It was reported incorrectly Sunday that investigators have not been able to enter the containment dome at the Three Mile Island nuclear power plant. The dome has been open for inspection since July 1980.

In the four weeks since the disaster at the Chernobyl nuclear power station, atomic power experts in the United States have pieced together enough details to begin to understand what happened in the worst radiation release in the history of nuclear power.

Although many key facts remain unknown, probably even to Soviet authorities, the picture is one of a complex chain of events ranging from the activity of subatomic particles deep in the reactor's core to the behavior of the entire global atmosphere.

What follows is a best-guess scenario, a synthesis of details and interpretations obtained from U.S. experts in nuclear power and radiation medicine, some of whom have recently visited the Soviet Union, from published documents on Soviet reactors of the type at Chernobyl and from official statements by Soviet authorities.

A full account, if ever available, would probably take months to assemble. Such delay in obtaining information, nuclear power experts said, is not unreasonable. It often takes weeks or months to reconstruct what happened in an airplane crash, and nuclear power plant accidents are much more difficult to analyze because spilled radiation keeps investigators at a distance.

It was not until several weeks after the 1979 accident at Three Mile Island (TMI), near Harrisburg, Pa., that a detailed picture began to emerge. Even now, much remains unknown because it has been impossible to send people inside TMI's highly contaminated containment dome.

The Chernobyl accident began at 1:23 a.m. local time, Saturday, April 26, when, according to one Soviet account, the plant was under the control of a relatively small crew less experienced than its daytime counterpart. The same situation occurred at Three Mile Island.

The Soviet reactor's power output had been reduced to 20 percent of its normal 1,000 megawatts in preparation for a total shutdown for maintenance. Tests were under way to check the reactor's systems that night.

The first event appears to have been a fire of unknown origin breaking out in the turbine room (adjacent to the reactor room) where high-pressure steam, heated by the reactor's core, spins turbine blades connected to a generator. The Crisis Escalates

When the alarm sounded, firefighters rushed to the plant, climbed 100 feet to the top of the building and began spraying water on the blaze. The fire was so hot, Soviet sources said, that it melted the asphalt on the roof, making the firemen's boots stick.

Shortly after the fire started, the crisis escalated to disaster when the reactor's core -- which contains the nuclear fuel -- lost its cooling water and began to heat rapidly.

The core is the heart of a nuclear reactor. It is where the fuel undergoes radioactive decay, with atoms of uranium splitting and giving off heat. Water is circulated past the fuel to pick up the heat and boil into steam to turn the turbine blades. In the process, the water also keeps the reactor fuel from getting too hot.

Chernobyl's core is a cylinder about 40 feet in diameter and 26 feet high. Most of the core is a stack of blocks of graphite, the crystalline form of pure carbon much like pencil lead. Passing vertically through the core are 1,660 metal tubes that contain the fuel and the cooling water. The tubes are an alloy of zirconium metal.

Within each tube is a cluster of 18 zirconium-clad rods of uranium oxide fuel. The cooling water is pumped into the bottom of each tube and flows up, between the fuel rods, made hot by the fission chain reaction, and the outer wall of the tube. At the top, the water reaches boiling temperature and the high-pressure steam that results is piped to the turbines. As long as the water flows, it keeps the fuel rods from overheating.

At least that is what's supposed to happen.

The fire in the turbine room either destroyed electrical power supplies to the water pumps or ruptured pipes, allowing water to leak out. In any case, the rods lost their cooling, and the technicians faced the single worst prospect a nuclear reactor can experience. Two Explosions

If it worked properly, an emergency cooling system kicked in, feeding water to the tubes. But Chernobyl's emergency supply is believed to have only enough water for about three minutes. Usually this is enough time to get the primary water pumps going again after a more routine mishap. This time it was not enough.

Heat accumulated rapidly in the fuel rods, driving their temperature toward the melting point. According to Soviet sources, there were two explosions in the reactor. It is not clear what caused the first, but it may have been a steam explosion, triggered when the fuel rods boiled the water faster than the steam could escape. The force may have burst pipes designed to hold the normal pressure of nearly 1,000 pounds per square inch.

The second explosion, igniting highly flamable hydrogen gas, occurred shortly afterward. It was just such a hydrogen explosion that was feared, but did not happen, at Three Mile Island in 1979. A Blast Through the Roof

There is no free hydrogen in a normally operating reactor. It forms when metals oxidize, or rust. When iron rusts, it splits water molecules, binding the water's oxygen to the iron to form iron oxide. The leftover hydrogen atoms become hydrogen gas. Because iron rusts so slowly, too little hydrogen gas forms to pose a danger.

