I don't know why. Please help me.

Why did the reactors at the Fukushima plant overheat?

Because of Nuclear power generation?

2 answers

http://www.google.com/search?q=+Why+did+the+reactors+at+the+Fukushima+plant+overheat&ie=utf-8&oe=utf-8&aq=t&client=firefox-a&rlz=1R1GGLL_en___US379

Read widely and take good notes.
This is information I found. Yet I don't understand.

Flooding the reactor sooner could have avoided the need to vent radioactive steam caused by overheating of fuel rods and an explosion caused when steam from the overheating water reacted with zirconium cladding that holds fuel pellets in the reactor vessel to produce hydrogen,