The radiation released from the Chernobyl reactor since Saturday has been greater than that given off by a typical atmospheric nuclear bomb test in the 1950s, according to the first detailed analysis of emissions from the accident.

The computer analysis of the amount of radioactivity and its spread shows a massive meltdown of the plant's reactor core that vented most of the radioactivity in the first 24 hours of the accident. The analysis was done by researchers at the Lawrence Livermore National Laboratory in California.

The explosion of a one-kiloton bomb releases radioactivity measuring about 150,000 curies, said Joseph Knox, head of Livermore's division of atmospherics. He calculates that 60 million to 70 million curies of radiation would have been inside the Chernobyl reactor at the time of the accident. Even the most conservative estimate would suggest that many millions of curies of radiation escaped.

Most of the radiation was dumped at the reactor site, elsewhere within the Soviet Union and in Eastern bloc nations. The radiation cloud, now diminished, is expected to reach the United States within days. Knox said that when it does, the amount of airborne radiation will be small and will not cause adverse health effects.

The cloud is expected to circulate around the earth several times before becoming undetectable.

Livermore's new information puts the time of the Chernobyl meltdown as early Saturday. On Sunday, Finland and Sweden experienced their peak levels of radiation exposure. Because of their distance from Chernobyl, and because rain and chemical recombination diluted the radioactivity, those two countries registered peak amounts that just breached the International Atomic Energy Agency's permissible limit for a year, not counting ordinary background levels.

Above that limit, health effects theoretically might be expected. But by Wednesday, airborne radiation levels of such elements as iodine-131 were about one-third of the permissible limit.

But, Knox said, the figures suggesting no substantial health hazard in Western Europe do not hold for Eastern Europe. Livermore will soon calculate an airborne radiation level for nations there, such as Poland. Knox said it is likely that levels in the eastern countries will be significantly above the permissible limit, and adverse health effects may be expected.

In making their calculations, the scientists used supercomputers to pull together information, including measurements of radiation from air and ground stations in several countries, weather patterns in Eastern Europe and Scandinavia, the nature of the radioactive elements, and the operating history of the reactor, including how long and at what power it had been on.

From this, they estimated the amount of radioactive material in the plant at the time of the accident, and what percentage was released. "We estimate that nearly all of the possible volatile fission products have been released," the Livermore report said.

In addition to the volatile radioactive elements -- those most easily vaporized and sent into the air if a reactor overheats -- elements rarely found to be released were not only emitted in substantial amounts, but found hundreds of miles away.