A leading Soviet scientist said today that "a turning point" had been reached in the situation at the Chernobyl nuclear plant, and Soviet television offered the first footage of cleanup operations inside the closed 18-mile zone surrounding the plant.

"The decision will be taken to resume the operation of the power station," said Yevgeny Velikhov, vice president of the Academy of Sciences and a director of the cleanup effort, in an upbeat assessment of the situation reported by the official Soviet news agency Tass.

"Until today," Velikhov said, "the possibility existed of a catastrophe, since a large amount of fuel and reactor graphite remained in an overheated condition. Now that possibility is no more," he added.

International Atomic Energy Agency official Morris Rosen, who visited Chernobyl last week, said Sunday in Vienna that efforts to cool the fuel would have to continue for several more weeks or months and that if they fail there could be further melting in the core of the reactor.

"If these means fail . . . the possibility of melting still exists," Rosen said in an interview with National Public Radio.

[Rosen also said figures for radiation levels that he gave at a press conference in Moscow Friday came from Soviet sources and that he could not be sure they were accurate. He said he could not get reliable information because the Soviets themselves did not have much. Further, he said, he had trouble making sense of some of the Soviet data.]

The five-minute Soviet television broadcast from inside the evacuated zone surrounding the plant featured a reporter riding through tree-lined streets, past boarded-up houses and cleanup crews wearing face masks. A commentator said all houses were sealed to prevent radiation from seeping in. The film included brief interviews with Velikhov and Ivan Silayev, a first deputy prime minister who is directing the cleanup with Velikhov, inside their Chernobyl headquarters.

State-controlled media have increased coverage of the accident since the first few days, when it was reported only in brief official statements.

The film showed officials wearing white coats working in the headquarters; police were directing the traffic of workers outside. The streets were nearly empty of people and vehicles, but the camera focused at length on a stork perched in a tree.

Work goes on from 6 a.m. until 1 the following morning "and even through the night," a TV commentator said. All workers televised outdoors were wearing medical-type face masks.

The area within an 18-mile radius of the reactor site was evacuated gradually, starting 36 hours after the explosion took place on April 26, according to previous accounts. The government newspaper Izvestia said today that about 92,000 have been evacuated -- an increase from the 84,000 evacuees reported by Ukrainian officials Thursday.

Velikhov refrained from indicating whether radiation has continued to seep out of the damaged reactor. Radiation levels in the Ukraine and Byelorussia "remain the same," according to a government announcement released by Tass. The statement seemed to refer to radiation readings released yesterday, which specialists do not consider dangerous.

Silayev referred to the Chernobyl incident as "a historic event." But he also was quoted by Tass as saying "the great catastrophes predicted in the West have not happened. The main danger has been eliminated. We can now work quietly."

Velikhov, quoted as saying that the cleanup has entered a new stage, gave a brief account of some of the methods workers are using to secure the damaged reactor. They are freezing the soil, supplying large amounts of concrete, with the aim of "burying" the damaged reactor, he said, and are also beginning to identify and eliminate areas of contamination.

Workers are also preparing to decontaminate residential houses, according to the government statement. In addition to the damaged nuclear plant there are three other reactors at the Chernobyl site.

Silayev told a Soviet interviewer that the other three reactors are capable of working and just have to be isolated from the damaged one. According to Velikhov, the station will be put into operation again "within a definite period whose duration will be determined by safety norms."

[The Associated Press quoted American bone marrow specialist Dr. Robert Gale as saying his four-member team was winding up the transplant stage of its work. "When one does a transplant, it is a very long and complicated procedure," he said. "Four to six weeks later, we need to evaluate the patients. That's the most difficult stage -- keeping them alive."]

Reuter reported from Frankfurt on another development:

Three West German technicians left for Moscow to teach Soviet technicians how to use two robot vehicles to investigate the disaster area at the Chernobyl nuclear power plant.

The unmanned vehicles, which are about the size of a small car and are especially designed for using in nuclear accidents, were flown to Moscow Saturday from West Germany. They are equipped to take soil samples and measure radiation and can be controlled from a distance of up to half a mile by radio and by camera