Energy Department officials have conceded that their prediction it would cost $150 million to meet a proposed federal safety standard for the suspected carcinogen beryllium - a cost so high that it could threaten national security - came from "a gross estimate based on rule of thumb" by beryllium producers.

An official who was part of the Energy Department task force that made the prediction said his group failed to consider other economic studies by the beryllium industry and the government. One of those studies placed the cost of meeting the standard in a plant producing beryllium for the government at $4.6 million.

In a controversial letter made public last week by The Washington Post, Energy Secretary James R. Schlesinger cited the $150 million figure as too high for industry to meet. He warned beryllium producers would shut down the plants that supply the critical metal, thus posing a national security threat.

Other federal officials challenged Schlesinger's assertion, noting that only a small part of the industry's production goes for government use. Elimination of the standard in the name of national defense, they said. Would leave an estimated 30,000 workers exposed to a suspected carcinogen. The officials also challenged Schlesinger's economic data.

The Occupational Safety and Health Administration, an arm of the Labor Department, is preparing to adopt the standard to lower workers' beryllium exposure from two micrograms to one microgram per cubic meter of air over eight hours.

Beryllium, a critical metal in the aerospace and nuclear weapons industries, can cause fatal respiratory disease and has been cited in federal laboratory studies as causing cancer in nine animal species.

Glen L. Taylor, a member of the Energy Department's beryllium task force, said his group based its $150 million compliance estimate on data gathered only from the beryllium industry. The two U.S. producers of ultralight metal - Kawecki Berylco Industries and Brush Wellman Inc. - have vigorously opposed the implementation of the standard.

Taylor said an engineering study his group used would, in effect, have created a whole new plant for the beryllium producers' entire production although only about 2 percent of Kawecki Berylco's output goes to the government and 10 percent of Brush Wellman's beryllium is similarly purchased. The $150 million estimate, he acknowledged, was "very rough."

Two other studies on compliance costs were not used by the task force, Tayler said. In one, done for Kawecki Berylco by Catalytic Inc., a Philadelphia engineering and construction firm, the compliance estimate for the metal manufacturer came to $4.6 million to upgrade the company's plant at Hazelton, Pa., which produces beryllium for the government.

A spokesman for Catalytic Inc. refused to comment on the study Friday. But papers filed by Kawecki Berylco with the Labor Department indicate that for the $4.6 million expenditure the company would lower the exposure level to most workers in the Hazelton plant below one microgram.

Tayler and industry officials said the plant would easily be able to meet the annual federal requirement of about 80,000 pounds of beryllium.

In the past both Energy Department and Kawecki Berylco officials have insisted there was no way the manufacturers could achieve the proposed one-microgram standard no matter how much they spent.

But Kawecki Berylco documents filed with the Labor Department last October note that the $4.6 million "could be expected to bring most of the jobs below the one-microgram level."

The beryllium manufacturer's brief said the company was still not sure whether it could meet the standard, but even if it could, it "would not simply embark on a program of spending millions of dollars . . . in an effort to lower concentration levels." The primary factor in such a decision, it said, was the economic return on its investment.

The Energy Department task force also rejected a second study that put the cost for both manufacturers to meet the proposed standard at $3.7 million. That study was done for OSHA by the Boston consulting firm of Bolt, Beranek and Newman. Tayler said yesterday the study was not considered "because it was superficial."

A BBN official said, however, the study conformed to engineering requirements set by the Labor Department and his firm stood by its findings.

Disclosure last week of Schlesinger's letters on the beryllium controversy to Labor Secretary Ray Marshall and Health, Education and Welfare Secretary Joseph A. Califano Jr. stirred an angry reaction among some officials.

Marshall, in a statement Thursday, said he would not comply with Schlesinger's request to halt implementation of the new standard. He called beryllium "one of the most toxic substances known to man."

Union officials said they were questioning the legality of Schlesinger's intervention in the standard-setting process. The formal record for comment on the standard closed last January. Anthony Mazzocchi, vice president of the Oil, Chemical and Atomic Workers Union representing beryllium workers, said Schlesinger's letters were "an abuse of the statutory process."

"To reopen the record now," Mazzocchi said in a statement, "would only further delay the issuance of this badly needed health standard."