But when zirconium gets hotter than about 1,600 degrees Fahrenheit in the presence of water, it oxidizes extremely rapidly, liberating large quantities of hydrogen gas in seconds.

American experts estimated that the sudden loss of coolant at Chernobyl would have caused the zirconium to reach this temperature, from its normal operating temperature of about 570 to 750 degrees Fahrenheit, in 10 to 12 seconds. Soviet officials said the heat produced by the fuel rods jumped from the near-shutdown low of 6 or 7 percent of capacity to 50 percent in 10 seconds.

When the heat ignited the hydrogen, it exploded with enormous force, bursting through the reinforced concrete roof of the reactor vault. Debris from the explosion shot more than a half-mile into the air, according to Soviet sources.

At that moment, probably no more than minutes after the fire in the turbine room started and before firemen could arrive, huge quantities of radiation began pouring through the hole in the roof.

Within seconds, the fuel rods became hot enough to melt the zirconium and ignite the graphite, starting a fire that would continue for days. A graphite fire, like charcoal briquettes in a backyard barbecue, burns with no flame but pours out great quantities of heat.

As a result, the fire became an effective device for leaching radioactive atoms from the exposed fuel rods and lofting them through the hole in the roof. No 'China Syndrome'

With the temperature soaring to more than 3,000 degrees, the fuel rods melted. Drops of molten uranium oxide, mixed with zirconium and other materials of which the rods are made, flowed to the bottom of the core, melting their way to the concrete floor of the reactor vault.

According to the "China syndrome," pools of molten radioactive fuel should burn their way through concrete and into the soil under the reactor. There is no evidence that this happened at Chernobyl. Under the reactor is a concrete slab six to seven feet thick. Most U.S. experts suspect that when the fuel hit this slab, it quickly spread out into a thin layer and the concrete absorbed enough heat to solidify the fuel again.

If the molten fuel had penetrated this layer, it would have fallen into a pool of water under the reactor. This so-called suppression pool is a safety feature into which any escaped steam from the reactor would be channeled and condensed back into water, thus reducing pressure inside the reactor vault. On the bottom of the suppression pool is another layer of concrete six to seven feet thick. Entombing the Core

All evidence suggests the meltdown did not penetrate the first concrete slab. Days later, as workers battled to extinguish the graphite fire, others would tunnel from the side into the suppression pool so that scuba divers could swim in and open valves to drain the pool.

Other workers would begin pouring concrete into the emptied pool to form a solid base for a huge, radiation-shielding concrete tomb that would enclose the reactor.

Above ground, the entombing process called for helicopter pilots to make hundreds of sorties over the reactor, dropping more than 4,000 tons of sand mixed with clay, lead and boron, all materials that would smother the graphite fire and block some of the radiation. Eventually, according to Soviet officials, the entire reactor would be encased in a giant concrete casket.

In the days before the helicopter runs and the tunneling operation began, however, massive quantities of radiation billowed into the sky.

Inside the reactor before the meltdown, among the nearly 200 tons of uranium oxide fuel, there were a few pounds of highly radioactive material -- every atom of which is a potential hazard to humans. These radioactive particles, byproducts of the reactor at work, include iodine 131, cesium 137, strontium 90 and tellurium 132.

In the dark, early hours of Saturday, April 26, the explosion that ripped open the roof also sent a shroud of smoke and a swarming gaseous mass of these radioactive particles high into the air.

Two workers at the plant died soon after the explosion, one burned by steam and another crushed by the debris.

Dozens of others, though they did not die that night, were already doomed. Their bodies absorbed huge amounts of radioactive particles and sustained damage from radiation outside their bodies. Three weeks later, 13 would have succumbed to radiation injuries, and more are expected to die. 300 in the Hospital

Death from acute radiation poisoning usually occurs within two months. Soon after the accident, the worst of the cases were numbered at 49. Thirty-five of the severest cases would be treated in Moscow by Dr. Robert Gale and his colleagues from the UCLA Medical Center, invited to help in bone marrow transplants for some victims. Altogether, three weeks after the disastrous night, 300 people would have been hospitalized with radiation illness of varying degrees of severity.

