxidizing and forming hydrogen?

?2011-03-20T01:52:09Z

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Many different paths lead to the breakdown & corrosion of these metal coatings at high temperature,
and hydrogen gas is a product. That is indeed what exploded.
The explosions indicate high temperatures- a dry reactor exposed to air.
The other radiation indicates a meltdown, in 4 reactors and more spent fuel pools.

Re-ignition- a fission reaction starting again- is indeed imminent.
Then metal explodes- including the zirconium casings, now part of molten pools.