The problems at the Fukushima No. 1 nuclear power plant have raised comparisons to Chernobyl, where the worst nuclear accident ever occurred 25 years ago.
But there are major differences that could have negative--as well as positive effects--on what unfolds at Fukushima.
The No. 4 reactor at Chernobyl exploded and a fire lasting for 10 days spewed radioactive materials into the atmosphere, spreading contamination over a wide area.
However, the possibility is low that a large fire will break out at the Fukushima plant.
On the other hand, the six reactors at the Fukushima plant have a much higher combined power output, and the volume of radioactive materials contained in the reactors is several times greater than at Chernobyl.
A large explosion rocked the No. 4 reactor at Chernobyl in April 1986 in the Ukraine. Exposed fuel rods in the core emitted large volumes of toxic radioactive materials. The Soviet government forcibly evacuated 116,000 residents living within a 30-kilometer radius of the reactor.
A study by the International Atomic Energy Agency 20 years after the Chernobyl disaster found about 200 people were exposed to fatal levels of radiation and several thousand people had either died or were expected to die from radiation exposure.
The outputs of the reactors at the Fukushima No. 1 plant range from 460,000 kilowatts at the No. 1 reactor to 784,000 kilowatts for the No. 2, No. 3 and No. 4 reactors. The combined power output is about three times the 1 gigawatt output at Chernobyl.
There is a high possibility of large amounts of toxic radioactive materials in the fuel rods, but the levels depend on how long the fuel rods have been used.
The type of reactor used is also very different.
The Chernobyl plant used graphite as a neutron moderator to produce nuclear fission of the fuel rods. The reactor had no core containment vessel. After the explosion, the graphite, which is made of carbon, burned along with the fuel rods in an exposed state.
The nuclear fission produced radioactive materials such as cesium, plutonium and iodine. The updraft from the subsequent fire carried the radioactive materials high up into the atmosphere and far away from Chernobyl.
The strontium that contaminated land surrounding the reactor accumulates in bones and is a cause of leukemia.
Like other industrial nuclear power plants in Japan, the Fukushima plant uses a light-water reactor. No graphite is used, and there are few combustible materials in the core. For those reasons, a fire like at Chernobyl is highly unlikely.
The worst accident involving a light-water reactor was the 1979 accident at the Three Mile Island nuclear power plant in the United States.
Keiji Miyazaki, a professor emeritus of nuclear engineering at Osaka University, said, "The events at the Fukushima No. 1 plant is very similar to what happened at Three Mile Island."
In the initial stages of the Three Mile Island accident, nuclear fission was stopped by the control rods, but the core could not be cooled, leading to a core meltdown in which the fuel rods and control rods melted.
The overheated core also melted stainless steel parts within the core container.
The 2 million people who lived within an 80-kilometer radius of the nuclear plant were exposed to radiation levels of an average 0.01 millisievert.
Hiroaki Koide, an associate professor at Kyoto University's Research Reactor Institute, said: "If the melted core should come in contact with water, it could cause a steam explosion. That might lead to the possibility of destroying the pressure container. A huge explosion could occur or it could end in a similar way to what happened at Three Mile Island."
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