Why did the hydrogen explosions happen at the Fukushima nuclear plant?.
Because of the explosion at reactor 1, the building shell violently collapsed on March 12th. The same type of explosion occurred at reactor 3 on March 14th, and at reactor 4 on March 15th.The wall of the building shell is ferroconcrete 1m-thick, so you can imagine how fierce the explosions were. These explosions are“hydrogen explosions”caused by leaked hydrogen and oxygen in the air inside the building shell.

What is a hydrogen explosion?

A “hydrogen explosion”occurs when air includes more than 5% in density of oxygen, and more than 4% in density of hydrogen,they then ignite with a resulting explosion. If the temperature is more than 500℃, it will ignite spontaneously and explode. This hydrogen explosion is thought to be because of some ignition rather than spontaneous fire.

Where did hydrogen come from?

A nuclear power plant is similar to an electric pot. It has a very high-temperature reactor core that boils water and generates steam. Vapor temperature is about 280℃. Though the power plants were shut down immediately after the earthquake, the nuclear fuel continued, and continues, releasing heat. (see "Why cool the reactors?")

Because of the loss of power, the circulating system of cooling water failed to work, and the water level of the reactor pressure vessel fell. Usually, fuel rods must be covered with cooling water, but this time the rods were exposed and its temperature rose around 1000 to 1500℃. Nuclear fuel is coated with a compound of zirconium called “zircaloy”. When zirconium is heated, it reacts vigorously with steam or water, is then oxidized and releases hydrogen.

Link: "Why cool the reactors?"

A hydrogen explosion occurred with the accumulation of hydrogen and oxygen

Hydrogen generated in a reactor pressure vessel is thought to have been leaked through pipes and orifices in the pressure vessel and the surrounding containment vessel into the building shell. It then mixed with oxygen and ended up with a hydrogen explosion.

The hydrogen explosion in reactor 4 was caused by hydrogen coming out from the spent fuel pool. In that pool, there were (and are) used nuclear fuel rods that released and still release decay heat. As the water level dropped, the used fuel rods were exposed to the air and hydrogen was formed. Though cooling water is being added into the pressure vessels and the spent fuel rods pools, the rods may be still partially exposed. This means the possibility of another explosion. On April 7th, nitrogen was injected into the containment area. Nitrogen is 78% of the air we breathe, and it is a nonflammable gas. By injecting nitrogen, it can prevent further hydrogen explosions.

Science Communicator: Moeko Tabata