Clinton Will Press for Worldwide Ban on Nuclear Tests : Weapons: Three of five known atomic powers back prohibition; China and Russia want right to experiment. Some in Pentagon sought to allow low-level explosions.

Taking a step “back from the nuclear precipice,” the United States will seek a worldwide ban on nuclear tests that will not except low-level explosions, President Clinton announced Friday.

He made his decision after Defense Secretary William J. Perry concluded that such low-level testing was of only limited value in maintaining the safety and reliability of America’s nuclear arsenal.

In a dramatic White House statement--made half a century after atomic bombs fell on Japan--Clinton told reporters that advanced laboratory experiments and computer simulations can guarantee that U.S. nuclear weapons will work, if needed, making test explosions unnecessary.

Some Pentagon officials had wanted an international test ban treaty, which is now being negotiated, to make an exception for low-yield tests; that provision would favor the United States because it has the technology to derive results from explosions as small as the equivalent of four pounds of TNT.

Clinton said he hopes his support for the “true zero” test ban will clear the way for a conference under way in Geneva to produce a treaty before the end of next year.

But U.S. officials said that difficult negotiations remain because Russia and China are holding out for permission to conduct at least some nuclear explosions.

“This is an historic milestone in our efforts to reduce the nuclear threat, to build a safer world,” Clinton said. “The United States will now insist on a test ban that prohibits any nuclear weapons test explosion or any other nuclear explosion,” even one that is for ostensibly peaceful purposes.

The United States has not conducted a nuclear test since it imposed a voluntary moratorium Oct. 1, 1992.

Clinton’s decision Friday was hailed by arms-control groups that had protested earlier U.S. efforts to write escape clauses and exceptions into the treaty.

“This position rejects the Pentagon preference for low-level nuclear tests and is an important boost to non-proliferation efforts and disarmament,” the Peace Action Education Fund said in a statement.

Other arms-control groups said the Pentagon really wanted an exception permitting tests of up to a yield of 500 tons. Although Perry insisted that his agency had never adopted such a position, the President’s announcement should quiet rumors that the military Establishment is unwilling to give up the right to test.

Clinton’s announcement means that three of the five acknowledged nuclear powers now support a ban on all nuclear tests, even extremely small ones.

France announced earlier this week that it would support a total ban on testing--once it completes an announced series of eight test explosions starting this fall at its South Pacific test range.

Britain is certain to go along with the American position because London long ago closed its own nuclear test facilities and tests its weaponry at the U.S. site in Nevada.

But China wants the right to conduct “peaceful nuclear explosions,” and Russia wants to set off test explosions with yields equivalent to several hundred tons of TNT.

John Holum, director of the Arms Control and Disarmament Agency, said it may be easier to persuade the Russians to go along with a total ban than it would have been to get them to agree to Washington’s previous position of allowing small tests.

Holum said that the Russians’ science is not advanced enough to give them useful information from tests with extremely small yields. For that reason, the previous U.S. position looked to Moscow like a formula that would permit the United States to conduct valuable tests while prohibiting Russia from doing the same.

Because the five nuclear powers have significant differences in their nuclear arsenals, their scientific capability and the size of tests they might need, Holum said, “any agreement would be discriminatory except for zero or hundreds of tons.”

Perry insisted that the Pentagon and the Joint Chiefs of Staff fully support Clinton’s “true zero” option even though they had wanted to retain some testing.

At a Pentagon news conference shortly after Clinton’s announcement, Perry said he had decided that the extremely small tests, which would have been permitted under Washington’s earlier position, “were of very limited value.” He said larger explosions, with yields of several hundred tons, “were desirable but not necessary for monitoring the safety and reliability of the stockpile.”

Clinton pledged to maintain a safe, reliable nuclear arsenal. Should a time come when the only way to do that would be to conduct a nuclear test, he said, he or a future President could pull out of the treaty. All arms-control pacts contain a clause allowing any nation to withdraw for reasons of “supreme national interest.”

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On the surface, there would seem to be little difference between insisting on a provision in the treaty permitting some testing and an assertion of the right to pull out of the treaty if testing is considered necessary.

But officials insisted that abrogation of the treaty would be a last resort that probably could be avoided.

“Exercising this right [to pull out of the treaty] is a decision I believe I or my future President will not have to make,” Clinton said in a written elaboration on his announcement.

“The nuclear weapons in the United States arsenal are safe and reliable, and I am determined our stockpile stewardship program will ensure they remain so in the absence of nuclear testing,” he said.