MOSCOW--A leading physicist in the cleanup of the damaged Chernobyl nuclear reactor was quoted yesterday as saying a turning point had been reached and that it was no longer possible that the situation could worsen.

"It's true that until today there existed the theoretical possibility of a catastrophe--a large portion of the (nuclear) fuel and reactor graphite was burning," the official news agency Tass quoted Yevgeny Velikhov as saying.

"This is now not the case," he was quoted as telling Soviet journalists in Kiev, 80 miles south of the plant.

The Kremlin issued a three paragraph statement last night saying work was under way to decontaminate housing near the power plant. Soviet television broadcast footage of the evacuated 18-mile zone around the plant, showing some decontamination workers in gas masks and other people in street clothes.

Velikhov said workers were trying to protect ground water from radioactive contamination. "A new phase of work has begun," Tass quoted him as saying. "Work is being done to decontaminate and encapsulate the radioactive material. This will ensure it won't fall into the ground water."

Velikhov, the vice president of the Soviet Academy of Scientists and a director of the Chernobyl cleanup, said soil was being frozen and cement was being poured with the goal of sealing off the damaged reactor. Officials will decide afterward whether to reactivate the power plant, he was quoted as saying.

Tass for the first time issued close-up photos of the damaged reactor. The black-and-white pictures, apparently taken from a helicopter, showed the upper part of the reactor building blown off and piles of rubble lying at its base. Nearby buildings appeared undamaged.