U. S. Invited Soviet Experts to Observe Nevada A-Test : Moscow Would Gain Little Data From Offer

Generous as it appears, President Reagan’s offer to let the Soviet Union monitor an American underground nuclear test holds little promise of new or valuable data for Soviet observers, several U.S. nuclear arms experts said Monday.

Reagan’s “unconditional” invitation, which the Soviets subsequently rejected, likely held scant appeal for them, these experts said, partly because Moscow probably already knows much of what an inspection visit would reveal--and largely because the offer is not unconditional at all.

“It doesn’t get them a great deal,” said Jack Mendelsohn, deputy director of the Arms Control Assn., a Washington-based private think tank. And even an official of the U.S. Arms Control and Disarmament Agency, speaking on the condition that he not be named, said the Soviets “may feel they have nothing in particular to gain” by accepting the American invitation.

The White House offer--billed as a way to better assure that nuclear explosions do not violate testing treaties negotiated during the Richard M. Nixon and Gerald R. Ford administrations--appears to offer a great deal at first blush.

The invitation would grant the Soviets entry to the Nevada Test Site north of Las Vegas, where an estimated 700 nuclear blasts have been triggered since 1945. In witnessing an explosion, the Soviets would gain first-time access to a military experiment whose very occurrence--now about 15 times a year--sometimes is kept secret even from the American public.

New Technologies Tested

Explosions conducted there include demonstrations of new nuclear weapons technologies such as X-ray lasers, quality-assurance tests of warheads taken from U.S. stockpiles, measurements of how nuclear explosions affect U.S. defense systems such as spy satellites and tests of safeguards designed to prevent accidental or terrorist-inspired nuclear explosions.

But the Soviets would carry none of those secrets away from a visit to the test site. The White House offer, while described as unconditional, carefully limits Soviet monitoring during a U.S. nuclear test to measurements of the explosive yield of the weapon being detonated.

That limited data, primarily seismographic information, could be used later to calibrate seismographs and other monitoring devices that the Russians now employ from afar to gauge the strength of U.S. nuclear explosions. The calibration is what U.S. officials are promoting as a means of better verification of nuclear explosions.

But beyond giving the Russians a slightly better yardstick to measure the size of U.S. nuclear tests--and the size of weapons being tested--such information has little strategic value, experts said.

In 1977 testimony before the Senate, Administration officials said that U.S. sensors could measure the yield of a distant Soviet nuclear blast only to within a factor of two, meaning that a 150-kiloton explosion would show up on American sensors as somewhere between 75 and 300 kilotons. The Soviets are thought to suffer a similar measuring problem.

Limited to Measuring Yield

“This would be strictly to measure the yield,” the U.S. arms control official said. “Were they to have access to some of the more sensitive aspects of the test, it could give them information on some of our weapons that we don’t want released.”

Another nuclear weapons expert said: “They’d probably be able to determine to a great deal the yield and characteristics of warheads, and that would help them to improve their verification in general.”

The expert, William M. Arkin of the Institute for Policy Studies, a Washington research organization, added: “But I can’t imagine that there would be any information gain that would change the balance of power.”

Some suggested that the White House offer was aimed at forcing the Soviets to grant a peek at their own long-secret testing grounds.

“I don’t think there’s any substantial benefit for the Soviets behind this proposal from the point of view of information gathering,” said Arkin. “But obviously, we know so little about Soviet nuclear testing . . . the size of warheads, and so on, that any quid pro quo would be a scientific boon for us.”