Two historians of the Atomic Age have turned up the first evidence that Japan tried to develop an atomic bomb during World War II.

The historians, Dr. Herbert, F. York Jr. of the University of California at San Diego and Dr. Charles Weiner of the Massachusetts Institute of Technology, concluded that the Japanese gave up their nuclear weapons development when they were unable after repeated attempts to isolate the isotope of uranium called U.235 that sustains a chain reaction and produces an atomic explosion. Their findings are reported in the current issue of Science magazine.

"I believe this evidence is only now coming out because the Japanese conducted a cover-up of what they'd done right after the war," York said yesterday. "I don't mean it was a plot; just that everybody involved in Japan wanted to forget it when they'd lost the war."

Historians have long thought that Japan did not research on the atomic bomb during the war, an impression the postwar Japanese government apparently encouraged.The first American atomic scientists into Japan in 1945 said they saw no signs of nuclear bomb development anywhere in the devastated country.

"There were no atomic piles, no uranium diffusion plants, nothing," said Dr. Ralph Lapp, one of the first nuclear scientists to enter Japan at the end of the war. "Even the magnets in their cyclotrons showed no signs of having been used in isotope separation, something we felt surely would show up if they'd been engaged in bomb research."

York and Weiner, however, from interviews in the last three years with Japanese scientists and from freshly translated documents and books, have found that Japan used the five cyclotrons it had at three universities to conduct basic research into the physics of an atomic weapon.

The historians also found that scientists at the University of Kyoto and Tokyo employed a heat process known as thermal diffusion in an attempt to separate the lighter U-235 isotope from the heavier and far more abundant U-238 isotopes both present in natural uranium.

The projects apparently failed repeatedly, York said, at least in part because thermal diffusion is the most difficult of four known theoretical ways physicists have of isolating the U-235 isotope and using it to make a bomb.

Science magazine says the chief figure in Japan's nuclear weapon research was a physicist Yoshio Nishina, who arranged for the army to finance weapons research as early as 1940. Science says that Nishina, who died in 1951, organized leading physicists and radiochemists into a group called the "Physics Colloquium," which reviewed the work going on at the universities of Kyoto, Osaka and Tokyo.

By March of 1943, Science says, the Colloquium concluded from the experimental failures at Kyoto and Tokyo and from the theoretical work being done at all three universities that the building of a bomb could not be accomplished.

"An atomic bomb would be impossible 'even' for the United States," the Collequium said in one account quoted by Science, which had access to the research done by York and Weiner. "The scientists viewed the project as extremely long term at best," "if not for this war then in time for the next one."

So little was known right after the war about Japan's attempts to develop a bomb that went the occupying American Army dismanded the five Japanese cyclotrons and sank them in Tokyo Bay there was an outcry from American scientists, who called the destruction "mindless."

Reconstructing what they now know, York and Weiner suggest that the destruction may have been justified even if the destruction orders were not based on inside knowledge that the cyclotrons had been used for weapons research.

"In hindsight, perhaps the Army gave the order to destroy the cyclotrons because they identified the machines with nuclear research," York said. "They must have recognized that there was some connection between the cyclotrons and the bomb."

Weiner said that, from all the evidence he has seen, the Japanese were as far as the Germans, or even farther, from developing an atomic bomb. He said the fact that they had no atomic pile to produce plutonium for weapons suggested that the entire program never went beyond the experimental stage.

Science magazine calls the York-Weiner findings "highly significant" to the history of nuclear weapons "and to the relationship that developed between Japan and the U.S. after the U.S. atomic bombing of the cities of Hiroshima and Nagasaki." Japan surrendered after those two bombings and said after the war that it would "never seek" to be a nuclear weapons nation.