A half-built particle accelerator, the largest pure-science project under way in the United States, should be scrapped to make way for a newer, bigger version despite the $200 million invested in it, a panel of physicists has told the Department of Energy.

The machine proposed for abandonment is now an empty 2 1/2-mile circular tunnel under the middle of Long Island at the Brookhaven National Laboratory. It was scheduled to cost as much as $450 million more to complete.

"What will we use the tunnel for now?" asked Arthur Schwarzschild, chairman of the physics department at Brookhaven. "People are talking about making it an indoor race track. My son says it would be a great place to store his car in the winter."

Schwarzschild is able to joke about the matter. But feelings around the laboratory have run strongly, from bitterness to fear, since the federal panel several weeks ago recommended scrapping the accelerator.

One federal science official said that while it is terrible to waste the $200 million that has been spent at Brookhaven, it would be worse to waste several hundred million more dollars and find that the machine is outmoded when completed.

Heated arguments about the unfinished accelerator have been carried on for a year or more. And Brookhaven partisans had hoped the machine being built there could serve as a prescursor to the proposed "super-conducting supercollider," an accelerator planned to be 75 to 120 miles in circumference and 100 times more powerful any in existence.

Though the final decision on the fate of the Brookhaven project has not been made by the Department of Energy, most scientists expect that the Brookhaven accelerator is doomed and that the giant new supercollider, or some version of it, will be built instead.

The Brookhaven project, nicknamed "Isabelle" and started in 1977, has been plagued with trouble for years. There were problems in designing the key elements in the accelerator, the 1,100 magnets that would collide atomic particles.

Magnet failures eventually knocked the schedule for completion back four years or more. The cost of the machine also escalated by $200 million, to $650 million.

Then, just as the technical problems were worked out and the machine was ready to be constructed, another blow came last fall.

Scientists in Europe discovered the key elementary particles for which the Brookhaven machine was being built to hunt. Thus, one of the major purposes of the machine vanished. Science had passed the machine by.

Finally, the federal panel's recommendation to shut down the project came. "We're all disappointed," said Nicholas P. Samios, the lab's director. "But what we're talking about now is what kind of new machine we can put in that tunnel . . . ."

Some 300 people were working directly on the project when the news came. "All of them were worried for their jobs," Schwarzschild said.

All 3,000 at Brookhaven were aware that the loss of the accelerator would be a serious blow to the prestige of their laboratory, since it is the lab's biggest single new project.

Now, the workers from that project may do other work at Brookhaven, or may help design and build the new supercollider if it is funded by the Department of Energy.

Brookhaven scientists say that much of the "Isabelle" money will not have been wasted if another machine can be built in the tunnel, or if the tunnel can be used as a laboratory and testing ground for the thousands of 20-ton magnets that will be needed for a supercollider.