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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.

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 diesel backup generators needed to provide power for the cooling system. Batteries provided power for only up to eight hours. Mobile generators were brought in to take over.

Still, it's too early to know for sure what sequence of events led to what outcome, said David Lochbaum, who directs the Union of Concerned Scientists' Nuclear Safety Program and has worked at three nuclear plants in the United States similar to the General Electric plants in Japan.

According to the 1979 Kemeny Commission report on Three Mile Island—the definitive document of that disaster—"equipment failures initiated the event," but "operator error" was the "fundamental cause of the accident." Emergency cooling systems were shut down, with dire consequences. Three Mile Island would have been a "relatively insignificant incident," the commission found, if the plant operators (or those who supervised them) had kept the emergency cooling systems on through the early stages of the accident.

At the Chernobyl reactor in Ukraine, an "ill-conceived, badly executed safety test" initiated the disaster, said Wilmshurst. A sudden surge of power triggered a steam explosion that ruptured the reactor vessel, according to a recent report from the United Nations. This allowed "further violent fuel-steam interactions that destroyed the reactor core and severely damaged the reactor building."

Understanding the Problem

The level of access to information about what is going on inside a reactor has increased in the decades since Three Mile Island and Chernobyl.

As Peter Bradford, who served on the U.S. Nuclear Regulatory Commission at the time of Three Mile Island, said this week, "At Three Mile Island, much of what we thought we knew on the third day turned out to be incorrect." The extent of fuel melting, and even the fact that a hydrogen explosion had occurred in the containment on the first day, he said, did not become clear for years. "There was all kinds of information . . . we didn't know," he said.

During the first few minutes of the accident at Three Mile Island, more than 100 alarms went off, and no system was in place to filter out the important signals from the insignificant ones, according to the 1979 Kemeny report. "Overall, little attention had been paid to the interaction between human beings and machines under the rapidly changing and confusing circumstances of an accident," the commissioners wrote.

By contrast, said Bradford, the level of computerization and information transfer available today could give Japanese officials much more insight to what happens in the four troubled reactors at Fukushima—at least in theory. "They've got so much more going on in terms of the earthquake and the tsunami that we didn't have at TMI, that I'm sure that the situation is every bit as confused," Bradford said.

Radiation Containment

Like the Three Mile Island plant, the Fukushima reactors have three barriers designed to prevent radiation leakage, including metal cladding surrounding the nuclear fuel, a reactor pressure vessel, and the primary containment vessel. Chernobyl lacked a containment vessel, said Wilmshurst.

Once radiation is released into the environment, it can contaminate vast areas. "Contamination levels are not linear," said Lochbaum. "Further away you don't necessarily get lower doses," he explained. Among other factors, prevailing winds can influence what areas are affected. In the Chernobyl accident, some areas 100 miles away from the facility had radiation levels higher than areas just 10 or 20 miles away.

"The Chernobyl pattern was quite erratic," said Lyman. Radiation was released "very, very high because of the nature of the reactor and graphite fire." Weather changed over a prolonged emission period, as a graphite fire burned for 10 days. So radioactive gases and particles were picked up by wind and carried high in the atmosphere over long distances before raining down on communities far from the source, he said.

Ultimately, the radiation released as a result of Three Mile Island was not high enough to present detectable health effects in the general population. That accident rated as a level 5 of 7 on the International Nuclear Event Scale, an "accident with wider [than local] consequence."

At Chernobyl, a level 7 "major accident," radiation exposure affected thousands of people.

Fukushima Daiichi has been elevated to level 4—"accident with local consequences. But it remains to be seen how much higher on the scale this incident will go. In Tokyo, 180 miles away from the plant, peak radiation levels were recorded at 23 times above normal at one point on Tuesday, but they reportedly dropped to about 10 times above normal later in the day.

Exposure in Perspective

In the United States, the average radiation dose from natural background and man-made sources, such as medical procedures and consumer products, is 620 millirems (mrem) per year, according to the NRC.

