For three decades, serious mishaps at the Savannah River nuclear weapons plant were kept secret for national security reasons and in some cases not reported to Washington, according to federal officials who are trying to change that attitude.

One senior Energy Department official has compared the approach to safety at the Savannah River Plant, near Aiken, S.C., to that which led to the explosion of the shuttle Challenger on Jan. 28, 1986.And one member of Congress says the facility, which has been closed since August, should not be allowed to reopen until the situation improves.

An Energy Department spokesman, Will Callicott, said that since the federal government began making atomic weapons in World War II, "there has always been a mind set, a culture, that we are doing work that is important for national security and perhaps that may override any obligation toward public accountablility.

"People have historically not been sensitive to the importance of disclosing things," Callicott said. "Clearly, there is a lot of information that did not make it up the chain to top management here at headquarters."

Without a detailed review of the record, he said, it would be impossible to determine exactly what information had reached Washington and what had been released to the public.

Energy Secretary John Herrington, since taking office in 1985, "has given heightened attention to the area of environmental safety and health," and for the first time placed these issues in the hands of an assistant secretary, Callicott said.

The Savannah River facility produces plutonium and tritium, which are used in making nuclear weapons. It is run by E.I. du Pont Nemours & Co. under contract from the Energy Department.

A Du Pont inspector, G.C. Ridgely, wrote in a recently released August 1985 memorandum that 30 "reactor incidents of the greatest significance" occurred there. None, however, were as serious as the May 1986 disaster at Chernobyl in the Soviet Union, according to congresssional aides.

One of the most serious mishaps at Savannah River was the melting in November 1970 of a rod used to start an atomic chain reaction, causing radioactive contamination of an adjacent room. It took 900 people three months to clean up the contamination, according to the memo.

"Serious radiation exposure could have occurred because no outside alarm existed" and an "alarm was ignored for two hours," the memo said.

Last Friday, Energy Department officials said the department had not been informed of the incidents, but on Monday senior department spokesman C. Anson Franklin said those statements were incorrect and that the incidents had been reported to the department's regional office in Aiken.

He said the information apparently had not been relayed to headquarters in Washington from the regional office.

The reactors at Savannah River were closed down after the most recent incident, in August, and had been scheduled to reopen Tuesday. But Energy Department officials assured members of Congress last week that production would not resume until safety was assured, possibly after 30 days to 45 days.

"If they restart those things without having briefed us and without having convinced us they have solved the safety and health issues, they will be in serious trouble in the Congress," said Rep. Mike Synar, D-Okla.

Synar, chairman of the Energy, Environment and Natural Resources subcommittee of the Government Affairs Committee, held hearings on the Savannah River facility last Friday.

He reacted angrily Tuesday to assertions that national security considerations ever justified keeping mishaps secret at Savannah River.

"Just because they claim it is national security doesn't make it national security," he said.