三里岛核事故分析-英文(DOC)

How Is Japan's Nuclear Disaster Different? Fukushima Daiichi may be no Chernobyl, but it has overshadowed Three Mile Island.

The control room at Chernobyl's Reactor No. 4 is shown here. Reactor design, wind patterns, communication and other factors can cause differences in the severity of nuclear accidents.

Photograph by Gerd Ludwig, National Geographic

Josie Garthwaite

For National Geographic News

Published March 16, 2011

This story is part of a special series that explores energy issues. For more, visitThe Great Energy Challenge.

For decades, Three Mile Island and Chernobyl have served as shorthand for the nightmare of nuclear power generation gone awry. In the wake of Japan's deadly earthquake and tsunami last week, the still-unfolding disaster of Fukushima Daiichi has come closer than any nuclear crisis in history to making it a fearsome trio. (Related Story: "Japan Tries to Avert Nuclear Disaster")

It remains to be seen how much damage will be caused by the crisis at the Fukushima nuclear power complex, where four of the six reactors have seen a range of woes including three explosions in four days, damage to two containment vessels, possible

overheating from spent fuel rods, and mounting peril for the last remaining 50 workers due to dangerous spikes in radiation emissions.

Yet it is already possible to outline key differences that set the current Fukushima situation apart from the 1979 Three Mile Island emergency near Harrisburg, Pennsylvania, and the disaster in Chernobyl, Ukraine, that unfolded seven years later. Reactor Type

Japan's Fukushima Daiichi nuclear power complex, which began operating in the 1970s, is made up of six boiling-water reactors, or BWRs—a type of "Light Water Reactor." (Using ordinary water, it is distinguished from "heavy water reactors," which use deuterium oxide, or D2O, instead of H2O.) Three Mile Island used another type of Light Water Reactor known as a pressurized-water reactor, or PWR.

Both of these reactors use water for two purposes. It acts as a coolant, carrying heat away from the nuclear fuel, and as a "moderator," slowing down the release of neutrons during fission reactions, explained Neil Wilmshurst, vice president of the nuclear sector at the U.S. Electric Power Research Institute, the industry's nonprofit research organization.

In a PWR, the water is kept under pressure. This means the temperature can be higher than the boiling point of water without generating a significant amount steam (a less efficient coolant), said Wilmshurst. So the reactor core operates at a higher temperature in these systems, and heat can be transferred more efficiently.

Boiling-water reactors operate at lower temperatures, and they tend to be simpler, with fewer parts, said Wilmshurst.

Chernobyl's reactors were a type called RBMK (for the Russian, "reaktor bolshoy moshchnosty kanalny"), which also used water for the coolant. But unlike the Light Water Reactors, the RMBK used graphite as a moderator. According to the World Nuclear Association, an industry trade group based in London, no other power reactor in the world combines a graphite moderator and water coolant as Chernobyl did, although Russia does have several RBMK reactors in operation.

Most nuclear reactors in the United States today use either BWR or PWR technology, which Wilmshurst and EPRI say are "equally safe." Both types of reactors have a kind of self-regulation or "negative feedback" loop: As the reactor gets hotter, the fission reaction slows down, decreasing power, said Wilmshurst. The RMBK design, on the other hand, "could go into positive feedback," where higher temperature begets more power, which in turn increases the temperature, and so on.

Accident Cause

At this point in the Fukushima disaster, Wilmshurst said, the tsunami appears to be the immediate culprit, since the plants shut down as they were designed to do following the earthquake. When the tsunami hit an hour later, it damaged the site infrastructure, he said. So while the earthquake had cut the reactors' external power supply, which is needed to keep coolant pumps doing their job, the tsunami killed the