THE FIRST NUCLEAR REACTOR

An unused football stadium, Stagg Field, with abandoned squash courts underneath the west stands at the University of Chicago was the perfect setting for a large, secret experiment. It was not a place anyone would look for world-changing science. Although preliminary studies of a graphite­moderated fission reaction had been studied at Columbia and Princeton,

the groups were combined and sent to Chicago for the definitive experi­ment by the sponsoring agency, the Office of Scientific Research and Development, directed by Arthur H. Compton.

With his methodical, thorough experimental ethic, Dr. Enrico Fermi directed the project, building 30 test assemblies out of larger and larger piles of pure graphite bricks, with interspersed cylinders of pressed ura­nium oxide. Everything was carefully considered and designed, from the allowable impurities in the graphite to the radius and length of each ura­nium cylinder, to the optimum spacing between uranium pieces, with calculations improving as larger and larger subcritical piles were assem­bled, observed, and dismantled. On November 16, 1942, when the team thought they had enough data to predict the size of a fully critical, con­tinuously running nuclear reactor, they started stacking layers of graphite and uranium on the wooden floor of the squash court, building “CP-1,” or Chicago Pile number 1.

Chemically pure graphite seemed the ideal moderator material to slow the neutrons down to fission speed. Under severe badgering from the eccentric Hungarian genius, Leo Szilard, three companies were encour­aged to produce some remarkably pure synthetic graphite. On the Ger­man side of the race to produce nuclear power, graphite had been quickly dismissed as a possibility. In Europe, graphite was mined, for use in pro­ducing pencils, and graphite was readily available, but its mineral impu­rities made it unusable. The Germans chose to use the exceedingly rare material deuterium oxide, or heavy water, as a moderator.

The Chicago pile was big. It was roughly a sphere, 25 feet (7.6 m) in diameter, looking like a huge basketball made of black Legos. Wooden timbers held up the bottom half of the thing, and the cadmium control — rods, intended to absorb neutrons to selectively kill the fission reaction, ran in from the front, through channels routed in the graphite. Electronic radiation counters and controllers for the control rod motors were piled up in the squash court balcony. On December 2, 1942, the reactor was fully built, checked out, and ready to be tested. Forty-two men and one woman, Leona Woods (1919-86), crowded onto the squash court balcony and a couple of stations on the pile to see the first sustained nuclear reaction. George Weil stood on the floor in front of the pile to move one critical control rod by hand. Harold Lichtenberger, W. E. Nyler, and A. C. Graves got to stand on a wooden platform in back of the reactor, holding car­boys full of cadmium salt solution, poised to drop them on the graphite and kill the reaction in case things got seriously out of control. Fermi,

Process of Chain-reacting Nuclear Fission

 

Control

rod

 

Control

rod

 

Escaped Absorbed Fast Moderated neutrons neutrons neutrons neutrons

 

Ге? New, unique fission products

 

© Infobase Learning

 

In a nuclear reactor, neutrons born from fission are slowed down to a crawl by traveling through a moderator, such as graphite. They can then cause other fissions or be lost to a control rod that is meant to absorb neutrons, or they can simply leak out of the reactor.

 

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Compton, Herb Anderson, and Walter Zinn, from Canada, sat at the con­trol desk. Norm Hilberry stood ready with an axe to cut the rope holding the zip rod, which would fall by gravity into the pile if they needed a quick shutdown. Everybody else was there to watch.

For most historical scientific experiments and discoveries, we have only a sketchy, incomplete account of the detailed activity, and we must rely on recollections and cleaned-up summaries of what happened. The worlds first reactor startup is a very rare exception. Neither recordings nor photographs were made, due to the military secrecy of the operation, but Leona Woods, the youngest person to witness the event, took detailed, minute-by-minute notes. Fermi was not a very talkative person, and the
gallery remained mostly in awed silence. Woodss written record of each action and spoken word is considered correct.

Подпись: Chicago Pile-1 (CP-1)
Подпись: © Infobase Learning
Подпись: The first nuclear reactor ever built was literally a pile of graphite bricks, with rounded cylinders of uranium evenly dispersed throughout. The three cylinders wired in front are neutron detectors used to monitor the fission activity.

At 9:45 a. m., the experiment began. Fermi called for withdrawal of the electrically driven control rods, somebody threw the switch, and the crowd hushed as the DC motor whined. Out slithered the cadmium rods from the middle of the slippery graphite pile. All eyes turned to the neu­tron counter dial and the pen chart recorder, which was keeping a record of the neutron activity in the pile on a continuous roll of paper, with a motor-controlled blue pen. The count rate stepped up a little, but nothing to write a paper about. An audio amplifier and a speaker were connected

to the counting equipment, giving an occasional click sound as a neutron strayed into the detector tube.

