MOSCOW (AP) _ One year after the world's worst nuclear disaster at its Chernobyl plant, the Soviet Union is pushing ahead with an ambitious plan to build 11 new plants, because, a top official says, it has ''no alternative.''

Soviet news media reported Saturday that at the same time, plans to expand the damaged Chernobyl power plant have been shelved, at least for now, apparently because of continued elevated radiation levels at the site.

According to Andranik Petrosyants, chairman of the Soviet Atomic Energy Commission, the 11 power stations under construction will by 1990 double the 1985 level of electric output from atomic plants.

Petrosyants said in an interview published by the official news agency Tass on Saturday, anniversary eve of the April 26, 1986, Chernobyl disaster, that the Soviets also plan other nuclear power stations, but did not say how many.

''Naturally, people feel worried in connection with the accident,'' Petrosyants said, referring to the explosion and fire that tore apart one of Chernobyl's four reactors. ''Yet there is no alternative to nuclear power engineering.''

The accident sent a cloud of radioactivity over large areas of Europe and around the world. Thirty-one Soviets died and more than 200 were stricken with radiation sickness.

In the Tass interview, Petrosyants defended nuclear stations as more environmentally sound than coal-burning plants and said safety measures have been taken at all Soviet nuclear plants with reactors similar to those at Chernobyl.

Since the Chernobyl accident, he said, ''reactor safety has been considerably increased. And it will be ensured, even in case of wrong action by personnel.''

Petrosyant's safety assurances were part of a concentrated campaign to portray the recovery from the Chernobyl accident as a triumph for Soviet officials.

Recent press reports and documentary films here have stressed efforts to improve nuclear plant safety, provide aid to about 135,000 evacuees, and decontaminate the plant and its surroundings, 80 miles north of the Ukrainian capital of Kiev.

Soviet officials were harshly criticized for keeping the accident a secret nearly three days. The first Soviet announcement of it came only after Sweden inquired about high radiation levels detected in Scandinavian countries.

Soviet officials have denied almost all Western press requests to visit the Chernobyl area. Official statements on the accident also have omitted some information and have sometimes contained contradictions.

Last Monday, for example, Leonid Ilyin, vice president of the Soviet Academy of Medical Sciences, told Soviet television that of 237 people diagnosed as having acute radiation sickness, 209 had recovered and 196 of them were back at work.

But on Friday, Tass quoted Valery Legasov, first deputy director of the Kurchatov Atomic Energy Institute, as saying only 65 were fully recovered.

Soviet officials also have declined to give details about a trial of those charged with responsibility for the accident.

In a telephone interview Friday with a small group of Western reporters based in Moscow, Chernobyl plant manager Mikhail Umanets declined to say when the trial would take place, how many people were under indictment and what charges they faced.

''All those responsible will stand trial,'' he said.

Umanets did disclose that construction was halted on two new 1,000-megawatt reactors at Chernobyl and that a decision on whether to continue expansion of the station was delayed at least until 1991.

He did not say why reactor construction had stopped. But the Moscow News, a small-circulation weekly newspaper, reported Friday that new construction at Chernobyl was prohibited because of ''enhanced radioactive background.''

Two of the plant's reactors are operating at full capacity, after decontamination efforts brought radiation levels within safety standards.

Decontamination is still under way at the third reactor, which is scheduled to return to service this year. The fourth reactor, where the explosion and fire occurred, was entombed in concrete.

''The design of the sarcophagus totally excludes the possibility of radioactive substances being released from the damaged reactor into the atmosphere,'' Petrosyants said in the Tass interview.