The United States is about to double the nuclear explosive power of more than 2,000 warheads now carried by Poseidon submarine-launched missiles as part of a major upgrading of the nation's strategic force, according to administration sources.

The unprecedented rebuilding program will increase the yields of about 300 of the 8-year-old Poseidon missiles.Another 192 already are scheduled to be replaced by newer, longer-rang Trident I missiles.

At the same time, it has been learned that Pentagon nuclear weapon planners are considering installing a bigger warhead on the new MX landbased, mobile missile that would turn it into a mirror image of the giant Soviet SS18. The SS18 has an estimated payload to five to eight megatons, depending on the number of warheads packed into the nose of each weapon.

One megaton is the equivalent of one million tons of TNT. The 1945 Hiroshima bomb was 12.5 kilotons, equal to 12,500 tons of TNT.

In the perverse logic of arms control politics, the plans to boost yields of the two missile systems may be chalked up as an attempt to assuage American critics of the newly signed SALT II strategic arms limitation treaty. They say it gives the Soviet Union an unacceptable superiority over the United States in warhead megatonnage.

But interviews with nuclear weapons planning officials indicate scientific and bureaucratic factors may also be playing a role in the decision.

The Poseidon enhancement program, for example, appears to have developed after a troublesome maintenance problem in the missile's inner workings turned up.

During recent annual reliability tests, the chemical high explosive in the Poseidon appeared to be of questionable performance.

Taking advantage of the need to replace the weapon's explosive charges, scientists also decided to replace the nuclear package using new designs developed since the original Poseidons were made.

The result: a doubling of the yield from 50 kilotons to 100 kilotons in more than 2,000 warheads.

The determination of what payload the MX would carry is still far from settled, according to officials close to the program.

When the president's decision to go ahead with the MX missile was announced June 8, Pentagon officials said it would likely carry 10 separately targeted warheads, each with a yield of about 350 kilotons.

The warhead mentioned was the Mark 12A, the same one being put on the Minuteman III.

Now, however, the Air Force, according to sources at one of the nation's weapons laboratories, favors a warhead with a 500-kiloton yield - one that was tested at full strength in 1973.

The test date is important for it took place before March 1976, when the so-called threshhold test ban treaty with the Soviet Union took effect. Since that time, the United States has not tested a device more powerful than 150 kilotons, the limit set by that agreement.

Although it is possible to test a device below 150 kilotons and then "scale it up" in scientific terms to be a 350 kiloton or larger warhead, some U.S. nuclear weapons designers have doubts about such a procedure.

A heated controversy still exists between the nation's two weapons laboratories - Los Alamos Scientific Laboratory in New Mexico and the Lawrence Livermore Laboratory in California - over the yield of the Mark 12A warhead.

Los Alamos developed the device and tested a version of it before the threshhold test ban went into effect. It did not test out to its projected 350 kiloton yield. When some adjustments were made, it was too late to test it at full strength - since the 150 kiloton limit was in effect.

A scientific panel had to project what the yield should be.

In the wake of the event, the former director of Los Alamos, Dr. Harold Agnew, announced a policy that his lab would not certify a weapon's yield unless it had been tested at full strength.

The Air Force lean toward the 500-kiloton warhead is said to stem mostly from the fact that the device has been fully tested.

Another unpublicized factor in the MX warhead determination is the competition for doing the design that has been under way for almost four years between Los Alamos and Livermore.

Over that period a reported 12 design proposals have been generated - in part because the Pentagon has several times changed the weapons characteristics it desires.

Prestige is also heavily involved. Livermore gained its reputation in the late 1950s and early 1960s as the builder of strategic weapons - the Polaris and Poseidon sub-launched warheads and the Minuteman land-based ICBMs.

More recently, Los Alamos has taken over the big weapons, winning design of the modern Minuteman Mark 12A, the sub-launched Trident and the air-launched cruise missile.

Los Alamos is also the designer of the 500-kiloton device the Air Force now favors.

Within the weapons building community, it is believed that Livermore expects to get the MX job.

Livermore, however, runs into trouble because the only moderate yield devices it has designed for the MX were tested since 1976 at below 150 kilotons, according to a government official.

That official, however, said Livermore does have designs for "megaton size" warheads that would be compatible with the MX.

The solution to the bureaucratic problem of which laboratory gets the design could be solved, according to this official, if the MX were given two different warheads. Los Alamos would provide eight or 10 warheads each with a 350- or 500-kiloton yield. And Livermore would develop the larger megation size with the MX carrying four to six of them.

That formulation almost matches what U.S. intelligence sources say the Soviets have done with the loading of their SS18. And it is one option being examined within the Pentagon for the MX.

Under the nuclear weapons building procedures, the Pentagon is expected to send the Department of Energy a letter in the next few months outlining the characteristics it wants in the MX warhead.

DOE then will have to choose whether one laboratory or both will get the job. The decision could also come just as the SALT II ratification debate reaches the Senate floor. So strategic arms limitation politics could well be part of the reason for whatever size warhead load is finally chosen.