Fermi, a man of few extraneous words, said, “Zip out.” It was just after 10:00 a. m. Zinn pulled out the gravity rod and tied its rope to the balcony. The neutron count rate rose noticeably.

It was 10:37 a. m. With his eyes locked on the neutron rate dial, Fermi called to Weil, “Pull it to 13 feet, George.” Weil, standing at the reactor face, carefully withdrew his small vernier control to 13 feet (4 m), marked on the side of the rod. The neutron count jumped. The crowd murmured, as slide rules slid and pencils scratched.

“This is not it,” predicted Fermi. “The trace will go to this point and level off.” He pointed to a blank spot on the pen chart. Slowly the pen moved up and leveled off, right where he said it would. The crowd was enraptured and studied the new flatline for seven minutes, then Fermi called to Weil for another foot of rod. Weil complied. The count rate increased but leveled out. For nuclear fission to be happening at a useful level, the rate would have to increase exponentially, tending to become a vertical line on the graph.

At 11:00 a. m., Fermi, seeming not in the slightest way impatient, called to Weil for another six inches of vernier control. At 11:15 a. m., a little more. At 11:25 a. m., another smidgen. After each movement, the count rate would increase slightly, and the clicks of neutrons hitting the detec­tor tube, amplified and put over a loudspeaker, became irritating. Fermi seemed to be enjoying the drama, as he correctly predicted each level out of the pen on the chart. He knew they were getting close. Just to be absolutely sure of things, Fermi ordered that the automatically actuated control rod be dropped in, to test the circuit. The safety rod banged home, and the count rate dropped abruptly, just as it should.

Satisfied, Fermi called for a restart, and at 11:35 a. m. the safety rod was reset in out position, and the vernier was carefully pulled out a little more. The count rate rose and rose. The crowd watched and waited, silent, enthralled by the rising neutron count. Suddenly, there was a loud bang.

Everybody froze. Then, as nothing seemed to be melting through the floor, they realized that the safety rod had automatically tripped, sending it rapidly into the reactor core to stop the reactions. The tripping point was set too low on the neutron rate meter. It was quickly adjusted.

Fermi announced that “I’m hungry. Let’s go to lunch.” Weil parked the vernier rod, the motor rods were driven in, the zip was lowered in, and the party broke for the dining hall.

Over lunch, not a word was said about neutrons, graphite, or the unspeakable substance uranium. Fermi just ate lunch, giving not a hint of a pep talk, as the others went on about anything except the “game.”

At 2:00 P. M., they assumed their positions in the squash court, and it took 20 minutes to warm up the equipment and withdraw the safeties to their previous condition. “All right, George,” called Fermi. Weil took this to mean restore the vernier to its last position, and he did so. The count rate was high.

At 2:50 p. m., Fermi called for another foot of rod. Out it came. The pen chart ran off the top of the graph, but they still weren’t exponential. Some­body clicked the pen chart range up by a factor of 10, to get the pen back on the chart. Everybody watched.

At 3:20 P. M., Fermi said “Move it six inches.” Weil pulled six inches. The random ticking sound on the speaker, indicating individual neutrons counted, was becoming frantic. What was once a series of clicks now sounded like air escaping or waves crashing on rocks. Over the hiss from the speaker Fermi called “Pull it out another foot.” Weil pulled.

Fermi, supremely confident, turned to Compton and said, “This is going to do it. Now it will become self-sustaining. The trace will climb and

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The Birth of the Atomic Age by artist Gary Sheahan (1893-1978). Although the room in which the first nuclear reactor was built was too crowded to take a picture in, this painting captures the moment of the first self-sustaining chain reaction at the University of Chicago in 1942 (Chicago History Museum)

continue to climb. It will not level off.” He pulled his slide rule and began calculating. He flipped his rule over and penciled temporary numbers on the back. He looked grim, as the count rate rose.

Three minutes later, Fermi made another calculation, and the crowd was jostling for position to see the count rate on the chart. Wilcox Over­beck began calling out the numbers from the chart as the pen traversed. Fermi, stone-faced and a picture of calm throughout the exercise, sud­denly closed the C-scale on his slide-rule and grinned broadly. “The reac­tion is self-sustaining,” he announced. “The curve is exponential.” The time was 3:52 p. m. on December 2, 1942. Power production by the direct conversion of matter to energy had been proven feasible.

It was a closely held secret. The construction of CP-1 was finally declas­sified on May 18, 1955, when Enrico Fermi and Leo Szilard were awarded the patent, number 2,708,656, for the nuclear reactor. The people of the United States were given their first glimpse of what their tax money had paid for in 1942. The experiment cost about $1 million, or, adjusted for inflation, $12.5 million.

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