A3 Hydrogen Explosion

Topic

As previously described, when fuel cladding reaches sufficiently high temperatures in the presence of water vapor, it reacts by releasing hydrogen gas. This anomaly arises when that hydrogen accumulates in enclosed spaces within the reactor building—particularly if the systems normally responsible for venting it or recombining it with oxygen in a controlled manner have also been compromised by the same general power loss that caused the overheating in the first place.

When hydrogen accumulates in sufficient concentrations and mixes with the oxygen in the air within those spaces, it forms a mixture capable of exploding upon contact with an ignition source as simple as an electrical spark from otherwise normally functioning equipment.

This mechanism is precisely what was observed in several of the Fukushima Daiichi reactors, where explosions occurred in the upper structures of the buildings—separate from the primary containment vessel—severely damaging the surrounding structure. However, these explosions were not the direct cause of the fuel meltdown; that process was already underway due to the loss of cooling before the explosions took place.

For this reason, this module—which begins by discussing hydrogen accumulating where it ought to be vented—concludes by explaining the logic behind the passive hydrogen recombiners found in modern reactors. Converting hydrogen back into water through a controlled reaction with ambient oxygen, without requiring external electrical power, is the exact solution to a risk that—as this very accident demonstrated—tends to emerge precisely when the active systems designed to prevent it are no longer available.

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