The Soviet Union last Saturday detonated an underground nuclear test that U.S. scientists believe exceeded the 150-kiloton limit for such tests agreed to by both countries in 1974. Yesterday, the United States formally questioned the Soviet Union about the test.
U.S. officials who disclosed information about the test yesterday said that a preliminary intelligence estimate put its size at about 200 kilotons but that later information and analysis could change that figure. The test certainly was in a range of 100 to 400 kilotons, these officials said.
Other sources said U.S. seismologists now feel there was an 80 percent probability that the test exceeded 150 kilotons.
The 150-kiloton limit was contained in the Threshold Test Ban Treaty signed in Moscow in 1974. However, the treaty has never come into force legally, pending the outcome of negotiations on a total ban on underground testing.
Both the United States and the Soviet Union have declared their intention to respect the 150-kiloton limit during these negotiations, although they are not bound to do so.
Sources inside the Carter administration speculated yesterday that if the Soviets did indeed exceed the 150-kiloton limit in Saturday's test, they may have done so to demonstrate to the United States that they will not adhere indefinitely to agreements that are not converted into legally binding treaties.
Other interpretations were that the Soviets simply decided to cheat, made a mistake in calculating the explosive force of a device they tested, or in fact did not violate the limit at all.
This last possibility remains real, according to official sources, because the seismologists who make these estimates, using data from listening stations around the world, practice "an art as much as a science," as one official put it.
The range of potential error is large, many sources agreed, though the evidence of a test bigger than 150 kilotons is strong.
Moreover, the two countries agreed as part of the Threshold Test Ban Treaty that because technical difficulties in controlling blast sizes and measuring them would be unavoidable, "one of two slight, unintended breaches per year would not be considered a violation."
Nevertheless, administration officials conceded that this latest Soviet test could cause difficulties, particularly during the Senate debate on SALT II, which will include the issue of whether the Soviets can be trusted to adhere to an agreement.
News of the test was made available by officials who said they feared that alarmist accounts of the test would soon be leaked, since information on it had been widely circulated inside the government.
The Soviet test occurred at Semipalatinsk, the traditional location for such explosions. During the last year, sources said, the Soviets have intensified their underground testing program, detonating about 50 percent more nuclear devices than the United States has detonated in its ongoing underground testing program.
Underground tests are used by both countries to perfect new warheads and explosive devices for their startegic and tactical nuclear weapons.
The United States gave the Soviet government a diplomatic note yesterday asking for a clarification on the size of Saturday's test. Twice on earlier occassions the United States has made similar requests, and the Soviets have responded by denying they violated the limit. The matter has ended there.