Experiment 50 years ago marked start of nuclear era

December 02, 1992|By Newsday

Fifty years ago today, 42 people gathered at an old squas court beneath football stands at the University of Chicago to initiate mankind's first self-sustaining nuclear chain reaction.

That first controlled release of energy from a pile of uranium and graphite irrevocably opened the door to a new age, an era in which the unleashed atom could be used either to power a city or destroy it in a blinding flash; in which radiation, capable of triggering cancer, could also help treat the disease.

The terrible ambivalence of that new age was not lost on some of the scientists who worked for months toward the breakthrough on Dec. 2, 1942. "Even though we had anticipated the success of the experiment, its accomplishment had a deep impact on us," physicist Eugene Wigner was to write later. "For some time we had known we were about to unlock a giant; still, we could not escape an eerie feeling when we knew we had actually done it."

For those beneath the stands at Stagg Field that afternoon, there were no doubts about the immediate goal. Led by Enrico Fermi, a brilliant Italian physicist, the Chicago team was part of a crash national effort to build a bomb exploiting the principles of nuclear fission, which had been discovered just four years earlier by two German chemists.

Though it released only half a watt of energy -- scarcely enough to light a flashlight bulb -- the Chicago "atomic pile" demonstrated that the controlled splitting of uranium atoms by neutrons was feasible. For each atom split, more neutrons were released, in turn splitting more atoms. Scientists could then be confident that -- under the right conditions -- enormously more fission energy could be liberated in an instantaneous, runaway chain reaction. The result would be a catastrophic explosion.

The Chicago pile was designed to avoid such a reaction. It contained natural uranium with low amounts of the fissionable uranium-235 isotope. And like a modern nuclear reactor, the pile had control rods made of neutron-absorbing material that could be quickly inserted to dampen the chain reaction.

The scientists knew that, as layers were added to the pile, there would be more atoms available to support a chain reaction. If the pile reached a "critical" size, there would be enough material to allow a self-sustaining reaction as neutron after neutron found another uranium atom to split.

Above the squash court, Dr. Fermi and other project leaders were on a small balcony where a control cabinet and monitoring instruments were situated. On the floor below, George Weil, a young physicist, stood near the emergency control rod, dubbed "ZIP."

At 9:45 a.m., Dr. Fermi ordered the electrically operated control rods withdrawn. A switch on the control panel was flipped. Counters began to click as the flux of neutrons in the pile increased. Shortly after 10 a.m., Dr. Fermi ordered "ZIP" withdrawn. Then Dr. Weil began to withdraw the last cadmium control rod by hand in incremental steps at Dr. Fermi's order.

Things were going smoothly when there was a startling crash. The emergency rod, ZIP, had slammed back into the pile automatically. There was no danger. The pile was still subcritical. The preset safety point that automatically triggered the rod had been set too low. Dr. Fermi knew it was time to break the tension.

"I'm hungry," he said. "Let's go to lunch."

At 2 p.m., after the team had reassembled at the squash court, the process resumed. The last control rod was removed to its earlier position. ZIP was withdrawn again.

"This is going to do it," Dr. Fermi said. Using his pocket slide rule, he calculated the rise in neutron counts. After several minutes of grim-faced concentration, Dr. Fermi closed his slide rule and announced: "The reaction is self-sustaining."

The others in the group -- 40 men and one woman, physicist Leona Woods -- watched tensely as Dr. Fermi allowed the reaction to continue. The steady rise in neutron counts was tracked by a needle on a circular drum recorder. Finally, at 3:53 p.m., Dr. Fermi ordered, "OK, ZIP in." The activity in the pile fell off.

Dr. Wigner brought out a bottle of Chianti wine, which Dr. Fermi shared with his colleagues.

For many of the younger scientists gathered on that frigid December day, the emphasis was on getting the job done, not the larger picture. "Some of the older people who hadn't been working quite as hard were able to think about the consequences with more sophistication than we did," one observer said.

Leo Szilard understood the darker significance of the accomplishment. He wrote, "I shook hands with Fermi and I said that I thought this day would go down as a black day in the history of mankind."

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