How a nuclear reactor overheats - KWWL - Eastern Iowa Breaking News, Weather, Closings

How a nuclear reactor overheats

NORTHERN JAPAN (KWWL) -- The problems at the quake-rattled nuclear plant in northeast Japan started as soon as the earthquake hit on Friday.  Usually water cools the fuel rods inside a nuclear reactor to maintain their temperature at about 500 degrees Fahrenheit.

But if the cooling fails, the temperature can rise to some 2,200 degrees.  That's hot enough to melt the fuel rods.

When the earthquake hit the first safety system to prevent a melt-down was activated.  Control rods rose into the nuclear reactor to stop the nuclear fission, but the fuel rods were still hot.  Water should have been circulated to cool them down but that didn't happen because of a power outage right after the quake.  So the second safety system turned on - the generator began spraying the rods with coolant.

But an hour later the emergency generator stopped, possibly because the of the tsunami, which hit around that time.

The third safety system, which converts steam traveling through the pipes into water, kicked in.  But the water level went down and the temperature continued to rise.  All three safety measures had failed.

A professor from Japan's Atomic Energy Commission who involved in the construction of the Fukushima plants thinks the cooling water somehow leaked from the reactor.  He said, "The reactor's coolants must have leaked somewhere in the building. We thought we had taken adequate precautions for a tsunami, but what happened was beyond our expectations."

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