Seven counter-terrorist experts, using forged credentials, infiltrated the government's Savannah River nuclear weapons plant, seized hostages and succeeded in taking over the control room of a large atomic reactor during a 1980 security test, informed sources said yesterday.

The mock raid, which was carried out by a special "threat assessment" team hired by the Department of Energy to test the security at the government's nuclear weapons production facilities, was described to horrified members of Congress last Friday at a classified hearing.

If the security experts who penetrated one of the three reactors at the South Carolina facility -- where all the plutonium and tritium for America's nuclear weapons is produced -- had actually been terrorists intent on sabotage, they could have triggered an incident that might have lead to a meltdown, sources said.

The three large reactors at Savannah River, moreover, were built in the early 1950s, and are not shielded by the domed secondary containment structures that surround the nation's commercial nuclear power reactors. It thus would be possible, by circumventing the filtering system, to engineer a massive radioactive release.

The threat assessment teams also found it relatively easy to infiltrate the government's plutonium production reactor at Hanford, Wash., and discovered problems of major concern in the safeguarding of materials at Rocky Flats, Colo., where parts for nuclear weapons are manufactured, the sources added.

The only weapons production facility to get high grades for security and safeguards from the counter-terrorist consultants was the Pantex Plant outside Amarillo, Tex., where America's nuclear weapons are actually assembled.

Sen. John Glenn (D-Ohio), who last week sent a classified letter to President Reagan urging him to act immediately to rectify serious problems in security at the nuclear weapons plants, said yesterday "the administration obviously has not been taking these major security deficiencies seriously."

Glenn said that the administration, which originally included $62.8 million for upgrading security at the bomb plants in the fiscal 1983 budget, later eliminated this amount entirely and did not seek any funding for this purpose in the supplemental appropriations bill Congress just passed over the president's veto.

"Now, they are apparently planning to stretch out these essential upgrades into 1984 and 1985 -- nearly 10 years after problems were first found," Glenn said.

The Department of Energy yesterday declined to comment either on security tests or on its plans for the fiscal 1984 and 1985 budgets. Cliff Webb, a spokesman for the Savannah River plant, also declined to comment on the security tests that have taken place at that facility.

Sources who attended last Friday's classified congressional briefing, which was called following release of a classified General Accounting Office report saying that serious security problems at the nuclear weapons plants had not been corrected, said members of the assessment teams described in some detail the manner in which they infiltrated the Savannah River plant.

They gained entry to the facility, a 300-square-mile reservation that borders the Savannah River, simply by turning off Route 125, a public highway that bisects the plant, sources said. While there are checkpoints at either end of the main highway that keep track of through traffic that is routinely allowed to cross the reservation, the team had no difficulty turning off the public road and the fact that they had not exited was not immediately detected.

They subsequently passed through interior checkpoints within the reservation, the sources said, by using forged credentials.

The top management of the plant, which had been notified in advance that a security test would take place, was so horrified by the ease with which the counter-terrorist team took over one of the nuclear reactors that they asked that the remainder of the test be canceled and turned into a training exercise for security personnel, the sources said.

In addition to the three active nuclear reactors, the Savannah River plant has a fourth large reactor scheduled to be returned to operation next year. It also houses two large plutonium reprocessing plants, which separate out the super-grade plutonium that is used in the nation's nuclear weapons, and a tritium plant that produces the tritium used in hydrogen warheads.

Sources who attended Friday's hearing said one of the major problems in security at all of the government's weapons facilities appeared to be the belief that a major terrorist attack, mounted by a half-dozen highly trained personnel using sophisticated infiltration and commando techniques, simply "couldn't happen in America."

"Too many of the guards simply cannot conceive of this type of threat," a source said.

The sources also said the orientation of the security forces at the plants appeared primarily directed toward preventing the theft of nuclear materials or weapons parts, and said they did not appear equally alert to the possibility of sabotage.