In an article about the Nuclear Emergency Search Team in Tuesday's Metro section certain information about a demonstration by the nonviolent environmental group Greenpeace was incorrect. The demonstration, which occurred on Aug. 21, 1980, concerned a protest of uranium mining practices and potential resulting pollution problems, and involved harmless quantities of uranium mill tailings. The incident did not constitute a terrorist threat. The concrete barrel used to house the tailings and the Geiger counter attached to the apparatus were described incorrectly as a "possible bomb" by police at the scene, who relayed their concern about potential danger to the Secret Service and NEST. Greenpeace engages in peaceful, direct action on ecological issues.

The thousands of tourists who poured into Washington for the Bicentennial celebration in 1976 probably never noticed the unmarked vans circling the streets around the federal buildings off the Mall. Though the men driving the vans were dressed in short-sleeve shirts and sport pants to resemble ordinary delivery men, they were actually nuclear experts on a secret government mission.

The drivers were all members of the Nuclear Emergency Search Team (NEST), a secretive, select group of U.S. Department of Energy scientists and technicians who respond to threats of nuclear terrorism.

On this particular day they were checking the radiation levels around federal buildings because the FBI was worried that a terrorist group might use the highly public event to threaten to explode or release nuclear material.

That fear proved false. But since then, the 250 NEST experts--30 of whom are stationed at Andrews Air Force Base--have criss-crossed the continent, from Pennsylvania Avenue to the Canadian Northwest Territories, responding to more than 20 threats of blackmail and to two nuclear accidents.

In only one extortion case did NEST experts find any actual nuclear materials. In 1979, a former employe of a Wilmington, N.C., nuclear power plant obtained uranium oxide and threatened to release it into the air unless he received $100,000 in ransom.

The incident ended safely--the man was caught, prosecuted and eventually sentenced to 15 years in prison. But NEST experts and FBI officials say the possibility of a truly serious threat of nuclear terrorism is no longer a question of if, but when.

"The technical know-how is out there for a group to create a device if it could obtain the materials," said Oliver B. (Buck) Revell, the FBI's assistant director for criminal investigations. Revell, a husky, silver-haired man with an imposing frame, would be one of the first officials to decide whether a threat is believable enough for NEST experts to be put on alert and eventually deployed.

"It doesn't have to be a tactical weapon to cause tremendous destruction," Revell continued. "Even a small, crude device has a destructive radius of at least a mile. That's not taking into consideration fallout."

So far, NEST has investigated two terrorism threats in the Washington area. In 1976, the anti-nuclear group called Green Peace said it had placed a nuclear device inside a van parked in front of the White House.

"We found a parked van with a 55-gallon drum full of concrete, with a recording device that went tick, tick, tick," recalled Herb Hahn, program administrator of the NEST operation at Andrews. "They said it was a nuclear bomb, but it was really a recording device."

In 1978, someone sent a package containing a dry, brown substance to a congressional office with a note saying the substance was radioactive. NEST checked it out: It was dirt.

FBI and NEST officials say what worries them most is the following scenario: A terrorist group, acting under the auspices of a Third World country with nuclear capability, smuggles a nuclear device or materials into this country, then threatens to explode or release them.

That scenario formed the plot of a recent novel called The Fifth Horseman, a copy of which Revell keeps in his office. In the book, terrorists backed by Col. Muammar Qaddafi of Libya, threaten to detonate a nuclear device unless world leaders agree to the establishment of an independent Palestinian state.

Although a nuclear device small enough to fit in a briefcase would be powerful enough to destroy a square mile of any major American city, the NEST experts say it would be easier, and probably more likely, for a terrorist group to try to contaminate water or release radioactive gases into the air.

"It is unlikely that a terrorist group could contaminate an entire city's water supply," said Revell. "But the fear of that is substantial enough to wreak havoc."

When a threat comes, usually to an individual, a police department or a company, it is immediately relayed to Revell's shop at the FBI.

If the threat is in writing, FBI experts examine the kind of paper it is written on, analyze the handwriting and have a psycholinguistics expert analyze such things as the letter-writer's choice of words and sentence structure for clues about the writer's state of mind and the region he or she may come from.

Ultra-sensitive lasers are used to lift fingerprints.

Meanwhile, energy department experts analyze the letter for technical data, such as how much the letter-writer shows he or she knows of nuclear devices.

Revell says he always puts the NEST experts on alert when a threat is received, no matter how unrealistic the threat may seem. NEST members are actually sent out only when "we have enough information to believe that there is some possibility of an actual device or material and we have a general idea of where it is."

If the threat was directed at an East Coast city, the NEST experts at Andrews would have to be ready to leave the base within two hours of deployment, Hahn said.

They would probably fly out on one of two NEST helicopters at Andrews equipped with radiation detection equipment. There is also a computer on board that interprets the collected data.

NEST's stock also includes a portable machine shop and photography laboratory that can be transported to the danger site in Air Force cargo planes if necessary.

Meanwhile, back at the FBI, Revell would open up a command center that has direct phone lines to the U.S. Attorney General, the State Department, the CIA and the White House, as well as computers that could call up information on well-known terrorist groups and television monitors of all local television stations.

If the threat were extremely serious, FBI Director William Webster would take personal control of the center and sit next to the direct phone line to the White House.

One of the more difficult questions the officials face is when to inform the public. Revell said there is no set procedure. It is the FBI, and ultimately the president, who makes that decision.

In the past when NEST has been called out on a threat, the public was not informed until the threat was over, and only then because the public first learned about the incident from some other source.

NEST officials try to appear inconspicuous when investigating a threat--thus the unmarked vans with radiation detection devices during the Bicentennial mission.

During a 1975 threat to the Union Oil Company of California, NEST experts searched six major company installations in the Los Angeles area dressed in business suits, their radiation-detection devices concealed in ordinary briefcases.

NEST and the FBI are guarded about the details they will divulge about their anti-nuclear activities. The very existence of NEST was not even known in the general public until three years after it was established in 1974, and information concerning it was revealed at a congressional budget hearing.

Today, NEST has a $14 million budget. Little is known about its members except that they include volunteer nuclear physicists and aviation mechanics who have other jobs within the Department of Energy.

The volunteers receive no extra pay, said Troy Wade, deputy assistant secretary for defense programs at the energy department.

One NEST volunteer, aviation mechanic Crestle Watson, 30, of Oxon Hill, said he heard about the program in 1976, when he was about to be discharged from the Air Force. "I did it for the challenge," he said.

Watson served on the NEST mission to Wilmington in 1979. Watson said from these and other missions, he is convinced that the U.S. could successfully handle any threat of nuclear terrorism. But, he quickly adds, "We're only human."