Once touted as a source of electricity so cheap that it would not have to be metered, nuclear power has fallen on hard times in the last decade and a half.

Anti-nuclear activists, who recently forced the state of New York to approve plans to tear down a nuclear generating station before it could be opened, have humbled the once proud and promising technology. Some even maintain that the last nuclear power plant has been built in the United States.Now, however, nuclear power could get a new lease on life because of growing concern over the greenhouse effect, which could cause disastrous climatic changes and a major disruption in food production.

Even some environmentalists are saying the United States needs to take a second look at nuclear energy because nuclear power plants produce no carbon dioxide, the chief pollutant contributing to the greenhouse effect. "Nuclear energy is going to have a continuing role to play in the global energy economy for at least the next 30 years," said Irving Mintzer of the World Resource Institute.

But unless nuclear power can overcome its nagging problems with safety, radioactive waste and nuclear weapons proliferation, it will continue to be a widely unpopular energy alternative, other environmentalists insist.

Some experts see reason for hope in new nuclear plant designs and experimental reactors that may prove to be less susceptible to core meltdowns and other malfunctions - including the sodium-cooled breeder reactor being developed at Argonne National Laboratory. But these reactors are a long way from commercial development.

If the greenhouse effect has caught the United States by surprise, nuclear energy may, ironically, have played a role.

Since the first commercial reactors went on line in the late 1960s, the growing controversy over their potential for sudden disaster has tended to obscure the possibly greater threat from the vast number of generating plants that burn coal, oil or gas.

These fossil-fuel plants, which generate about 80 percent of the nation's electricity, each year produce enormous amounts of air pollution, including millions of tons of carbon dioxide. The carbon dioxide acts like a blanket, trapping heat at the Earth's surface.

If the greenhouse threat starts to manifest itself more clearly, the role of villain in the energy drama could shift from nuclear reactors to fossil-fuel plants - which produce other pollutants that create the acid rain that damages trees, lakes and buildings and that injure health.

Medical researchers estimate that air pollution from fossil-fuel plants causes 50,000 Americans to die prematurely each year.

"One dose isn't killing people, but over a lifetime of exposure to air pollution from fossil plants, people are living shorter lives than they otherwise would," said Richard Ayres of the Natural Resources Defense Council in Washington.

Nuclear generating plants, on the other hand, do not produce carbon dioxide or other air pollutants. Furthermore, no civilian has ever been killed by radiation from a commercial nuclear power installation in the U.S.

Why, then, has nuclear power had such a hard time in this country? The reasons are complex, involving a mixture of primordial fears, arrogance, mistakes, bad timing and accidents.

Nuclear power was introduced to the world in 1945 in the form of fearsome atomic bombs, a debut that was bound to cause a lingering mistrust of this new form of energy.

Furthermore, it was mysterious. It produced invisible radiation that had the potential for harming future generations. Books and movies fed this fear with depictions of monstrous mutations.

Arrogance also played a role. Nuclear engineers, the federal government and many electrical power companies pushed for a rapid deployment of nuclear generating stations. By 1974, utilities had ordered a total of 150 large nuclear plants capable of producing one-third of the country's electrical output.

But hard times hit. One out of three of those plants was canceled, and no orders for new nuclear plants have been placed since. Including plants built before 1974, there are 108 nuclear generating stations licensed to operate in the U.S., producing about 18 percent of the nation's electrical needs. Another 12 plants are under construction.

Two things put the brakes on nuclear power's rapid expansion: the rise of the antinuclear movement and the Arab oil embargo of the 1970s.

Nuclear power became a symbol for those people who thought technology was running amok. The backlash against technology fueled the antinuclear movement, causing delays in the construction of nuclear plants and a consequent increase in costs.

At the same time, the oil crisis scared the nation into conserving energy. The demand for electricity fell, causing a cancellation of some nuclear plants and further delays in building others. As a result, new nuclear plants became more expensive to build and operate than coal-fired plants.

Confidence in nuclear power was seriously weakened by the core meltdown at Three Mile Island in 1979 and the Chernobyl disaster in the Soviet Ukraine two years ago. People began to believe that operator error made nuclear plants unmanageable.

The two biggest threats to nuclear power today are on Wall Street and Main Street, said the World Resource Institute's Mintzer. Wall Street considers utilities with active nuclear programs to be economically shaky, while on Main Street people have the NIMBY (Not In My Back Yard) syndrome, he said.

"It is very hard, given the new perspective that we have as a result of the greenhouse issue, to say no nuclear forever and forever," said Joseph Goffman of the Environmental Defense Fund. But the present generation of nuclear plants cannot be counted on to bail society out of the greenhouse mess, he added.

"If there is a next generation of technology that makes nuclear power safe and cheap, then we need it," he said. "But the key word is next generation."