U.S. negotiators will return to the Geneva arms talks later this month with orders to explore Soviet leader Mikhail Gorbachev's promise of "radical proposals" to reduce strategic nuclear arms in return for limits on President Reagan's strategic defense initiative (SDI) research program, government officials said yesterday.

The negotiators will go armed with a number of questions about two areas raised by the Soviet leader in an interview with Time magazine and in a meeting Tuesday in Moscow with eight U.S. senators:

What types of reductions in strategic weapons are to be proposed, and would they apply both to launchers and to warheads? Until now, the Soviets have talked about reductions of up to 25 percent, but refused to make any specific proposal. They also have hinted, but left uncertain, that they would apply the limits to warheads as well as launchers.

The United States has renewed its earlier proposal to reduce strategic warheads by one-third, coupling it to a limit on total explosive power permitted either side in land-based missiles -- a proposal designed to prevent one side from having the ability to destroy the other's missile force.

What exactly did Gorbachev mean when he said he would permit "fundamental science" research on space technology up to, but not including, the "designing stage" when "models or mockups or test samples" of weapons are field-tested?

In a study released in April, the Pentagon said it believes that the 1972 Antiballistic Missile Treaty permits field testing of "experimental devices to demonstrate technical feasibility and gather data prior to reaching the stage of prototype or breadboard model of an actual ABM component or weapon system."

Officials warn that Gorbachev's statements are only propaganda until they are presented in a serious form.

"What's needed is for the Soviets to translate their many public statements into actual negotiating proposals in Geneva," State Department spokesman Charles E. Redman said yesterday.

Redman also said that Gorbachev's statements could be interpreted to mean that the strategic weapons reduction proposals would come only after U.S. agreement to limit SDI research. "In that sense," the spokesman said, "the Soviets are still posing preconditions for serious negotiations."

Meanwhile, the administration continued preparations yesterday for the resumption of the arms talks in Geneva on Sept. 19.

At the White House, national security affairs adviser Robert C. McFarlane presided over an interagency meeting of senior arms control officials developing positions on issues, one official said. Another meeting of that group is scheduled for Friday and a full National Security Council meeting chaired by the president is set for next week.

Gorbachev's statements on the SDI research issue prompted the most comment yesterday. Up to now, the Soviets have maintained at Geneva that they want to bar all activity leading to a space-based defense system, including laboratory research.

Outside the Geneva talks, several Soviet officials have said publicly that they recognized that laboratory research could not be barred, but the Gorbachev formulation takes those statements a step further.

By leaving open tests until a country created "models and mockups" of weapons, he "leaves a lot of room for wiggle" for both sides to test outside the laboratory, one official said yesterday. Another pointed out that the Soviets need to have some leeway for their own space program activities.

Officials also are watching closely for details on the Soviet proposal for deep reductions in strategic weapons. At the last round of the Geneva talks the Soviet delegates floated the idea of a sublimit for each type of strategic delivery system -- the so-called triad of bombs, submarine-launched intercontinental missiles and land-based ICBMs.

"They proposed that no one leg would exceed some fixed percentage" of the entire strategic force, an official said, adding that they never said what that limit would be.

"If the offer is sweet enough," one official said of Gorbachev's pronouncements, "the question will be whether the president is prepared to abandon his baby."