STOCKHOLM, Sweden, April 29, 1986 (UPI) - Swedish nuclear experts today said the accident at a giant Soviet nuclear power plant may have led to a reactor meltdown.
A Soviet diplomat in Helsinki admitted the accident was ''the worst ever in the world.'' He did not elaborate.
''The accident may very well involve a reactor meltdown,'' said Lars Erik de Geer of Sweden's military research establishment. He said that at worst such an accident might lead to the evacuation of the city of Kiev.
Another Swedish nuclear expert said, ''The composition of the radioactive particles indicates there may have been some damage to the nuclear core.''
Hours before Moscow acknowledged the accident at the Chernobyl nuclear power plant north of Kiev, Swedish officials suspected such an atomic power plant accident was responsible for depositing a carpet of nuclear dust over Northern Europe.
Swedish authorities reported the radioactivity over Sweden had dropped 50 percent today from Monday's levels.
But a spokesman for the radiation institute said the drop does not necessarily mean the radioactive leak had ceased, explaining that weather conditions could thin out or change the direction of the nuclear cloud.
The institute warned Soviet-bound tourists to stay at least 60 miles away from the ill-fated nuclear plant and to avoid the area between Kiev and Lithuania.
An unprecedented statement was issued by the official Tass news agency after Sweden, Finland, Denmark and Norway reported high levels of radioactivity they believed had emanated from a nuclear accident in the Soviet Union.
The Tass statement did not mention casualties from the accident at the plant 80 miles north of Kiev, the Ukranian capital with a population of 2.35 million in the western Soviet Union. The plant, which had at least four reactors, is about 1,000 miles south of Stockholm.
''We have registered radioactivity just about everywhere we have looked,'' said Ragnar Boge of the Swedish Radiation Institute.
The level of radioactivity in Sweden and Denmark was five times above normal, while Finland reported levels up to 10 times higher. Parts of Norway recorded radiation levels 60 percent above normal.
Swedish Energy Minister Birgitta Dahl said the radioactive cloud drifted from Kiev north and then northeast over Finland, on to middle and southern Sweden, then to Norway and Denmark.
Olof Hermander, chief of the Swedish nuclear inspection board, said the discharge probably resulted from a Soviet reactor overheating to the point where isotopes escaped into the atmosphere, a process that can lead to a meltdown of the reactor.
''The radioactivity apparently came from a civilian nuclear power plant,'' said Swedish Energy Minister Birgitta Dahl. ''We can trace the emissions over a curve towards the southeast.''
Swedish officials had discounted the possibility that a nuclear test explosion set off the radioactive cloud, noting an underground test would have been registered by Sweden's sensitive Hagfors seismological institute.
A U.S. NATO expert said it was highly unlikely the cloud could result from an atmospheric test, as ''that would involve a violation of the atmospheric test ban treaty.''
Sweden received no prior information of the nuclear discharge, said Dahl, and had planned to demand a full explanation to this ''unacceptable'' break with international agreements.
In Moscow, a Swedish diplomat said he telephoned three government agencies including the Atomic Energy Commission ''and they had no reasonable explanation for it (the radiation).'' Hours later, Tass issued its statement.