Nuclear weapons scientist now believe they could pack 12 or 13 powerful, silo-busting warheads on the proposed land-based MX intercontinental ballistic missle where little more than a year ago they worried about fitting 10, according to Pentagon and Department of Energy sources.

The strategic arms limitation treaty (SALT II), now up for Senate approval. limits the United States and Soviet Union to 10 warheads on each new land-based ICBM such as the MX. The limit was chosen by U.S. negotiators because it was the largest number of warheads tested on a missle by the Soviets and more than was contemplated at the time by U.S. weapons builders.

The ability of scientists at the two government-financed nuclear weapons laboratories -- in Los Alamos and Livermore, Calif, -- to come up with warhead designs for the MX that meet or exceed limits previously thought unreachable shows how quickly weapons technology is advancing.

Another example of expanding strategic weapons capabilities occured this past weekend when the first refitted Poseidon submarine, with new, longer-range and more powerful Trident missiles, slipped out to sea on its insitial operational patrol.

The Trident missile has a range of about 4,000 miles whereas the Poseidon missile it replaced went about 2,500 miles. The longer range permits the missile-launching sub to roam an area more than twice as large as in the past. It also will be able to reach some Soviet targets, if necessary shortly after leaving its home port in Kings Bay, Ga.

In addition, the Trident missile carries eight warheads, each with the explosive power of 100 kilotons, equal to 100,000 tons of TNT. The 1945 Hiroshima A-bomb was 12.5 kilotons.

Although the older poseidon missile had more warheads -- 10 to 14 -- each had the power of 40 kitotons.

Three more Poseidon subs will be outfitted with Trident missiles in the current fiscal year, and eight others over the next two years.

However, developments in land-based missile warheads best illustrate how rapidly nuclear arms can be advanced.

Livermore produced in the early 1970s the Mark 12 warhead for the Minuteman III missile with a 175-kiloton yeild. By 1974, the Air Force wanedt to double that yield without making the warhead larger. Within two years, Los Alamos scientist had just about done it.

The Mark 12A was slightly heavier -- by some 35 pounds -- and could produce a 340-kiloton blast.

Three Mark 12As fit on a Mintueman III. Originally Pentagon officials hoped that the MX would carry the Mark 12A. The number hoped was 10; 12 or 13 is the number they now could get.

If SALT fails in the Senate, the United States could put more than 10 warheads on the MX. But the Soviets would also be able to add warheads to their giant SS18 missile that now carries 10. Neither side could be certain where that competition would end, U.S. scientists say.

Officials from the Arms Control and Disarmament Agency and the State Department say the Soviets want SALT II approved in part because they believe U.S. technology would win that race.

A second MX warehead design is under consideration. It is larger than the Mark 12A, with a more powerful explosion -- some 500 kilotons.Initially it was believed an MX could carry eight of these warheads. But scientists have redesigned it so that 10 of these bigger nuclear bombs would fit on the MX -- giving it the same coverage of targets as the Mark 12A.

Scientists from both nuclear weapons labs are to meet this week in Germantown with Air Force and Energy Department officials to plan 12 more months of study for the two MX designs.

"We have anothr year to see what they can come up iwth," said one official involved in the project. Energy Department officials want less nuclear material used to achieve the same explosive results -- thereby cutting down on the need for additional plutonium production.