Those who launched the nuclear age urge caution, care

Nuclear power has come a long, long way in the short time since Enrico Fermi and his scientist colleagues achieved the first sustained man-made nuclear chain reaction on a University of Chicago squash court in December 1942.

Now, millions of people lean on nuclear power for electricity and for a variety of scientific and medical uses. The development of nuclear weapons has triggered global debates over the merits of a nuclear freeze.

But the recent two-day conference at the University of Chicago campus, called to commemorate the 40th anniversary of Dr. Fermi's achievement, was not exactly a self-congratulatory celebration.

Yes, many of the scientists who pioneered the development of nuclear power were guest speakers. And now and then they waxed nostalgic over the excitement of those early days and their role in the adventure.

But the dominant message from almost every speaker was of the need to pause and reflect - to decide how much farther to go and under what constraints.

At a ceremony beside Henry Moore's ''Nuclear Energy'' sculpture, which now marks the Fermi reactor site, University of Chicago physics Prof. John A. Simpson, who worked on the bomb project in the 1940s, says between five and eight nations now have nuclear weapons and twice that many may have the capability of producing them by the end of this decade.

''There can be no winners in a nuclear war,'' he insisted, as students bearing placards reading ''No More Nuclear Weapons'' and ''Jobs Not Bombs'' listened as attentively as the hundreds attending the symposium. ''But there is still time, late as it is, for a rededication, a reappraisal, and a renewal of our purpose to seek a world freed from the fear of nuclear holocaust, free to harness nuclear energy safely for the benefit of mankind.''

The importance of stemming the further spread of nuclear weapons was underscored by several scholars. Albert Carnesale of Harvard University's Kennedy School of Government urged a strengthening of international safeguards and security guarantees to dampen the interest of other nations in acquiring nuclear weapons.

Though this group of scholars and scientists appeared keenly aware of the dangers in adding to the world's nuclear arsenal, several speakers suggested that public concern over the safety of nuclear power as an energy source is overblown and irrational. The result, they said, has contributed in part to the current standstill in nuclear plant construction and could have adverse consequences for the American public.

The US currently has 76 operating nuclear power plants, more than any other country. But Dr. Carnesale notes that no new nuclear plants have been ordered in the last four years and that all orders placed during the last decade have been canceled.

''It's clear that nuclear power has reached at least a hiatus and probably an impasse,'' he says. ''Does it matter if nuclear power has reached a dead end? I believe the answer is yes. If there aren't any orders (for plants), the US won't be able to maintain the nuclear power option. . . . The best scientists in the field would be the first to leave. . . . The risks of operating on-line plants would be increased, not decreased.''

Alvin Weinberg, director of the Institute for Energy Analysis at Oak Ridge Associated Universities, says if those risks and the slim probability of a reactor catastrophe are currently weighed against those of other energy sources, nuclear power emerges as ''adequately safe.''

''The first nuclear era is over, and the question is whether or not there will be a second one,'' says Dr. Weinberg. ''That depends perhaps crucially on the extent to which the reactor-safety issue is resolved.''

Certainly, the sharply reduced national demand for electricity and the steep capital expense of building new power plants are also key factors in the nuclear power plant slowdown.

Dr. Carnesale suggests the risk in the multibillion-dollar investment and the possibility that public policy or regulation could change during the 12-to-14 -year construction period may require fundamental changes in government policy. He urges a careful rethinking, for instance, of whether the government should assume some of that dollar risk. One possibility: running the plants as a nationalized system and selling the electricity to public utilities.