Why did Japan's nuclear plant overheat?

Answer 1

Electrical generators are cooled with hydrogen gas. If the containment of the hydrogen, either the tubes leading from the electrical generators to the cooling water supply fails due to expansion/contraction of the tubes during foundation movement, leaks might occur and the hydrogen, when mixed wit air, becomes explosive.

Most hydrogen created in the nuclear fission reaction are kept within the sealed nuclear tubes are are a very minor source of hydrogen for explosive activity.

Answer 2

Fukushima Daiichi overheated because it was deprived of cooling water.

When the earthquake struck, the plant shutdown properly, and the emergency core cooling systems and backup diesel generators started normally.

Unfortunately, the resulting tsunami damaged the diesel generators and part of the plant's systems, and the cooling water for the emergency cooling system was lost.

Even though the reactor was subcritical, decay heat remained and that overheated the core. Decay heat also overheated the spent fuel rods which were located outside of the reactor in the spent fuel pool, which also depended on power to circulate cooling water.

Answer 3

There are two sources of heat in a nuclear power plant.

There is the heat created from the release of binding energy (Strong Atomic Force) during fission. That is the heat that makes steam that turns the turbine that makes electricity.

And there is decay heat from mixed fission byproducts which are created as a result of that fission.

The control rods stopped the fission. No problem.

The mixed fission byproducts, however, continue to generate heat for a long while as they decay, which is why cooling water is necessary even when the plant is shutdown. Take that cooling water away, and the fuel rods overheat.

Note that there are two places of concern. First, inside the reactor, which is where the control rods are. Second, in the spent fuel pool, where spent fuel from the reactor is moved after it has been removed from the reactor during a refueling operating. By that time, a lot of the mixed fission byproducts have decayed, but there is still residual decay heat.