Chernobyl accident may not be as bad as some reports said. Claims that the Chernobyl accident will devastate Soviet grain production are probably exaggerated. And it is possible that there really have been only two accident-related deaths so far. A nuclear specialist looks at the disaster's possible causes -- and the consequences.

Indications are that the accident at Chernobyl, while serious, may not have been as disastrous as some early reports indicated. Although radioactive isotopes of cesium and iodine have been detected abroad, there has been no plutonium, indicating that a complete core meltdown did not occur.

Computer models being run by the Safety and Reliability Directorate and the National Radiological Protection Board in Britain and Westinghouse Corporation in the United States all show that, based on readings taken in Scandinavia, the accident probably released only about 1 percent of the core inventory of radioactivity.

This release would be on the borderline for causing early deaths, according to John Gittus, head of safety at Britain's Atomic Energy Authority. Thus, it is possible there have been no deaths yet other than the two that the Soviets have announced.

Contamination of the area up to three miles from the plant will be sufficient to keep people from living or farming there for years unless a major cleanup is conducted. But this contamination will be concentrated in the initial direction of the plume north of the station.

Claims that the ``breadbasket of the Soviet Union,'' the grain-growing area south of Kiev, will be badly affected are probably wildly exaggerated. What contamination will arise will come mainly via the rivers that flow south, and is unlikely to be extensive. The contamination in Europe is only six times the normal background level, at worst, and is not a significant health risk. The very high levels of ground contamination in Sweden probably arose because a rain storm coincided with passage of the centerline of the contaminated plume over the country.

No one knows for certain if the Soviets have succeeded in putting out the fire at Chernobyl. If water were used, a very large amount would be needed. The radiation levels probably mean that remote vehicles would have to be used.

The Chernobyl station consists of four operating 1000-megawatt RBMK reactors, with another two under construction. Each reactor rests on a welded steel structure in a metal-lined concrete pit. The graphite blocks of the core are built up in this cavity, with channels for the fuel and safety systems. The fuel, containing 180 metric ton of uranium, is contained in 1,693 vertical pressure tubes.

In normal operation, water is passed through the pressure tubes containing the hot fuel and is turned to steam, which drives the turbine generators. The problem with this reactor design is that the red-hot graphite of the core is separated from the steam only by the quarter-inch pressure tube containing the fuel.

The safety implications for nuclear power plants in the rest of the world are limited, because the Chernobyl reactors are of a water-cooled, graphite-moderated design, the RBMK, only built in the Soviet Union.

Although it is not clear exactly what caused the accident, there are two likely scenarios:

Most probably, there was a rupture of a single pressure tube. The escaping superheated steam reacted with the red-hot graphite, producing a number of gases, including carbon monoxide and hydrogen. Either the initial reaction caused a pressure buildup in the graphite which later blew the core apart, or the hydrogen seeped into the containment building where it later reached a critical concentration and there was a violent explosion. This blew the roof off the containment, destroying the safety systems, and the graphite caught fire.

In the second scenario, there could have been a catastrophic failure of one of the huge welded pressure vessels, known as steam drums, that separate steam from the reactor's primary circuit for delivery to the turbines. Rapid failure would have led to a large explosion, and would also cause total loss of coolant possibly within a few seconds. The result would be overheating, fuel failure, and vaporization of parts of the core.

The author is deputy editor of Nuclear Engineering International.

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