In the 36 hours before evacuations began near the damaged Chernobyl nuclear-power station, thousands of persons may have incurred enough radiation exposure to produce illnesses that could become apparent in the next few weeks or, in some cases, years from now, experts on radiation sickness said yesterday.

About 25,000 people were in the town of Pripyat, three miles from the reactor, when the disaster began with a chemical explosion at 1:23 a.m. local time, April 26.

By the time evacuation began, about 2 p.m. the next day, radioactive fallout had spread as far as Sweden, about 700 miles.

"It's very hard to say anything definitive, given the lack of hard information, but I can't imagine a scenario in which people could be so close for that long without taking a fairly significant dose," said Dr. Roland Finston, director of health physics at Stanford University.

Radiation specialists said that estimating the degree of exposure sustained is difficult because it depends on how much time people spent outdoors and their clothing. Individuals also vary greatly in susceptibility to radiation damage to their body's cells.

Another unknown in the Chernobyl accident is how long after the explosion the reactor building's walls were breached. Several hours may have passed before fire destroyed enough of the building to allow radiation to escape.

If the building remained intact, confining radioactive particles from the damaged reactor for a few hours, the experts said, some radiation could have decayed before the remainder was released to the environment. Many of the different kinds of radioactive atoms produced in a reactor decay within seconds, minutes or hours.

Finston said some hospitalized victims may die within days or weeks of damage to the gastrointestinal tract, the chief killer of those exposed to high doses but who are not severely enough exposed to die within hours of damage to the nervous system.

Soviet officials have said 18 people remain hospitalized after "extreme radioactive exposure."

Those who received lower doses, including many who may have had few or no symptoms in the days immediately after the accident, can be expected, at least one to two weeks after exposure, to show signs of damage to bone marrow, the tissue that makes blood cells.

The number of infection-fighting white cells drops, and the victim falls prey, like those with acquired immune deficiency syndrome, to various infections. Loss of hair and hemorrhaging are common.

Finston said he has heard from colleagues that the 18 severely exposed people have received marrow transplants but added that such treatment is likely to help only victims who show signs of marrow damage fairly quickly, within weeks.

Dr. Herbert Abrams, a Stanford radiation specialist, said many who escape acute signs of radiation sickness may succumb to cancers in later years.

"Compared to those who develop acute symptoms," he said, "a much larger fraction are now at risk for long-term effects such as cataracts, leukemia at about four years and, of course, there could be some mental retardation" in children born of pregnancies now in early stages. Decades from now, a rise in other forms of cancer is likely, he said.

Although firm estimates of the health consequences from the Chernobyl accident are impossible, the experts said they base their assessments on several known factors.

A key one is the distance between a person and the reactor. Standard Nuclear Regulatory Commission charts indicate that if an American reactor were to release a large amount of radiation in an accident much like that at Chernobyl, a person outdoors one mile from the reactor would absorb a dose of 1,000 rems in 24 hours.

The dose would usually be lethal, though perhaps not for weeks. Five miles away, the dose would drop to 300 rems, rarely lethal but severely damaging to marrow.

Soviet officials have not said how much radiation was released at Chernobyl but did report, a few days after the accident, that the intensity at the site had dropped to 200 roentgens per hour. The units of measurement are different, but roentgens are comparable to rems.

Reckoning back to what may have been the radiation intensity when the reactor building was breached, the experts said this suggests that the NRC tables may be roughly applicable.

If so, anyone from Pripyat outdoors for 24 of the 36 hours before being evacuated might have taken a dose of 300 to 1,000 rems. But, because most probably spent more time indoors, their dose was probably much less.

Other factors may also have reduced exposure. According to one report, the reactor was being shut down and some of its fuel removed when the accident happened.

"Both of these could have mitigated the exposure," said Frank von Hippel, a Princeton University specialist in reactor safety. Shutting down the reactor reduces the rate of nuclear fission in the fuel rods, which lets radioactivity decay. Thus, less radioactive material would be in the reactor when the accident occurred.

"Still," von Hippel said, "we know there had to be a lot left, and the people around there had to have gotten a dose that is going to have some consequence."