Nuclear fission

• 1. Introduction

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  Nuclear fission, the splitting of an atom's nucleus into two or more smaller nuclei, passed its most important test on the 2nd of December 1942. Enrico Fermi had already pulled every control rod but one out of a towering stack called Chicago Pile-1. Slowly, he drew out the last one. The neutron counter clicked faster, and the pen recorder climbed with it. Then Fermi announced, "The pile has gone critical." The reaction had reached a multiplication factor of 1.006, which meant its neutron intensity was doubling every two minutes. The process behind that moment had been recognized only a few years earlier. It would go on to heat homes and cities, and to drive the most destructive weapons ever built. How did a puzzling chemical result in Berlin reveal it? What actually happens inside a nucleus when it divides? Who first saw that the split could feed itself, and what did they fear? And why did nature turn out to have run the experiment first, billions of years ago?

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• 2. The Barium Puzzle

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  In 1934, Enrico Fermi's group in Rome bombarded uranium with neutrons and announced that they had made new elements with 93 and 94 protons. The group dubbed them ausenium and hesperium. Fermi would win the 1938 Nobel Prize in Physics partly for this work, yet not everyone was convinced. The German chemist Ida Noddack suggested in 1934 that the nucleus might instead break into several large fragments. She was a renowned analytical chemist, but she lacked the physics background to grasp how enormous her suggestion was.

  After Fermi's publication, Otto Hahn, Lise Meitner and Fritz Strassmann began similar experiments in Berlin. Then history intervened. Meitner, an Austrian Jew, lost her Austrian citizenship when Austria was united with Germany in March 1938. She fled to Sweden in July and kept up a correspondence by mail with Hahn. By coincidence, her nephew Otto Robert Frisch, also a refugee, was in Sweden as well.

  A letter from Hahn, dated the 19th of December, described his chemical proof that some products of the uranium bombardment were barium. Barium has an atomic mass about 40 percent lower than uranium's. No known form of radioactive decay could account for so large a drop. Hahn suggested that the nucleus had burst, but he could not say what physical process lay behind it. Frisch was skeptical. Meitner trusted Hahn's skill as a chemist, and she knew that Marie Curie had separated barium from radium for many years, so the techniques were well established.

  Meitner and Frisch concluded that the uranium nucleus had split roughly in half. Frisch proposed calling the process fission, by analogy with the way a living cell divides into two, which biologists then called binary fission. According to Richard Rhodes, the idea arose in a discussion with the physicist William A. Arnold, who had suggested "binary fission." Hahn and Strassmann's paper announcing the discovery appeared on the 6th of January 1939.

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• 3. News Crosses the Atlantic

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  Niels Bohr carried the explanation across the Atlantic when he travelled to Princeton University to lecture. At Princeton, I. I. Rabi and Willis Lamb, two Columbia University physicists working there, heard the news and took it back to Columbia. Rabi said he told Enrico Fermi, though Fermi gave the credit to Lamb. Soon afterward Bohr himself went from Princeton to Columbia to find Fermi.

  Fermi was not in his office, so Bohr went down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the shoulder and said, "Young man, let me explain to you about something new and exciting in physics."

  Scientists at Columbia saw at once that they should try to measure the energy released when uranium split. On the 25th of January 1939, a Columbia team ran the first nuclear fission experiment in the United States, in the basement of Pupin Hall. They placed uranium oxide inside an ionization chamber, irradiated it with neutrons, and measured the energy it gave off. The results confirmed that fission was happening. They also hinted strongly that the isotope uranium-235 was the one splitting.

  The next day, the fifth Washington Conference on Theoretical Physics opened in Washington, D.C., under the joint auspices of George Washington University and the Carnegie Institution of Washington. The news spread further there, and many more experimental demonstrations followed.

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• 4. Anatomy of a Split

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  A typical fission event releases about 200 million electron volts of energy, and that energy comes from roughly 0.1 percent of the uranium nucleus's mass. Meitner and Frisch reached that estimate in their paper of the 11th of February 1939. They compared the nucleus to a liquid drop that stretches and divides. On the 1st of September 1939, Bohr and John Wheeler used the same liquid drop model to put numbers on the process. They described an energy surface shaped like a saddle, and the saddle's rim was a barrier of about 6 million electron volts that an incoming neutron had to help the nucleus cross.

