In 1986, the Chernobyl nuclear reactor in Ukraine (then still a part of the Soviet Union) exploded, dynamite-like, from a runaway nuclear chain reaction. The explosion was enough to destroy most of the reactor building, but that was it. The disaster that followed came not from the explosion but from the enormous amount of radioactive debris that was released. The number of excess cancers caused by this radiation is plausibly estimated at 24,000. Fortunately, many of these are thyroid cancers and are readily treatable.
Unlike at Chernobyl, the reactor at Fukushima did not explode. A buildup of hydrogen gas caused the upper building to explode, but the reactor itself survived the tsunami, turned itself off, and sat safely for several hours. Even though the chain reaction had stopped, there was enough radioactive material in the core to generate dangerous levels of heat, but cooling pumps initially kept the situation from getting out of hand.
The most modern reactors don't need these pumps; they are designed to use the natural convection of the water to keep it circulating. But the Fukushima reactor was not the most modern kind. It depended on auxiliary power systems to keep the pumps going. These worked well, despite the devastation of the earthquake and the tsunami, and they kept the reactor cool.
But the auxiliary cooling systems were not expected to work forever; they were designed to last about eight hours, after which normal power was supposed to be restored. That plan did not anticipate the enormous destruction of the infrastructure that was wreaked by the tsunami. The emergency power ran out and the fuel overheated, much of it melting. It was the loss of electricity that killed the plant.
The meltdown at Fukushima led to a huge release of radioactivity—much greater than had happened in the prior U.S. nuclear reactor accident at Three Mile Island in 1979. The release of radioactivity was second only to that of the Chernobyl accident.
One horrible fact of the Chernobyl disaster was that the public was not protected from debris laced with iodine, cesium and strontium until a great deal of harm had been done. Residents continued to drink milk from cows that had eaten contaminated grass. By contrast, officials at Fukushima ordered evacuation of the accident vicinity and banned the consumption of food grown in the region.
Since the accident in 1986, the Chernobyl region has experienced high rates of illness and premature death, but careful analysis by the United Nations has attributed most of these problems to the high cigarette and alcohol consumption that has long been endemic to the region. "No evidence or likelihood of decreased fertility among the affected population has been found," a 2005 UN investigation concluded, "nor has there been any evidence of congenital malformations." The report went on to say that the largest public-health problem created by the Chernobyl accident was the psychological overreaction of residents, who had been traumatized by exaggerated fears of induced cancer and other defects.
—Dr. Muller is a professor of physics at the University of California, Berkeley. Adapted from his new book, "Energy for Future Presidents: The Science Behind the Headlines."