One millisievert (mSv) is equal to 100 millirems. The Japanese Ministry of Health Labor and Welfare on Wednesday lifted the maximum allowable exposure for nuclear workers to 250 mSv, from 100 mSv, the Associated Press reported. According to the Nuclear Energy Institute, radioactivity at the plant hit a dose rate of 1,190 mrem per hour Tuesday evening, but dropped to 60 mrem per hour six hours later.

The Chernobyl accident caused acute radiation sickness in 134 of the 600 workers who were at the site on the morning of the initial explosion and received high doses of radiation—80,000 to 1.6 million mrem, according to the UN report and the U.S. Nuclear Regulatory Commission. Of this group 28 people died within three months. Two others died due to injuries from the fire and radiation. And eventually as many as 4,000 people are expected to die as a result of radiation exposure from the Chernobyl plant, according to the World Health Organization.

From a public health perspective, Chernobyl's greatest impact was an epidemic of thyroid cancer (more than 6,000 cases so far) among children and adolescents exposed to radiation, often by drinking contaminated cow's milk.

Crisis Communication

The global nuclear power industry today, said Wilmshurst, has "come together" to share information in an effort to help resolve the dangers at Fukushima. There is significantly more communication within the industry now than there was during the disasters at Three Mile Island and Chernobyl, he said.

Communication during a nuclear crisis, of course, must extend beyond industry, and in this area plant operator Tokyo Electric Power (Tepco) is facing harsh criticism. On Tuesday the director general of the International Atomic Agency, Yukiya Amano, called for Japanese counterparts to facilitate stronger communication. According to the Kyodo News Agency, Prime Minister Naoto Kan admonished Tepco executives in a meeting Tuesday after he learned about an explosion from TV, rather than receiving a call from Tepco. He reportedly demanded to know, "What the hell is going on?"

As the Three Mile Island emergency unfolded, officials attempted to reassure the public that the "danger was over," even as efforts to cool the reactor and stabilize the plant proved ineffective. And in Chernobyl, information hardly flowed at the speed of Twitter. In the view of the London-based World Nuclear Association, an industry group, Chernobyl "was a direct consequence of Cold War isolation and the resulting lack of any safety culture."

The U.S. Environmental Protection Agency wrote in a 1986 journal article on the accident that, "Chernobyl was a secret disaster at first." In fact, the earliest evidence for the international community that a major nuclear accident had occurred came from Sweden, where the discovery of radioactive particles on nuclear plant workers' clothing instigated a search for the source of radioactivity. The following day, the Soviet news agency confirmed the accident at Chernobyl plant but did not offer details, according to the EPA account. "The resulting information vacuum fueled rumors of all kinds, from fatality estimates to speculation about fires in adjoining reactors."

As the crisis in Japan intensifies, officials have come under fire for statements that in hindsight seem to underestimate the escalating threat. Arjun Makhijani, president of the Institute for Energy and Environmental Research, criticized Japanese authorities for "working from a standard nuclear industry playbook whose byline seems to be, 'What, me worry?'"

Makhijani is calling for "a frank appraisal of what is known and not known and the potential range of damage and consequences." This would afford the public more confidence in the pronouncements. As it is, he said, "verbal reassurances about low radiation levels stand in stark contrast to repeated increases in the radius of evacuations."

As the Wall Street Journal reports, Japan's government has complained about the slow release of information from Tepco. Edwin Lyman, a physicist in the Union of Concerned Scientists Global Security Program and former president of the Nuclear Control Institute, commented in a call with reporters Tuesday that Tepco's briefings are becoming "less and less transparent."

"There's clearly a kind of erratic quality to the information coming out by the Japanese," Lyman, said. But this could be explained, he said, by the fact that they're still struggling to find out what's going on. "There's a staggering amount of confusion on the ground," added Union of Concerned Scientists nuclear expert Ellen Vancko.