A missile armed with a one-megaton nuclear warhead, probably launched from a Soviet submarine cruising in the North Atlantic, detonates near ground level at the Science Center. Within seconds, Memorial Hall and the Yard disappear into a crater more than 200 feet deep. The third largest library in the world is flattened, its collection buried under a thick crust of radioactive soil thrown up from the blast's hole. Little is left standing between the Quad and the Charles River.

But causalities are minimal. No students, no professors die. Along with about 100,000 other Cambridge residents, they have received a civil defense warning, and they escape to bunkers in a tiny hamlet in western Massachusetts.

Harvard has no civil defense plan of its own. In the event of a nuclear war, the survival of the University's population would depend on a newly revamped scheme that state civil defense officials predict would reduce the civilian death toll in Massachusetts by 40 percent. But many citizens, city officials and area congressmen have opposed proposed increases in civil defense spending, calling the mass run-and-hide scenario implausible and wasteful. The debate has raised the crucial question of whether Americans should devote large amounts of money and intellectual energy to trying to survive a nuclear attack.

Coordinated by the Federal Emergency Management Agency (FEMA), new national civil defense plans announced recently concentrate on "crisis relocation"--evacuating 145 million people from 450 sites expected to be principal targets in a Soviet nuclear attack. The refugees would receive instructions under the federal plan to hop in their cars and drive en masse to various rural "host communities."

Cambridge sits in one of the 450 "high-risk areas." But when FEMA officials presented the city's relocation route to the Cambridge City Council at a public hearing last spring, the nine councilors rejected the plan, the first such rebellion by a local government. A spokesman for the Cambridge Civil Defense Agency, who refuses to be identified, says that preparations in the city "are in limbo, the whole plan is in a state of flux."

In Washington, meanwhile, the Reagan Administration's proposal to raise civil defense spending to an estimated $4.2 billion in the next seven years is meeting stiff resistance from a Capitol Hill contingent led by Sen. Alan Cranston (D-Calif.). FEMA authorities complain that without more funding, they cannot move from planning to implementing any measures.

"We have never claimed we had a perfect plan," says FEMA official Russell Clanahan. "But the alternative would be to not do anything and let people die needlessly."

Until 1973, the only U.S. civil defense program was the marking and stocking of fallout shelters. Constructed on orders from President John F. Kennedy '40, the shelters were filled with water, specially packed biscuits and candy. Many of the signs designating shelters still mark. Harvard buildings, but none of the shelters have any provisions. The biscuits were sent to the famine-stricken population of Bangladesh in 1975, and the other stores were thrown out.

Crisis relocation replaced the urban shelters as a national policy after the severity of the nuclear blast itself and the firestorms that would accompany it were recognized in the late 1960s. "If a bomb hits, there is not going to be a bomb shelter." Hallice says, "there's just going to be a big hole."

The Cambridge relocation plan is typical of the evacuations 200 FEMA regional officials are planning around the country. In the face of what one administrator casually calls a "gradually increasing crisis." 100,000 Cantabrigians would travel 100 miles west on Route 2 to Greenfield. Mass. In that small town of 20,000 evacuees would presumably be far enough away to avoid the blast itself and would take shelter for the days of heavy fallout in predesignated buildings."

To accommodate the thousands of city dwellers without cars, including most of the Harvard student body, the train that normally runs from Boston to Greenfield would make frequent stops in Cambridge and repeat its two-and-one-half-hour run several times a day, officials say. Once in Greenfield, evacuees would live in municipal buildings, motels and private businesses with about 20 square feet per person. Few would have beds.

But a recent FEMA survey indicates that 95 percent of Greenfield's families would share their homes with Cambridge families--a major source of satisfaction for FEMA planners. They add that no one will be forced to comply with any of these plans at either end. "There is nothing that says you have to go, and nobody is going house-to-house to check," says Forbes.

State and local civil defense officials are convinced the evacuation can take place with "a planned and non-chaotic movement, Clanahan says. Douglas Forbes, FEMA's chief planner in Massachusetts, adds that the entire operation will take about 60 hours to complete.

Forbes explains that Cambridge residents driving to the western refuge will have little trouble with traffic jams because the plan calls for a flow of cars at about half the rate that a FEMA study indicates is normal at rush hour.

Planners admit that there will be some panic but discount its effect on emergency measures. "People panic when they don't know what is going to happen," Clahahan says, "but if they know what the plan is, they will usually carry it out."

To prevent a stampede for that single Greenfield train or an hysterical demolition derby on Memorial Drive. Forbes says local police will seal off the major routes leaving the city. "We want to reduce this as much as possible," says Forbes.

But when state and local planners presented the Cambridge proposal at last spring's hearing, the city council was completely unconvinced. Led by Saundra Graham and David A. Wylie, the councilors questioned the feasibility and propriety of civil defense measures, harping on the inevitable carnage expected after a nuclear attack, regardless of the sleep-over scheme in Greenfield.

The council concluded that "the sole means of protecting Cambridge citizens from nuclear warfare would be for nations with nuclear arms to destroy those arms and renounce their use." In an effort to increase public debate on the issue, the council prepared a booklet. Cambridge and Nuclear Weapons, and mailed it to 30,000 homes last summer. The 10-page pamphlet, packed with basic information on nuclear arms, exhorts Cantabrigians to "draw your own conclusion. Take action."

FEMA officials downplay the significance of the council's rejection. "The planning process will simply not take place in Cambridge." Clanahan says. He adds that only 11 of 3000 cities participating in preparations have opposed the planning. Forbes describes the Cambridge reaction as "a political move." Councilors, he adds, "are not going to sit around at ground zero or near ground zero and watch those things pop."

Regardless of their plans, however, civil defense officials acknowledge that without enough advance notice, "if a nuclear bomb hits anywhere where it's supposed to hit we're all dead," as Capt. Chester Hallice, director of the Cambridge Civil Defense Agency, puts it. FEMA officials are banking, therefore, on the expectation that Soviet leaders would try to evacuate their population before any nuclear attack on the United States, alerting analysts here to sound the alarms. "The Soviets only have blast shelters for 25 percent of their urban population." Clanahan explained, "they would lose 100 million people they did not have to with a pre-emptive strike."

"Civil defense is not military, it is a passive defense measure," Clanahan says. "If we had the capability to evacuate at least as quickly as the Soviets, civil defense might actually deter a nuclear attack by denying a disparity in vulnerability."

If warheads began falling tomorrow, Hallice remarks, the safest spot in the Northeast would be near the Canadian border, at a scenic retreat called Bear Lake. "But there are no shelters of any kind up there," he adds. "So if the fallout doesn't get you, the bears probably will."