Every form of energy is conditionally unsafe in one way or another. Fossil fuels -- oil, natural gas, coal -- threaten the environment and even in the short term can cause catastrophic fires and explosions. Electricity, managed incorrectly, is lethal. Hydroelectric dams can burst. Even the wind can blow too hard and the sun can burn human skin or sere the earth.
What about nuclear energy? Obviously, the lesson of Chernobyl is still with us and a cause of suffering for thousands of irradiated human beings. This inherent danger in all forms of energy is what physicist Edward Teller meant when he said ''Nuclear reactors are not safe.'' Such a comment coming from one of the world's leading proponents of nuclear energy is clarified when Mr. Teller adds that nuclear reactors ''can be operated safely.''
And there is the key. Can nuclear reactors be operated with such absolute safety to human beings and the environment that they will attract the environmentalists' support instead of their historically implacable opposition? The answer seems to be that nuclear power plants can be built to be safe even in the event of human error or mechanical failure. Safe reactors may, however, involve some of efficiency.
The evidence is building that the long-term use of fossil fuels -- coal, oil and even natural gas -- may strain the environment beyond its capacity to support life as we have known it. Certainly solar energy and wind power ought to be exploited to their full extent, but in an industrialized world they can never play more than a marginal role. Hydroelectric power, for practical and environmental reasons, seems to have reached its peak.
The industrial revolution was powered by fossil fuels. This, along with the combustion of wood, increased the carbon dioxide content of the air by 14 percent between 1860 and 1960, and the acceleration continues. Before industrialization, the balance of carbon dioxide was maintained by its ''digestion'' in photosynthesis, especially in the ''oxygen factory'' of the great rain forest.
Now the accumulation of carbon dioxide and other gases appears to be responsible for the so-called greenhouse effect, in which these gases trap the sun's heat, preventing its dispersion back into space. The resulting global warming -- if in fact it is happening, and it seems to be -- is a frightening prospect for our planet's future. Meanwhile, an immediate and measureable threat -- air pollution -- is choking the world's largest cities.
Can nuclear energy reverse the trend? France now produces 70 percent of its energy from nuclear power, with the resulting near-elimination of carbon dioxide, sulfur dioxide and nitrogen oxide emissions from power generation. Environmentalists there are beginning to see nuclear power as their best friend.
Prospects for a new generation U.S. nuclear plants grow with the reality that electrical demand has grown by 50 percent since the oil embargo of 1973. But, if there are to be new plants, Paul E. Gray, president of the Massachusetts Institute of Technology, quoted in Popular Science magazine, says they must meet three requirements.
''First, plants must be able to pass a tough, realistic test to ensure that even a total cooling system failure will not result in a meltdown or a dangerous release of radioactivity. Second, plants must be designed to be passively safe, without the active intervention of operators. Third, the political and technical problems of radioactive fuel waste disposal must be solved.''
He concludes, ''Technological advances in reactor designs and materials in the past 20 years make all three possible.''
In the current reactors, where does the danger lie? Reactors produce energy, heat, by fission, the splitting of the nucleus of the uranium atom. In most reactors, water used as a coolant is converted to steam, which turns turbines that produce electricity. If for any reason the water stops circulating, the reactor core can overheat and the vessel which contains the uranium can rupture, spewing radioactive materials into the atmosphere (as happened at Chernobyl). To reduce the possibility of failure, the pumps which keep the water moving are backed up by redundant systems.
In recent years there have been advances in ''passive'' safety features, systems which turn themselves on in the event of any kind of failure. One, developed by Westinghouse, has gravity-fed tanks above the reactor that automatically open in the event of a power failure or other accident. Critics then ask: Suppose the valves of the gravity tanks fail to open?