  The two pieces are rarely equal. Most often the nucleus breaks into fragments of roughly 95 and 135 daltons, a mass ratio of about 3 to 2. Occasionally, in two to four events per thousand, three positively charged fragments appear instead, in what is called ternary fission. The smallest of those can range from a single proton up to an argon nucleus.

  For uranium-235, about 169 million electron volts of the total shows up as motion of the fragments. They fly apart at about 3 percent of the speed of light, driven by electric repulsion. An average of 2.5 neutrons also escape from each split.

  Not every heavy nucleus responds alike. Uranium-235 has 143 neutrons, an odd number, and an extra neutron settles into a pairing that releases 1 to 2 million electron volts of added binding energy. That is enough to split it even when the neutron is slow. Uranium-238 has 146 neutrons and needs a fast neutron above about 1 million electron volts. Nuclei in the first group are called fissile, and those in the second are called fissionable.

  Some nuclei need no neutron at all. Flyorov, Petrzhak and Kurchatov discovered spontaneous fission in Moscow in 1940. Yet fission matters little to the universe at large. Stars, and the elements that make up planets and living things, come from fusion.

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• 5. The Chain Reaction Fear

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  In February 1939, Hahn and Strassmann used the word Uranspaltung, meaning uranium fission, for the first time. In that same second paper they predicted that the process would free additional neutrons. That prediction opened the door to a nuclear chain reaction, an idea the Hungarian physicist Leó Szilárd had first formulated in 1933 after reading Rutherford's disparaging remarks about drawing power from neutron collisions. Szilárd had tried and failed to get a chain reaction going with beryllium.

  On the 25th of January 1939, Eugene Wigner told Szilárd about Hahn's discovery. Szilárd wrote, "All of the things which H. G. Wells predicted appeared suddenly real to me." Once Hahn and Strassmann's paper was published, he warned Lewis Strauss in a letter that the energy released would be very much higher than in any earlier case. It might lead, he wrote, to "large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs."

  Szilárd urged Fermi in New York and Frédéric Joliot-Curie in Paris not to publish on the possibility of a chain reaction, lest the Nazi government learn of it. Fermi agreed to censor himself, with some hesitation. Joliot-Curie did not. In April 1939 his team in Paris, including Hans von Halban and Lew Kowarski, reported in the journal Nature a figure of 3.5 neutrons per fission of uranium. Szilárd and Walter Zinn found about two. Fermi and Anderson estimated about two neutrons for each neutron captured.

  In August 1939, Szilárd, Edward Teller and Wigner feared the Germans might exploit the chain reaction. They persuaded Albert Einstein to sign a letter to President Franklin Roosevelt. Alexander Sachs delivered it on the 11th of October. Roosevelt grasped the point quickly, saying, "Alex, what you are after is to see that the Nazis don't blow us up." He ordered the formation of the Advisory Committee on Uranium.

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• 6. Building the Pile

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  In April 1939, Fermi and Szilárd set a goal of creating a chain reaction in natural uranium, rather than first separating its isotopes. Their first attempt used five hundred pounds of uranium oxide from the Eldorado Radium Corporation. It was packed into fifty-two cans, each two inches across and two feet long, and set in a tank of manganese solution. They confirmed that more neutrons were emitted than absorbed. But the hydrogen in the water soaked up the slow neutrons that fission needs, so they turned to graphite, whose carbon captures far fewer.

  In April 1940, Fermi confirmed carbon's promise for a slow-neutron chain reaction, using graphite bricks from the National Carbon Company at the Pupin Laboratories. He defined a reproduction factor, k, to judge each test. John Lilley describes it as the ratio of neutrons in one generation to the number in the one before. Below 1.0 the reaction dies out, above 1.0 it grows, and at exactly 1.0 it holds steady.

  The early numbers were discouraging. In September 1941, Fermi's team reached a k of only 0.87. By April 1942 they had raised it to 0.918 by removing moisture from the oxide. In May 1942, one of the exponential piles at Stagg Field reached 0.995, and Fermi planned a full-scale pile. Purity became the obsession. The graphite had to be free of boron, which absorbs neutrons strongly. The oxide came from Mallinckrodt Chemical Works. Pure uranium metal from the Ames process allowed Frank Spedding's "eggs" to replace the oxide pseudospheres.

