A budget account for a Reagan-era program to build a new generation of exotic nuclear weapons for the "Star Wars" missile defense effort was eliminated yesterday by House and Senate defense-spending conferees, and some legislators and independent experts said the action could hobble the weapons component of the Strategic Defense Initiative (SDI).

The so-called "nuclear directed-energy weapons" program to build lasers and projectile guns capable of destroying Soviet missiles in flight had played a key role in President Ronald Reagan's 1983 decision to launch SDI, which had been heavily promoted to Reagan a few months earlier by H-bomb scientist Edward Teller.

Since then, however, the highly classified program has evidently achieved few scientific breakthroughs, causing it to lose some scientific and political support. House Armed Services Committee member Rep. John Spratt (D-N.C.), who initially proposed yesterday's move, said many scientists now believe the program is not likely to produce a deployable weapon in the forseeable future.

In combination with a separate action to cut SDI's current budget by roughly 24 percent, the nuclear weapons decision signaled new congressional disaffection with the entire missile defense effort, which grew at a faster rate than virtually every other military research project under Reagan.

Scientists at the three major U.S. nuclear weapon laboratories once said they hoped the program would culminate in deployment of a "third generation" of nuclear arms to follow the massive arsenals of atomic and hydrogen bombs developed by the United States during and shortly after World War II.

Unlike earlier devices, the new weapons were to have been designed to wreak destruction in what scientists called a highly "tailored" manner, using nuclear explosions to produce highly concentrated and carefully focused energy such as optical and X-ray lasers or swiftly moving projectiles that could destroy targets such as Soviet missiles at a great distances.

Teller initially called the X-ray laser being developed by his protege Lowell Wood "the most novel and potentially the most fruitful" SDI device. He told the White House in a 1984 letter that "a single X-ray laser module the size of an executive desk . . . could potentially shoot down the entire Soviet land-based missile force" carrying thousands of nuclear warheads.

But no such weapon has emerged from the program, which received roughly $1.5 billion from the Energy Department under specially earmarked appropriations since 1985. The new defense bill accepted by the conferees eliminates $192 million in specially earmarked funds and, if approved by the House and Senate and signed into law by President Bush, will allow future research -- but no development work -- on exotic nuclear weapons for SDI only at the expense of other non-SDI nuclear weapons work.

The entire nuclear weapons research and testing budget also will shrink, and language in the conference report is expected to order the Energy Department to spend up to $160 million of these funds on safety improvements for existing nuclear weapons.

Rep. George Brown (D-Calif.), a longtime opponent of the weapons effort, said "you can safely say this represents the demise of Star Wars" as it was initially conceived, because "the centerpiece is no longer being seriously considered."

The program seemed headed for trouble even before the latest congressional action. The number of people working on it has shrunk in recent years by more than one-third, to roughly 1,000, and its budget was also reduced by one-third from a peak of $317 million in 1987. Many officials never overcame embarrassment that the SDI effort, ostensibly established to render nuclear weapons "impotent and obsolete," included such a large effort devoted to creating new ones.

A lobbying campaign by Teller and Wood to enlarge the program's budget several years ago was briefly successful but ultimately backfired when it prompted the resignation of a senior official of Lawrence Livermore National Laboratory, who charged that its scientific promise had been exaggerated.

Spratt noted that even Teller and Wood were now "off pursuing other programs," such as the controversial Brilliant Pebbles concept of deploying tens of thousands of small non-nuclear missile interceptors in space. Neither of the two scientists could be reached for comment yesterday, but Energy Department spokesman James Fox said the agency applauded elimination of a special account for the program.