For nearly four years, a report giving "some precise details" of the secret devices that trigger hydrogen bombs was open to inspection and copying by numerous Americans and foreigners using the public library of the Los Alamos Scientific Laboratory in New Mexico, the Department of Energy acknowledged yesterday.

Maj. Gen. Joseph K. Bratton, who runs the nuclear weapons program for DOE and is its acting assistant secretary for defense programs, termed the security breach "serious."

"We have egg on our face," he said.

The data in the document "could be damaging" and are "of value to someone who is interested in designing a bomb" even though the paper is 23 years old, Bratton said in a telephone interview.

The 29-page report quotes "some precise numbers" in a summation of results of various devices used to test H-bombs in the 1950's, Bratton said.

But he emphasized, the data are "far from enough" to enable anyone "to design or build a trigger mechanism." He said the information in the report is "sketchy" and depends heavily on related reports that never have been publicly available. The report has no diagrams or illustrations, he added.

The document carried a "secret" stamp on the top and bottom of each of its 29 pages until the old Atomic Energy Commission declassified it on July 30, 1975.

Blaming a clerical error, Bratton said that only Part VI, consisting of several pages on nuclear propulsion of spacecraft, was supposed to have been declassified.

In March 1978, Sen, John H. Glenn (D-Ohio), chairman of a Senate subcommittee on nuclear proliferation, held hearings on DOE practices regarding protection of nuclear weapons secrets. As a result, DOE assured Glenn that it was undertaking a thorough review of materials in its libraries to be certain that classification procedures were adequate.

The erroneous declassification "should have been caught by the review," Bratton conceded. "It's an embarrassment."

Glenn said he finds it "hard to believe that a document so clearly concerning weapons development didn't get picked up by that review."

He said he was reopening the subcommittee investigation and may hold new hearings. "This breach of security concerning some of our nation's most important military secrets is shocking," Glenn said, adding:

"If the document is truly as sensitive as DOE says it is, it means that for the past four years any nation trying to develop advanced nuclear weapons had virtually free access to vitally important information which could take years off the time needed to develop a hydrogen bomb."

The new developments are an outgrowth of the effort by the Progressive magazine to publish, and of the government to suppress, an article on the workings of the H-bomb.

Howard Morland, who wrote the article, says he got all of the information for it from public sources. But a federal judge, saying the article contains concepts that are confidential under the Atomic Energy Act, granted the government a preliminary injunction to prevent publication.

The magazine and Morland, as well as the American Civil Liberties Union, which is representing Progressive editors Erwin Knoll and Samuel H. Day Jr., are appealing to the 7th U.S. Circuit Court of Appeals in Chicago.

In a letter delivered to Assistant Attorney General Barbara Babcock shortly before noon Wednesday, ACLU lawyer Bruce J. Ennis said that a volunteer researcher, Dimitri A. Rotow of Alexandria, had been sent to the Los Alamos library to confirm what Morland had learned there last year: "That basic concepts of fusion weapons . . . are already in the public domain; in fact, they have been put there by the government."

Going to the library's card catalogue a week ago last Monday, Rotow and an aide quickly found the document at issue, which was designated "UCRL 4725." Each of its 29 pages had been perforated by a declassification marking.

Rotow made a photocopy of the report, mailed the copy to the ACLU, and returned the document to its place on a shelf. Subsequently, he sent out additional copies, including one to The Washington Star, which published an article on it yesterday.

Rotow had aroused instant attention from the library staff, because he'd used the facility in 1978 to develop startling information on weapons design for Glenn's hearings. If he'd been a foreigner, he quipped to a reporter yesterday, he wouldn't have been noticed.

Later last week, Rotow again removed the document from the shelf, but at closing time left it on a desk with other papers under a note saying, "Do not remove . . ."

According to DOE's Bratton, a security-cleared employe of the laboratory walked by the desk, saw the paper protruding from the pile, was alarmed by its nature, and notified authorities. As a result, the library was closed down for yet another documents classification check.

In an earlier affidavit, acting laboratory director Robert Thorne had asserted that the Morland article for the Progressive contained information "not publicly available."

But Thorne and other government officials who made similar sworn statements in the litigation "did not know - or care - what was on the government's own library shelves," the ACLU's Ennis protested to the Justice Department.

Accusing the government of trying to "cover up" evidence needed for the Progressive's defense and of "official misconduct and interference with the preparation of our defense," Ennis submitted demands including immediate reopening of the library and restoration to its shelves of all materials hitherto publicly available.

If the Justice Department doesn't agree to the demands by noon today, the ACLU will go to court for orders to protect the evidence it needs and possibly for dismissal of the Progressive case, Ennis said.

A department spokesman said that the demands would be answered with a letter. DOE's Bratton said,"We acted in good faith."

Disputing DOE's account about how the report was discovered among Rotow's papers, Rotow said it was buried out of sight and could not have been noticed on a passing glance. He also asserted that the report would be more than 50 times as helpful as the suppressed Progressive article to a foreign power wanting to build an H-bomb efficiently and quickly. Hundreds of other public documents in the library would be just as useful, he said.

Bratton, however, said he "just can't agree" that other public documents at the library could be as valuable as UCRL 4725. CAPTION: Picture, Dimitri A. Rotow with a photo copy of "UCRL 4725." By Linda Wheeler - The Washington Post