Information is coming out now about the Fukushima Dai-ichi building blasts. Engineers and executives struggled about when, and how to vent the flammable mixture from the reactor buildings.
People might well wonder why Units 1, 3, and 4 all suffered big hydrogen explosions. How could hydrogen achieve the conditions necessary for explosion so easily? Many accidental, combusible mixtures of fuel gas and air in everyday life never catch fire or blow up.
It helps to know that hydrogen-air mixtures are remarkable in their ability to ignite. Here's a NASA hazard analysis for hydrogen as a gas and a cryogenic liquid.
Flammability range: Its lower and upper combustibility limits, when mixed with air at standard pressure, are 4% and 75% by volume. Compared to other fuel gases like propane and methane, the lower limit isn't that remarkable, but the upper limit is. Among common fuel gases, only acetylene has a higher upper flammable limit. Hydrogen mixtures with air are explosive in the 18-59% range.
Energy of ignition: The minimum ignition energy for hydrogen-air mixtures is only a tenth of that needed to start gasoline, methane and propane fires. A spark not visible to the human eye -- less than you feel after walking across a carpet on a winter's day and touching a doorknob -- is enough.
Permeability through flaws in pipes and fittings: High.
Is it burning? It can be hard to tell. Often the human eye can't detect a hydrogen fire in daylight, if nothing else like plastic or grease is burning. Industrial firefighters use infrared flame detectors.
What's it add up to? The fact that hydrogen needs special attention doesn't mean it's "unsafe" compared to other fuel gases. It's got a safety edge, in fact, when it comes to hazards posed by leaks in open air. As readers of Hindenpunk literature know, hydrogen has the lowest density of all gases. It rises and therefore disperses rapidly into the open air when released from an outside source like a leaking pipe flange. That's safer than a gas that spreads along the surface, like chilly methane boiling off a liquefied natural gas tanker spill.