The Soviet Union said Friday that a powerful nuclear explosion rocked an atomic weapons complex in the Ural mountains in 1957, forcing the evacuation of more than 10,000 people but killing no one.

The accident, kept secret by authorities until now, created a radioactive trail 65 miles long and 5 to 6 miles wide, the official news agency Tass said.More than 30 years later large areas around the town of Kasli, 60 miles north of the city of Chelyabinsk, were still contaminated and water reserves were undrinkable, it said.

Western experts have long suspected there was a serious nuclear accident in the area at about that time.

The blast, in a tank containing radioactive waste, was apparently widely known to Soviet scientists soon after it occurred in September 1957.

It did not become public knowledge in the West until 1976 when dissident Soviet biologist Zhores Medvedev mentioned it.

Medvedev, responding to wide interest and controversy prompted by his revelation in an article on another subject, gave more details three years later in a book titled "Nuclear Disaster in the Urals."

He wrote that hundreds of people died from radiation sickness after the blast, which he blamed on nuclear reactions and overheating in an underground dump for nuclear waste.

Tass gave the first official acknowledgement of the blast in a brief report on a debate in Chelyabinsk over construction of a new nuclear power plant in the southern Urals.

"The local press carried an account of a news conference by First Deputy Minister for Medium Machine Building Boris Nikipelov who described a chemical explosion near the city of Kasli," it said.

"A defense enterprise intended for developing atomic weapons was built in the southern Ural region," it added. "In 1957, a tank containing radioactive waste exploded."

Tass said around 2 million curies of radioactive elements were discharged, compared with the 50 million curies registered after the Chernobyl nuclear plant accident in the Ukraine in April 1986.

Tass reported no one was killed but said more than 10,000 people were urgently evacuated from the contaminated zone.

A $300 million cleanup campaign was carried out, and by 1978 economic activity had been restored to more than 80 percent of the contaminated zone.

The remaining 20 percent of the area was turned into a reserve, it added, although it insisted the "radioactive situation" had been assessed as safe over the past 15 years.

Tass quoted Nikipelov as saying the choice of the site for the new nuclear plant was not accidental.

"Indeed, some water reservoirs had remained contaminated since 1957," it said. "The plant will evaporate water thus purifying the reservoirs."

Tass said the accident was never reported as it occurred at a defense factory.