At the time of the explosion, nuclear power was widely used across Europe and in the United States. In the light-water reactors favored in the United States, water is used as both the moderator and coolant, circulating constantly among the reactor fuel rods. But, in the RBMK (reactor bolshoy moshchnosty kanalny, or high-power channel reactor), the Soviet model that was in operation at Chernobyl, water is used as a coolant, but the moderator is graphite; the fuel rods and the control rods run through chunks of the graphite. The difference in moderator becomes important under the conditions that triggered the Chernobyl accident: In a water-moderated reactor, steam forming in the water will simply slow the reaction by increasing the moderating activity of the water. But steam in the cooling water can increase reactivity in a graphite-moderated reactor. As the cooling water turns to steam, it absorbs fewer neutrons. This means that more neutrons will pass to the graphite, which will slow them down and reflect them back, increasing fission. This process causes the power level to rise, which in turn increases steam formation, and the process can quickly escalate. This feature of the RBMK reactor—the propensity of steam formation and reaction intensity to reinforce each other in fatal positive feedback—is called a “positive void coefficient”(or “positive reactivity coefficient” or “positive-void effect”).

The reactor was, therefore, inherently more dangerous and susceptible to explosion than those in use in Western Europe and the United States. Why did the Soviet Union build such reactors when better ones were available? First, in an autocratic society, there is little free dispute or criticism; if the engineers in the industry learn of a better way to do things or see inherent dangers in the way things are being done, they are unlikely to voice their concerns if another design is favored by the political bureaucracy. Second, in a closed society like the old Soviet Union, where access to information was tightly controlled, Soviet engineers were isolated from their Western European and American colleagues and thus had limited access to innovations in technology.

The problems for reactor Unit Four began shortly after midnight on April 25, 1986, when the engineers decided to run a dangerous experiment to find out if they could squeeze extra work out of the reactor at very low power. The danger was that all the safety systems, which were programmed to shut everything down at once when the power falls, had to be disabled in order to conduct the experiment; if anything went wrong, there would be no way to stop the reaction. And something did indeed go very wrong.

After fiddling with every control on the reactor for over an hour to try to keep the power level where they wanted it, the engineers completely shut off the steam supply to the generator they were testing to see how long it could run on mechanical momentum alone. The first result was that the cooling pumps, which were being powered by that generator, started to run down. The water began to boil out of control, and then, because of that positive void coefficient, the power surged.

The operators spotted the surge and reacted immediately to lower the control rods into the core of the reactor to shut down the reaction (the record indicates that corrective action was taken less than a minute from the initiation of the experimental procedure). But it was already too late. The drive mechanism was slow, for lack of power; when the rods were released to fall of their own weight, the meltdown was already in progress, and they never reached the heart of the reactor. As the intensely

Pripyat, Ukraine, Twenty Years After Chernobyl Explosion. The town of Pripyat still displays empty houses two decades after the 1986 explosion at the now closed Chernobyl nuclear power plant (in background). Radioactive particles settled over the surrounding communities, killing 31 people and injuring and displacing thousands more. Chernobyl remains a warning of the worst-case-scenario of the construction of nuclear power plants, particularly near humans.AP IMAGES.

hot fuel melted the reactor, parts if which dropped in great pieces into the cooling water, a thermal explosion destroyed what was left of the reactor and most of the building. The blast blew off the thousand-ton lid on the reactor core, tore open the building's side and roof, and sent several tons of uranium dioxide fuel, burning graphite, and fission products (such as cesium 137 and iodine 131) off into the night in a three-mile-wide plume, starting numerous fires.

In the ten days following the explosion, some 50 million curies of radioactivity were released into the air. Beginning with the three engineers who had run to see what had happened to the reactor immediately after the explosion, by September 1986 thirty-one people had died; many of the operators and firemen who dealt directly with the explosion and its fires died of radiation burns and poisoning, and the soldiers and volunteers who labored valiantly to cover the exposed core suffered the rest of the casualties. There were roughly 1,000 immediate injuries. Some 135,000 people within a radius of 19 miles of the plant had to be evacuated from their homes in the Ukraine. Ambient radiation continued to increase for weeks from the decay of the melted core.

About 7,000 kilograms of radioactive materials from the core of reactor Unit Four were released into the environment—50 to 100 million curies of radioactive isotopes. The city of Kiev, with 2.4 million people, fared better than some had feared because the winds blew away from the city during the worst period. But several wind shifts brought the nuclear cloud over nearly all of Europe, extending as far north as the Arctic Circle, as far south as Greece, and as far west as the British Isles. Potentially health-threatening levels of radioactive materials were deposited more than 1,200 miles from the plant, in at least twenty countries. The accident first came to light in Sweden on April 28, when technicians noticed atmospheric traces of radioactive gases, mostly xenon and

krypton, that could have come only from the Soviet Union. Shortly thereafter radiation was found in scattered regions throughout Europe.

What consequences have ensued from this nuclear disaster? In addition to the thirty-one deaths from radiation poisoning, some 500 people had to be hospitalized with some form of radiation poisoning. Up to 24,000 of the evacuees received serious doses of radiation. Radiation-caused disorders, including cancer, have been documented in this population, including forty cases of pediatric thyroid cancer, ordinarily a very rare disease, among children from the contaminated villages near Chernobyl. Over the long term, for the region outside the nearest direct exposure, the effects of the disaster remain uncertain.

There are three major health threats from exposure to radioactive materials. First, there is direct exposure, resulting in burns and massive internal injuries, especially to all areas where cells divide rapidly; this type of radiation poisoning killed the operators and rescue workers around the reactor. Second, damage can result from inhaling radioactive dust; many of the citizens of Pripyat may have been injured by such inhalation. Third, there are the radioisotopes that come to rest in the drinking water and the food supply, entering the food chain through the rain and the grass. These are potentially the most worrisome. Iodine-131, entering the body through food or water, was the major threat immediately after the accident; it concentrates in the thyroid and was certainly responsible for the cases of rare thyroid cancer in the children. But iodine-131 has a half-life of eight days and was largely gone from the area in a month or so. Strontium-90, also released in the explosion, has a half-life of twenty-seven years, but it was not present in large quantities.

The worst danger came from cesium-137. It was carried on wind high above the ground and fell where the rain did, along a broad swath of territory from the central Ukraine north across eastern Belarus. Almost 13,100 square miles of agricultural land, dotted with small cities, are contaminated with radioactivity at levels of five or more curies per square kilometer. Cesium contamination forced farmers to destroy produce as far away as Lapland, in northern Sweden, and in Italy and Wales. There is no way of knowing how much damage to health can result from trace contamination of this carcinogenic element. Estimates of cancer deaths attributable to Chernobyl run between 5,000 and 50,000; the wide discrepancy in the estimates indicates how little is known about the long-term health effects of cesium-137.

SEE ALSO Bhopal ; Future Generations ; Nuclear Power ; Pollution ; Precautionary Principle ; Russia and Eastern Europe ; Risk Assessment ; Waste Management .