Newly released U.S. intelligence documents show that two major explosions at a top-secret Soviet nuclear facility in the southern Ural Mountains killed or burned hundreds of persons in the late 1950s and left a large tract of land lifeless and contaminated with nuclear fallout.

It is not clear from the heavily censored Central Intelligence Agency files whether a 1958 blast at the Russian nuclear installation at Kyshtym was an atomic explosion. However, Soviet scientists apparently did set off a nuclear blast at the site during the following year which caused a wide fallout in the area.

A CIA informant traveling in the Kyshtym region in 1961 described the scene of the two blasts in these stark terms, according to the documents.

"We crossed a strange, uninhabited and unfarmed area. Highway signs along the way warned drivers not to stop for the next 20 to 30 kilometers because of radiation. The land was empty. There were no villages, no towns, no people, no cultivated land: only the chimneys destroyed houses remained.

"I asked the driver to stop because I wanted to drink water. The driver refused. 'One doesn't stop here. You drive quickly and cross the area without any stops.' he said."

The documents were obtained this week under a Freedom of Information Act request by Ralph Nader's Critical Mass Energy Project and by The Washington Post. The CIA released 14 of the 29 documents it had the incident and said some of the withheld reports were too sensitive to be released even with deletions.

The documents made available by the CIA confirm a report last year by exiled Soviet scientist Zhores Medvedev that thousand of persons were killed or suffered radiation sickness when buried nuclear waste at a site in the Urals overheated and exploded in 1953. Medvedev said the explosion released a radioactive cloud covering hundreds of miles in the area.

Other U.S. sources speculated last year that the explosion may have occurred at a site west of the Urals where they said it was believed plutonium-contaminated waste was stored in vats above ground. The sources said he vats may have been toppled by an earthquake.

The latest CIA document's make no mention of an earthquake. The note that the Kyshtym facility was closely guarded and closed to all outsiders except for a select group of Communist Party members and their dependents who were brought to the site from across the Soviet Union. The plant apparently manufactured components for nuclear weapons.

According to a CIA summary of information about the plant, hundreds of persons were exposed to radiation and injured in the 1958 explosion. In October of 1959 Soviet scientists apparently conducted an atomic test over the plant site contaminating food for at least 100 miles.

One CIA informant reported that after the 1958 blast, food purchased in Chelyabinsk, nearly 100 miles away from the plant site was being carefully checked by authorities and in some cases destroyed because it was radioactive. The informant said some villages near Kyshtym were burned to the ground by authorities and the inhabitants evacuated with only the clothes on their backs.

The CIA records note that the raw data supplied to the agency by its informants had not been avaluated for accuracy. Several informants' reports, for example, give different dates for the Kyshtym explosions than those contained in the CIA's own information summary.

The overall picture of the area following the blast is described by various informants, however, as one of devastation.

One report dated 1957-58 said all food stores in the region were shut and food was brought in two days later by truck and train. "The resulting queues were reminiscent of those during the worst shortages during World War II," the reports says. Some residents, it says, became hysterical with fear after the appearance of "mysterious" diseases while others walked around wearing radiation counters.

Another informant told the CIA in 1960 that after what was apparently the second blast in the area a fine red dust filtered down on the region around Chelyabinsk. "Very qucikly," the informant said, "all the leaves curled up and fell off the trees."

The blast - which one CIA document indicated may have been a 20-megaton bomb deliberately set off over the site - was marked by a bright flash and a shaking of the ground in the area.

Victims of the nuclear explosion were treated in a local hospital where a wing was sealed off for them and all outsiders kept away. One informant at the hospital during that time reported, "Some of them were bandaged and some were not. We could see the skin on their face, hands and other exposed parts of their body to be sloughing off . . . It was a horrible sight."

In an interview yesterday Nader said the blast was apparently the first serious nuclear accident involving massive casualties. Nader accused the CIA of deliberately holding up release of the documents about the blast for 20 years in order not to frighten persons in this country who might be concerned about nuclear development here.

"This information would have made people in the U.S. very concerned over nuclear installations, whether civilian or military," Nader said. He said also that he would seek to obtain at least some of the material withheld by the CIA on security grounds.

A CIA spokesman had no immediate comment yesterday on Nader's charges.