How does a nuclear power plant work?

The plants use radioactive material, such as uranium, to produce energy through what is called a chain reaction. In the nuclear reactions, atoms burst into pieces, and among the products are radioactive particles and heat. The heat can be used for power, as in burning coal or oil, by making steam to drive turbines that generate electricity.

How is the Soviet reactor different from most of those in America?

Most American reactors have all their fuel -- uranium pellets stacked in rods -- in one large pool of water inside a special steel and concrete containment building. At the Chernobyl plant, each of 1,693 fuel rods is in a separate water-filled tube inserted into a large graphite block -- all inside a building with no special containment structure.

What is a meltdown?

It occurs when radioactive nuclear fuel melts. The melting may begin when the plant's cooling system breaks down or equipment to control nuclear reactions fails. At the Soviet plant, graphite helps keep nuclear reactions under control by absorbing neutrons, the atomic particles that trigger the reactions.

What happens in a meltdown?

The molten fuel is so hot it turns the cooling water around it to steam. The fuel partly vaporizes, sending dangerous radioactive particles into the air with the heat and steam. If not held within a containment structure, the radioactive particles released into the air come down as radioactive fallout.

Why is there a fire at Chernobyl?

The material used to dampen and control the nuclear reaction is graphite, a material heavy with carbon like charcoal. The fuel rods, constantly cooled in water-filled tubes, sit inside a great array of graphite blocks. When the fuel overheats and breaks out of the pressurized tubes, it can set the graphite blocks on fire.

How does radiation get out of the plant?

Once fuel begins to melt and a graphite fire starts, great pressure can build up inside the building, causing leaks. Or explosions connected with the meltdown may rupture the roof or walls of the buildings. Radioactive particles are drawn up and out of the plant like fire up a fireplace.

Can dangerous radiation spread far? Is there any danger of fallout in America?

The most dangerous radiation would fall to the ground at or relatively near the plant. But, depending on the severity of the meltdown and graphite fire, an array of dangerous radioactive particles could be carried for scores or hundreds of miles. The greater the distance from the plant, the less serious the hazard. The amounts found so far in Sweden, 700 miles away, are not dangerous to health. No harmful amount is expected to reach the United States.

How does Chernobyl compare to Three Mile Island?

A very small amount of radiation was released into the air at Three Mile Island, and no adverse health effects have been found in the population around the TMI plant. For Chernobyl, no reliable figures are available, but based on estimates from the radiation detected in Sweden, a thousand to a million times as much radiation has been released at the Chernobyl site.

What does radiation do to people?

The effects depend on the amount, just as with the more familiar kind of radiation, sunlight. Particles of radiation penetrate the skin: they can enter a very short way and some can go deep into the body. Over a long term, the way the particles damage cells and genetic material can cause cancer. Extremely heavy exposure can cause burning of the skin and a poisoning that affects the blood. Large doses of radiation can be fatal.

What amounts of radiation can cause sickness?

Although small amounts given to a large population can raise the likelihood of cancer in that population, sickness immediately triggered by radiation begins at whole-body exposures of tens to hundreds of "rads," the units used to measure radiation. The radiation recorded at the Three Mile Island accident was estimated at many thousands, possibly a million, times less.

What about food and water?

At Chernobyl, there undoubtedly will be contamination of land and water, including the nearby Pripyat River that feeds Kiev's municipal reservoir. Radioactivity can enter the food chain several ways, such as when cows eat contaminated grass, concentrating the radiation in their milk. Fallout on farm crops could render them inedible. Areas getting the most contamination may have to be evacuated for many years, because radioactivity can take decades to decay.