Here are the main points of how the two accidents differ.
ARE THE TWO DESIGNS THE SAME?
Unit 4 at Chernobyl was a water-cooled and graphite-moderated reactor -- a combination that can and did yield a runaway chain reaction. A series of gross errors and misjudgment by operators resulted in an explosion and fire that catapulted radioactivity into the upper atmosphere.
The resulting release of radiation has been compared to 10 times that released by the 1945 U.S. nuclear bomb attack on the Japanese city of Hiroshima.
The boiling water reactors at Fukushima do not have a combustible graphite core. The nuclear fuel in reactors No. 1, No. 2 and No. 3 was allowed to melt at least partially, but operators have since succeeded in cooling both the reactors and the spent fuel pools and no chain reaction is happening now.
As long as cooling operations continue and Japan can prepare tanks fast enough to store the contamination overflow, Japan can still hope to buy time to figure out how to bring the reactors to a cold shutdown.
HOW DO THE CONTAINMENT STRUCTURES DIFFER?
Chernobyl had no containment structure and nothing stopped the trajectory of radioactive materials into the air.
Fukushima's reactors are built on granite foundations and are surrounded by steel and concrete structures. The reactor vessels and containment structures, as well as some of the pipes leading from the reactors, are likely to have been damaged by the March 11 tsunami and recurring earthquakes. But with radiation levels now down to a sliver of what they were at the peak, experts say that the structures are still holding.
Chernobyl contaminated an area as far as 500 km (300 miles) from the plant, and an area spanning 30 km (18 miles) around the plant is still an exclusion zone and uninhabited.
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