The Department of Energy has termed untrue a report in yesterday's Washington Post that counter-terrorist experts, as part of a 1980 security test, had staged a mock infiltration of the government's Savannah River nuclear weapons plant, had seized hostages and had taken over the control room of a large atomic reactor.

"No such infiltration or seizure occurred, nor was a control room overtaken," said Assistant Secretary Herman E. Roser. "The allegations are completely inaccurate and false."

A senior congressional source, however, confirmed yesterday that the mock raid at the South Carolina plant reported by The Post had been described on Sept. 10 at a classified hearing of a House Energy subcommittee.

Cox Newspapers, meanwhile, reported in editions being prepared for today that the independent investigations unit that sent counter-terrorist teams to test the security at nuclear weapons facilities has been disbanded by two longtime Energy Department officials whose operations in the field were exposed as having security gaps.

The report, by Joseph Albright of Cox's Washington bureau, said Robert L. Morgan, the head of the Savannah River operations office at the time of the 1980 test, was called to Washington at the start of the Reagan administration to run the government's nuclear weapons-making program temporarily.

In one of his first moves, Morgan ordered the counter-terrorist teams not to visit any more nuclear weapons factories, Cox reported.

Later in the year, Roser, then head of the Energy Department's Albuquerque operations office, was nominated to fill the job Morgan had been holding temporarily, Cox reported. As soon as Roser, whose Albuquerque office also had been criticized by the special investigators, was confirmed in his post last January, he abolished the Office of Assessments that had recruited the outside experts, Cox said.

Rep. John D. Dingell (D-Mich.), chairman of the subcommittee that received the closed briefing, issued a statement yesterday confirming that the subcommittee had heard testimony at the hearing elaborating on a recent classified General Accounting Office report highly critical of the adequacy of safeguards and security at key weapons facilities.

"We continue to be concerned about the attitude of certain Department of Energy officials in this matter," Dingell said in the statement.

Both Dingell and Energy Department officials said yesterday that the GAO report found no evidence that the physical security at any weapons facility had been breached "for the purpose of theft, sabotage or diversion."

The Post story made clear that the episode involved an official security test and not an actual incident.

Roser, in his statement, said the incident described in The Post's story "apparently refers to an inspection conducted over two years ago at the Savannah River Plant. There was no mock penetration as part of that review."

While Roser declined to elaborate on his statement, Assistant Secretary Robert C. Odle Jr. said The Post's story "basically involved activities that took place in September, 1980, when we went out and hired some people to take a look if it the Savannah River Plant was safe, and if not how it could be improved."

While The Post story reported that the management of the plant had been notified in advance of the security test, Odle said, "Any lay observer would assume reading the story that a bunch of outsiders went in, nobody was told and they took over the place . . . . That just didn't happen.