The Chernobyl nuclear power plant may have sustained at least a partial meltdown of its fuel-containing core, according to American scientists interpreting yesterday's reports that Swedish scientists had detected forms of radioactive fallout usually indicative of such an event.
This would be similar to the 1979 incident at the Three Mile Island atomic power plant, this country's worst accident at a commercial nuclear facility, except that the Pennsylvania plant has a steel-and-concrete "containment" building that enclosed the reactor and prevented radioactive atoms from escaping into the environment. The Soviet facility is not believed to have a containment facility.
The evidence of a meltdown at Chernobyl, according to James McKenzie, senior staff scientist at the Union of Concerned Scientists, is the fact that radioactive atoms of iodine and cesium were detected in the atmosphere over Stockholm. Both elements are produced in the nuclear fission reaction that creates a reactor's heat, but they normally stay in the solid-fuel rods. The atoms could be released only if the fuel melted.
Fallout containing radioactive iodine can pose a health threat because the element often finds its way into the grass that dairy cattle eat and becomes concentrated in their milk. In previous incidents of this sort, health officials in various countries found it necessary to dump milk that contained too much radioactive iodine. Eventually, however, radiation decays and subsequent milk production is safe.
"There are all kinds of mishaps that could lead to a core meltdown, if that's what happened, but basically what causes it is that the fission is producing heat faster than your cooling water takes it away," McKenzie, a physicist, said.
As a result, heat builds in the core and may reach the fuel's melting temperature. Vapors from the hot fuel, carrying radioactive atoms, can boil out of the reactor and into the surrounding air.
Atomic power plants make electricity by using the heat of a controlled nuclear fission chain reaction to turn water into steam whose pressure, in turn, spins turbines connected to generators. If the fission is not controlled well enough or if the water, flowing in pipes near the fuel, does not carry away enough heat, a meltdown can result.
U.S. nuclear plants have emergency core-cooling systems that are supposed to go into action if core temperatures get too high, but some have been found defective. It was not immediately known whether the Chernobyl plant had such a cooling system.
The exact cause of a meltdown at the Soviet plant, however, is hard to pinpoint, McKenzie and several other U.S. experts on nuclear power plants said, because the Chernobyl reactors are of a Soviet design very different from any in the West.
They are water-cooled, graphite-moderated reactors. Four are believed to have been in operation at Chernobyl, each generating 1,000 megawatts of electricity. The design is believed to be fairly old, and several authorities said the Soviet Union has been developing a more advanced reactor that is more like American designs and includes containment domes.
"The plumbing on them is quite different from the reactors we have," McKenzie said. "One thing is pretty certain, though. There's no way this could have been a nuclear explosion like a bomb. The fuel they use is a very low-enriched uranium that couldn't explode."
Scientists differed widely in their opinion of how much radiation was released.
Because Soviet sources are silent on this subject, the only source of evidence is the intensity of radioactivity measured in Stockholm, about 700 miles northwest of the plant, and other West European centers. Reports differed yesterday but they generally placed the levels well below anything that would cause a significant health hazard.
According to The Associated Press, Ed Zebroski, chief nuclear scientist for the Electric Power Research Institute in Palo Alto, Calif., said, "If the radioactivity is a few millirem 700 miles away, I'd hate to be within 10 miles" of the damaged plant.
Other scientists noted that winds can carry the radiation from the source into a high arc well above nearby areas that then descends some distance away. According to the National Oceanic and Atmospheric Administration, the prevailing winds near the Chernobyl plant were from the south at 5 to 10 miles per hour, but it could not be determined whether the winds kept the radiation close to the ground or lofted it higher.
Generally speaking, the higher the radiation is carried, the farther it spreads and the more diluted it becomes.
Although the official Soviet statement referred to casualties, it was not clear that these were people injured by radiation. The American scientists said there may have been something like a steam explosion that injured workers in the plant as it disabled the core-cooling system, leading to the meltdown.