President Reagan's vision of an effective shield against a Soviet nuclear attack "does not appear feasible" without cooperation from the Russians, including agreed reductions in offensive nuclear weapons, according to a study released yesterday by the congressional Office of Technology Assessment.

"Assured survival of the U.S. population appears impossible to achieve if the Soviets are determined to deny it to us," the OTA said, because of inherent advantages that offense would have over defense in a nuclear war.

The detailed study, which was done by OTA's staff aided by an advisory panel of supporters and opponents of the president's Strategic Defense Initiative research program, warned that without a negotiated agreement on the introduction of new defensive systems, the two superpowers "might easily suspect the other of attempting to gain a military advantage" and institute a new arms race in both offensive and defensive weapons.

The Soviet Union has thus far expressed adamant opposition to the idea of a negotiated agreement to introduce new defensive systems, although, according to the Reagan administration, it is also conducting active research on defensive technologies.

U.S. security would increase, the study said, if it proves feasible to build a defensive system that would protect population centers and American missile fields but only if an agreement with the Soviets could be reached to coordinate weapons reductions and the introduction of defensive systems. However, the study notes, "no one has as yet specified in any detail just how such an arms control agreement could be formulated."

"It is too early to predict the likelihood of success" in SDI research, the study said, but added that the end of 1986 would be a good time for decisions on future funding of the program.

A related OTA study of antisatellite weapons, also released yesterday, found that an arms control agreement could "reduce" the prospect of a race in such weaponry, but it "could not guarantee the survival of U.S. satellites attacked by residual or covert Soviet antisatellite weapons."

This study also found that an effective antisatellite arms control treaty "would place significant restrictions on the testing and deployment" of SDI, since many of the same technologies would be used in both systems.

The two reports are being released at a time when Congress is debating future SDI and antisatellite funding, and U.S. and Soviet officials are discussing their roles in arms control negotiations.

The SDI study says the program raises a fundamental issue of whether the 1972 Antiballistic Missile Treaty "continues to be compatible with our national interest" because it prohibits a nationwide missile defense system, which is the goal of the Reagan program.

It argues that the United States must eventually choose between seeking Soviet agreement to revise the treaty or withdraw from it. The study found, however, that the current U.S. "middle ground" objective "may be the most difficult to attain" -- seeking to bolster the ABM treaty to prevent near-term Soviet ABM testing while publicly declaring American intention to abandon it later "when we are ready."

The study then noted "the inherent paradox" that the administration plans to continue development of SDI even without Moscow's cooperation, while at the same time recognizing that an agreement with the Soviets would be necessary if the long-term SDI goal is to be met.

The study takes issue with some past beliefs of both supporters and opponents of the administration programs:

*Current Soviet ballistic missile defense technology "probably does not exceed our own," the report finds, but adds that because there is an ABM site around Moscow, the Soviets "are almost certainly better equipped in the near term to deploy a large-scale 'traditional' [land-based ABM] system than we are."

The study quickly adds that although the Defense Department has said the Soviets may be preparing such a nationwide system, "officials of the CIA . . . do not judge it likely . . . in the near term."

*Although the Soviets are "vigorously developing advanced technologies" for futuristic, space-based strategic defense, "in terms of basic technology capabilities, the United States is clearly ahead of the Soviet Union in key areas required for advanced [SDI] systems."

The study questions the president's publicized statement that he would be prepared to share SDI technology with the Soviets if it proved successful. It points out that giving such information away might allow Moscow to find weaknesses in the U.S. system and that the SDI technologies "have application in other military areas that we may not want to help the Soviets develop."

*The study addresses the uncertainty over just what the administration's SDI goal really is. The president has often referred to it as a program that if successful would render nuclear weapons obsolete. The study, however, says this is not the real objective of the administration's current program. Using a 1985 Pentagon definition and "some of the president's language," it ascribes to SDI the more limited goal of making ballistic missile defenses cheaper than new offensive weapons, thus paving the way for negotiations with the Soviets to reduce missiles.

The 324-page study of the SDI program, along with a 146-page report on the antisatellite program, were done at the request of the House Armed Services Committee and the Senate Foreign Relations Committee.

The aim of the studies, according to OTA officials, was to "pose the issues in a fair point of view" so that members of Congress could understand the policy options as they come up in legislation.

The 21-member advisory panel, made up of such diverse defense specialists as Reagan administration consultant Colin Gray and former defense secretary Robert S. McNamara, reviewed the study as it progressed as well as the final drafts of the report.