Summit likely to speed talks on strategic weapons. Experts see outline emerging for cuts in long-range forces. SUPERPOWER SUMMIT

From the pre-summit maneuvering in Washington, a cautious consensus is emerging that in addition to signing an agreement eliminating intermediate-range nuclear weapons, the superpowers may make significant progress next week toward a treaty to eliminate long-range, strategic nuclear weapons. Arms control experts, both inside and outside government, are sketching the broad outlines of an agreement that Soviet leader Mikhail Gorbachev and President Reagan might reach during the summit.

There are, to be sure, still substantial areas of disagreement and disputes over details - any of which could torpedo an agreement. But some in the administration are nurturing hopes that Mr. Reagan and Mr. Gorbachev can lay the groundwork for signing a full treaty cutting strategic warheads by about 50 percent in time for a summit early next year in Moscow.

The hope is that outstanding issues will be ``significantly narrowed'' during the summit, says a senior administration official.

Perhaps the clearest articulation of what the administration hopes to achieve came during a breakfast meeting with Kenneth Adelman, head of the Arms Control and Disarmament Agency. The scenario he outlined to reporters dovetails with a number of other forecasts made by nongovernmental experts.

John Steinbruner, director of foreign policy studies at the Brookings Institution, predicts that there is at least a 50 percent chance that the two leaders will emerge with an agreement outlining the terms of a strategic arms reduction (START) treaty.

Mr. Adelman says the superpowers could agree that neither side will deploy a space-based missile defense system for a fixed period. (The Soviets favor 10 years, the US seven.)

That would not place undue restrictions on the administration's plans for the Strategic Defense Initiative (SDI) because it will not be ready for deployment until at least the mid-1990s.

These years would mark what one senior administration official terms ``an era of predictability,'' in which both sides could begin a phased reduction in long-range nuclear weapons.

Adelman says the reductions were basically outlined at last year's summit in Reykjavik, Iceland, and future reductions would build upon those formulations. They would leave each side with a total of 6,000 nuclear warheads, on up to 600 delivery vehicles. Of these, 1,540 could be so-called ``heavy warheads'' - on large, multiple-warhead missiles. The total number of warheads on ballistic missiles would range from 4,800 to 5,100. Warheads carried on bombers would also be counted, according to a formula agreed upon at Reykjavik. In addition, the US would expect a 50 percent cut in the overall nuclear ``throw weight'' of the Soviet nuclear arsenal.

``That,'' Adelman says, ``is the outline of a START agreement.''

Clearly, there are disagreements on how to verify such a treaty. And the Soviets would also expect some limits on US sea-launched cruise missiles.

Moreover, a core area of disagreement would remain, revolving around the SDI program and the 1972 Antiballistic Missile (ABM) Treaty. Specifically, the Soviets want to restrict the kinds of SDI tests that could take place during the period of nonwithdrawal from the ABM Treaty - either by specifically elaborating the tests that would and would not be permitted, or insisting on a US commitment to a narrow, restrictive definition of the ABM Treaty that would foreclose certain kinds of tests.

Gen. Nikolai Chervov, one of the Soviet military's disarmament experts, told a Washington press conference that there is a ``fundamental difference in the positions of the two sides'' on the matter, and ``it is not possible to work out a compromise.''

The two sides also disagree on what happens when the period of nonwithdrawal from the treaty is over. Would the US then be free to deploy SDI? Or would the ABM Treaty still preclude such a deployment?

Adelman suggests that the two sides might simply ``agree to disagree.'' But some analysts suggest that the Soviets might simply attach reservations or stipulations to a START agreement, noting that if the Reagan administration or its successors expand testing of SDI beyond the present, narrow limits - or eventually deploys a system - then the START agreement becomes null and void.

That would place tremendous pressure on future presidents not to expand SDI testing or move toward deployment, especially when that might provoke the Soviets not to carry through with major nuclear arms reductions.

Adelman says the US would not agree to such a provision or stipulation in the treaty. But a number of former US disarmament officials note that the US would not have to agree.

``The Soviets could attach a reservation to the agreement,'' says Michael Krepon, an expert on arms control verification at the Carnegie Endowment for International Peace, simply stipulating its intent to withdraw from the treaty when the US takes actions not to its liking.

``Nations enter into statements of unilateral intent all the time,'' Mr. Krepon says.

Ralph Earle, a former US arms control official who now heads the Lawyers Alliance for Nuclear Arms Control, is skeptical. ``I don't think it's going to work, and I don't think anyone's going to try it.''

But, as Krepon notes, ``summits are unscripted. Things happen when the big guys get together.''

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