Superpower minuet: Who takes next step?

The Reagan administration's plan for deploying the MX missile combines two of the most basic elements of warfare: concrete and confusion. But it is this simple yet untried combination that is generating heated debate among the experts on nuclear weapons and strategy.

The capsules in which the missiles are to be placed are nothing more than beefed-up fortifications - steel bars and poured concrete six to eight feet thick - designed to withstand the terrific pressures that would come with an enemy nuclear strike. By bunching 100 new ''Peacekeeper'' missiles near Cheyenne , Wyo., the Air Force hopes to create such uncertainty - and in time of war, confusion - that Soviet leaders would hesitate to launch a nuclear first strike.

Ironically, both superpowers are borrowing from each other's strong points in fashioning their strategic arsenals. The Soviet Union has made much use of US high technology to build extremely accurate intercontinental ballistic missiles. Now, according to senior Defense Department officials, the United States is using intelligence gathered on Soviet missile-silo hardening to increase dramatically the protection of American ICBMs.

Just as it did before the President's official announcement this week, the ''dense pack'' basing plan has legions of critics. Many criticize it on technical grounds, warning that the missile (now renamed ''Peacekeeper'') could not survive, let alone endure, a Soviet attack. Others say it could violate existing arms control treaties.

The basic principle on which dense pack (officially called ''closely spaced basing'') rests also is not new. This is the theory that enemy warheads, if forced to attack targets in close proximity, will destroy or throw each other off course with their own blast, radiation, and debris. Called ''fratricide,'' this phenomenon has been studied by weapons planners for years.

The Air Force says at least half of the 100 missiles placed 1,800 feet apart in a 14-by-1.5-mile array would survive a Soviet attack. Because this theory could be proved only under actual wartime conditions or with atmospheric tests of nuclear weapons (which are banned), no one knows for sure whether it would work.

There also is uncertainty about whether the ''super-hardened'' silos could protect the MX from anything less than a direct hit, as Pentagon officials assert. The Air Force has been testing to one-quarter scale hardened missile capsules placed in areas larger than a football field covered with high explosives. Officials say the tests have been successful at pressures several times those to which existing Minuteman missiles are hardened. Tests at half-scale have shown that the capsules can be hydraulically pushed through many feet of rubble and positioned for launch. More tests are scheduled for December.

''Obviously, if it's going to work, you have to convince yourself that you can put those capsules out there in such a way that they're sufficiently hardened so they're highly survivable in the sense that one RV (reentry vehicle) could not take out two of those capsules with any high degree of confidence,'' says a high-ranking Pentagon official. ''That has been a major challenge, both from an analytical point of view and from a test point of view.''

The combination of hardened silos and closely spaced basing presents a dilemma for the Soviet Union, US officials say. Hardened silos require a ground burst of high yield to destroy them. Yet such a burst creates the very conditions that could bring about fratricide. Pentagon planners hope to force the Soviet Union to direct an unacceptable percentage of their strategic force (including submarine-launched missiles) against the MX dense pack, thus leaving the other legs of the US strategic triad intact and deterring such an attack.

The Pentagon acknowledges that the Soviet Union could eventually threaten the MX by developing highly accurate, low-yield, earth-penetrating weapons that would be able to avoid the effects of fratricide.

For this reason, the Defense Department is continuing to research possible countermeasure additions to the dense-pack plan: adding more empty silos as a form of deception, developing a ballistic missile defense system to intercept and destroy attacking warheads, or placing the MX capsules more deeply within the earth. These could add considerably to the system's current $26.4 billion price tag, not including maintenance.

Congress has 30 days to reject the administration's plan for deploying the new Peacekeeper missiles. If it does not, the first test firing of the missile would be in 1983, and initial operational deployment would be in 1986, with complete deployment by 1989.