  Starting on the 16th of November 1942, Anderson and Zinn worked in two twelve-hour shifts, and the pile reached 57 layers by the 1st of December. It held 771,000 pounds of graphite, 80,590 pounds of uranium oxide and 12,400 pounds of uranium metal, along with ten cadmium control rods.

  The design leaned on a lucky feature of fission. Some neutrons are emitted late, from decaying fission products, with half-lives up to several minutes. These delayed neutrons give operators time to react. Without them, the early reactors might have run away before anyone could shut them down. As a safeguard, Fermi added control rods hung from electromagnets and triggered by radiation counters, so they would drop into the center of the pile on their own. By late 1942, the United States had begun an all-out effort to build atomic weapons.

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• 7. The Bomb Race

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  In March 1940, at the University of Birmingham, Frisch and Rudolf Peierls wrote a memorandum on a "super-bomb." Mark Oliphant encouraged them. They reasoned that a pound or two of uranium-235, in a volume smaller than a golf ball, would set off a chain reaction faster than the material could vaporize. They estimated the explosion would reach temperatures above the interior of the sun and pressures above the center of the earth. The cost of separating isotopes, they argued, "would be insignificant compared to the cost of the war." The first meeting of the MAUD Committee took place on the 10th of April 1940.

  In December 1940, Franz Simon at Oxford proposed gaseous diffusion as the best way to separate uranium isotopes. On the 28th of March 1941, Emilio Segrè and Glenn Seaborg reported strong signs that plutonium-239 fissions with slow neutrons. That opened another path: a reactor running on ordinary uranium could breed plutonium as a substitute explosive. In October 1941, the MAUD report concluded, "it will be possible to make an effective uranium bomb."

  On the 6th of December 1941, Vannevar Bush and James Conant reorganized the Uranium Committee. Harold Urey took gaseous diffusion, Lawrence took electromagnetic separation, Eger Murphree took centrifuges, and Arthur Compton took theory and design. By the 20th of August 1942, Seaborg's team had extracted plutonium using ultramicrochemistry techniques.

  In 1943 the U.S. Army Corps of Engineers took over the effort, known as the Manhattan Engineer District and led by General Leslie R. Groves. Hanford, in Washington, had the first industrial-scale reactors and made plutonium. Oak Ridge, in Tennessee, concentrated on uranium enrichment. Los Alamos, in New Mexico, was the scientific hub for bomb design, and J. Robert Oppenheimer directed the science overall.

  The physics set hard limits. According to Rhodes, an untamped core even twice the critical mass would fission less than 1 percent of its material before it expanded enough to stop the reaction. A tamper reflected neutrons back and slowed that expansion.

  In July 1945, the first atomic device, "The Gadget," was detonated in the New Mexico desert in the Trinity test. It ran on plutonium made at Hanford. In August 1945, "Little Boy," a uranium-235 bomb, and "Fat Man," a plutonium bomb, were used against the Japanese cities of Hiroshima and Nagasaki.

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• 8. Power, Waste and Oklo

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  As of 2019, 448 nuclear power plants worldwide provided a capacity of 398 gigawatts electric, and about 85 percent of them were light-water cooled reactors. Their principle is simple. Heat from fission passes into the coolant, then to a heat exchanger, and the steam it makes drives a turbine. Fission gives roughly a million times more energy than burning the same amount of methane. Reactors are built mainly for one of three jobs: power, research, or breeding new fuel. A fast breeder reactor makes plutonium-239 from abundant uranium-238.

  The cost is waste. Fission products are on average far more radioactive than the heavy elements that fuel the reactor, and they stay that way for significant periods. Lilley notes that the radioactive decay of fission products is a second release of energy. It is why reactors must keep being cooled after shutdown and why waste must be handled with great care. Neutron absorption that does not cause fission also makes plutonium and minor actinides, whose radiotoxicity is far higher than that of the long-lived fission products. Reprocessing aims to recover usable material from spent fuel. Industry calls a process that fissions nearly all actinides a closed fuel cycle.

  Nature, it turns out, got there first. Francis Perrin discovered the Oklo Fossil Reactors in Gabon in 1972, and Paul Kuroda had postulated such things in 1956. At three ore deposits, sixteen sites show where self-sustaining fission took place about 2 billion years ago, moderated by ordinary water. It worked because natural uranium then held about 3 percent uranium-235, against 0.7 percent today. Modern light-water reactors must enrich their fuel back up to that same 3 percent.

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