The fictional scenario is like the nuclear-age nightmares that have long haunted military and civilian planners: A Navy helicopter carrying three nuclear weapons crashes on the outskirts of a park in a small Virginia town, killing the entire crew.

While local police and firefighters quickly contain the resulting fire, Naval officers are horrified to discover that radioactive debris from one of the nuclear bombs has been scattered on an unsuspecting populace.

This week, showing a flair for detail that could rival a Hollywood director, the Pentagon is staging just such an event as part of an elaborate disaster game on this windy desert about 95 miles northwest of Las Vegas.

More than two years in the making and costing $3.6 million, the Nuclear Weapon Accident Exercise (NUWAX-83) is the Pentagon's most ambitious test yet of the country's ability to cope with a nuclear accident. It is a topic that Pentagon officials rarely discuss in public.

More than 1,100 federal, Virginia state and local officials have been flown here to observe responses to the mock emergency. Actual low-level radiation has been placed around the crash site to authenticate conditions that would be faced by clean-up crews.

In its search for realism, the Pentagon has tried to leave little to the imagination. A "Welcome to Virginia" road sign greets visitors outside the fictitious town of Port Gaston (pop. 7,000) in the mythical county of Jefferson. Virginia state police cars, flown to the exercise site in military cargo planes, cruise the town's dirt roads.

Abandoned Department of Energy buildings there have been transformed into the "Catfish Cafe and Fish Market," "Seven Seas Investment Bank," and "Anchor's Away Pawn Shop." Around the corner, there is the "Neptune Tattoo Parlor."

"It's no easy task to transpose an urban Virginia village to the Nevada desert," said Col. Frank Powell, the NUWAX chief of staff. "It's as realistic as we could possibly make it."

For all its surreal qualities, the simulated disaster is no fantasy. Since 1950, there have been 32 nuclear accidents, most of them involving the transportation of weapons. In 1966, a B52 bomber carrying four 20-megaton nuclear bombs collided with its refueling tanker, spreading radioactive materials over a Spanish farm. Two years later, another B52, also carrying four bombs, crashed near Thule, Greenland, and scattered radioactivity across the Arctic ice.

In the most recent accident, in 1980, an explosion in a Titan II missile site in Damascus, Ark., propelled a nuclear warhead 300 feet out of its silo.

The Pentagon's Defense Nuclear Agency has conducted NUWAX exercises twice before, on a smaller scale. The Reagan administration's military buildup is giving new priority to the concerns.

"In the future, we are anticipating an increase in the transportation of nuclear weapons," said one department official at a briefing.

To demonstrate that it knows how to deal effectively with a potential problem, the Pentagon today took reporters out to its test site, barely 24 hours after the simulated helicopter crash had been sprung without warning on the NUMAX participants.

Port Gaston is a dusty eyesore covered with sagebrush and occasional yucca plants and cactuses. Nuclear detection crews with antiradiation suits scour the area for radioactive debris.

Marine guards with space-age masks and M-16 rifles patrol a cordoned-off area around the helicopter's remains in "Captain Nemo Memorial Park."

Despite the Pentagon's fondest hopes, the scene appears something less than a textbook case in crisis management. Admiral Joseph Frick, commander of the Norfolk Naval Base and the on-site commander here, says he is not happy that his men have yet to locate all the radioactive bomb parts. "It's of concern to me," he says ominously.

Then, however, Frick punctures the pretended realism. Of course, he says, "We didn't work all night."

Small mishaps abound. When Frick tried to send back his final report on the disaster to Washington last night, his teletype machine broke down. "This happens once in a while in real life," he says.

From there, the reporters are taken to a makeshift information center where Dale Smith, the designated military public affairs officer at Port Gaston, has had his hands full dealing with individuals playing the role of the media.

"There have been a lot of concerns that we were covering up and hazarding the population. . . that everybody within a 200-mile radius was gone," says Smith.

The news reports only fed hysteria among the citizens of Port Gaston. One woman, fearing her son had been playing in Captain Nemo park when the helicopter crashed, attacked and kicked a Marine guard who tried to prevent her from entering the area. Another man tried to steal a guard's rifle.

The authenticity of the Pentagon's test actors poses some problems. "There's a real question -- 'How far do you go in restraining the people ?" says Robert C. Hart, an Norfolk police officer playing the role of a Port Gaston officer.

There are other puzzling questions for the players. In the mock disaster, Port Gaston is ordered evacuated and Pentagon officials assuage public concern by putting out the word that the radioactive debris can be cleaned up easily and the town restored to normal fairly quickly.

Radioactive contamination is "more of a political problem than a health problem," says Walter C. Fesler, a radioactivity specialist with the Defense Nuclear Agency on the scene. "People are afraid of radiation because of lack of knowledge."

But a Department of Energy official on the site disagrees. If the Port Gaston crash had been real, the clean-up would take "months, maybe years" and cost millions of dollars, he says.

Meanwhile, the Virginia participants have had their own problems adjusting to the test. The Jefferson County administration building, staffed by local officials from Tidewater, was abruply cleared when a snake was discovered in an office.

Hart said he is enjoying the exercise, but has trouble believing in its authenticity. "The scenery is beautiful, but its not like Virginia," he said. "There are no trees."