Death from radiation exposure is not, as one might think, a result of burns. Rather, the radioactive particles enter the body by inhalation, swallowing or skin absorption, then break up into fast-moving fragments that rip through cells to cause various types of havoc. A deadly dose of such particles is 1,000 or more rems taken in over a few days. This amount causes intestinal tissue to break down, with massive bleeding and fluid loss followed by rampant infection. Most of those who have died of radiation poisoning so far probably received more than 1,000 rems.

Victims who received doses between 600 and 1,000 rems would suffer severe damage to the blood-producing bone marrow. Nineteen of these victims received marrow transplants. Death from damage to the bone marrow would be expected to occur within two months. After that, they would be considered survivors, though they would remain seriously ill for some time.

Those receiving 100 to 600 rems could face serious medical problems, including thyroid damage and suppression of the blood cells and the immune system.

In addition to the immediate radiation sickness, Gale said the Soviets estimated that 50,000 to 100,000 people received significant doses of radiation. Among them, some small fraction would be expected to get cancer within a few years. Cancer cases triggered by radiation will continue to appear for several decades, with a large majority expected in the Soviet Union or neighboring eastern European nations. Radiation Over the Ukraine

One early U.S. estimate, by Frank von Hippel of Princeton University and Thomas Cochran of the Natural Resources Defense Council, said that about 5,100 people might be expected to die of cancer eventually from the accident. They said 10 times that number might get cancers but not die of them.

When the explosion took apart the reactor core, it released thousands of rems per hour of radioactivity inside the plant, several hundred near the plant, and one reading equivalent to 36 millirems at the perimeter of a zone around the plant some distance from the reactor.

The explosion took most of the radiation high into the Ukrainian sky, where that night's northwest wind blew the lethal cloud toward the nearby village of Pripyat and on over the city of Minsk.

About half of all the radiation the reactor would eventually expel was blown off in the cloud from the first day. It is believed that the cloud was saturated with hundreds of millions of curies worth of radioactivity, more than a typical atomic bomb produced in U.S. tests in the 1950s. About 80 million curies of iodine 131 were sent into the atmosphere in that cloud, compared with about 150,000 curies of the same element given off by a typical atomic bomb in an atmospheric test.

As the cloud traveled, radioactive particles fell out. When it mingled with some rainstorms, more particles came down across the Ukraine, over Warsaw and the Polish countryside.

By the next morning, Sunday, April 27, radioactivity counting equipment began to tick across Sweden, more than 750 miles away. But no one was there to read the counters. In Finland, there also were rising spikes of radiation on monitors, but if the levels were noted, the information was not reported. Evacuation Orders

In Moscow, a panel was being established to look into the accident. But in the Ukraine, local officials apparently did not have a clear sense of its scale. They were misled, officials said, by some relatively low radioactivity readings in the zone around the plant after the accident.

Orders came in to evacuate a 6.2-mile radius around the plant. Buses were brought in, and between 2 p.m. and 4:20 p.m. local time, the town of Pripyat, three miles from the reactor, was emptied of its 25,000 people. Including at least three other villages, about 49,000 people were cleared out of the area. Six days later, the evacuation area would be widened to a circle with a 18.6-mile radius. The total number evacuated would reach 92,000.

The Soviets did not let the rest of country or the world know about the accident on the first or second day. Swedish officials called and told the Soviets of their readings of radioactivity. They had concluded, by the profile of different elements they had counted, that a nuclear plant accident of large scale had occurred. But by 6 p.m. Monday, officials in Moscow still denied knowledge of the accident.

At 9 p.m., however, they issued a short statement that said a nuclear power reactor had been damaged and an unspecified number of casualties had resulted. Spreading Radioactivity

In the days to follow, various currents of wind would carry the radioactive cloud into a series of branching streams across Europe, Asia and, eventually, over North America. Where rains fell, some of the radioactive particles would be brought to the ground.

In the first few days after the accident, winds blew the main cloud of radiation into a northwesterly stream over Scandinavia. By Day 4, other wind currents had carried some of the radiation south and west, back into the Soviet Union.

Within two weeks, the radiation, vastly diluted and decayed, had dispersed over much of the northern hemisphere. Nowhere outside of the vicinity of the reactor, however, were the levels high enough to cause acute radiation sickness. In most places where fallout was detected, it was far too low to cause medical problems, the doses being the equivalent of a fraction of one chest X-ray.

Epidemiologists, however, were beginning to estimate long-range effects such as increases in the incidence of cancer. As the decades go by, researchers in several countries are expected to track the medical effects closely, carrying out what is certain to become the largest study of the health effects of radiation exposure on human beings since the United States detonated an atomic bomb at Hiroshima in 1945.