CHAPTER EIGHTEEN

XENON-135

FROM JANUARY 1943 THROUGH THE SUMMER OF 1944, FERMI divided his time between three places: the newly relocated Met Lab in Argonne Forest, just west of Chicago; Oak Ridge, where the vast isotope separation plants were taking shape and where Fermi and a team from DuPont built the first plutonium research reactor; and the eastern desert of Washington State near the village of Richland, where the large-scale plutonium production reactors began their deadly work. Though he made the occasional trip to other locations—to Berkeley, and to the new village of Los Alamos, New Mexico, soon to become the epicenter of bomb design and construction—the majority of his time was spent with Met Lab colleagues who had worked with him in the squash court under the stands of Stagg Field.

IN FEBRUARY 1943, AFTER THE LABOR PROBLEMS THAT BEDEVILED the construction of the lab facilities were resolved, the entire Met Lab moved to Argonne. In the short period between December 2, 1942, and the February 1943 move, Fermi and the team continued to experiment with the pile, at one point driving the pile up to two hundred watts.

In February, the pile, originally dubbed CP-1 (Chicago Pile 1), was reconstructed at Argonne, in a new and more flexible configuration that became known as CP-2. Fermi reconfigured it so it had a central core that could be removed to test out different lattice structures and different quality materials. When it went critical in May 1943, its reproduction factor was much higher than CP-1. It took a little over one minute for CP-1 to double in power. CP-2 took about five seconds to do the same.

In CP-2, not only had Fermi created a prototype for the eventual production of plutonium, he also created a veritable neutron factory. He would never again have to fuss with delicate neutron sources in glass bulbs. All the neutrons he needed for research existed in the heart of CP-2. Beginning almost immediately, with the enthusiasm of a child with a new toy, he ran systematic experiments to test neutron reactivity on a variety of targets. He found a way to create beams of ultraslow neutrons and studied neutron diffraction and refraction in great detail. He also developed ways to test the purity of graphite and uranium now being produced in unprecedented quantities for the huge plutonium production reactors.

Working closely with Anderson, Zinn, Marshall, Libby, and the rest of the team, Fermi developed new detectors that measured the precise number of neutrons produced in the reactor. These detectors enabled the team to carry out new studies with unprecedented precision.

Work at Argonne required new routines for everyone. Fermi could no longer walk the half mile or so to his lab. Argonne was some twenty miles southwest of Hyde Park, along fabled US Route 66, and the drive, with Baudino at his side, took the better part of an hour. The hour-long drive drew the two men—opposites in so many ways—close together. Fermi might have initially resisted the idea of a bodyguard as unnecessary, but they soon became friends. Occasionally, Baudino lent a helping hand, stacking graphite bricks or moving blocks of paraffin. Libby, Anderson, and Marshall, who normally slept in a primitive dorm on the lab site, sometimes rode with their boss when they needed to get into town, and they discussed the day’s objectives as they drove through the cornfields and prairie land between the city and their new workplace. Fermi got to know the vast fertile plains that make up the American heartland. He was fascinated by the landscape, which was so very different from what he was used to in Italy.

Laura adjusted well to the new environs. Chicago was different from New York, and she liked her new home. She enjoyed socializing with the growing group of physicists and their spouses at the Met Lab—a very social group, even under the constraints of official secrecy. The party she hosted on the evening of December 2, 1942, was but one of many she hosted during this period. The Fermis did not drink very much at all—when they had wine, it was always with a bit of water added—but they served alcohol to their guests. They enjoyed playing party games like charades, which Enrico took very seriously. Old friends like the Ureys and Mayers made Laura feel even more at home. Laura also got along well with Leona Libby, although one senses Laura was a bit jealous Enrico was spending more time with this attractive twenty-three-year-old than he was with his own wife. During a snowstorm, Leona was a passenger in Fermi’s car as he drove home from the lab late at night. When Leona suggested they might have to stop the car and sleep there overnight, Fermi objected, saying that it might harm his reputation. The young woman, by now married to John Marshall and pregnant with his child, suggested that perhaps she should be worried about her reputation and because she wasn’t, why should he be? He asserted with characteristic confidence that his reputation was more important and they drove on. Unspoken, perhaps, was his concern over what Laura might have said the next morning.

Leona’s pregnancy posed some problems at Argonne. The health and safety officers assigned there would certainly have prohibited her from working at the new pile if they had known. She was, however, able to persuade those around her to keep the secret and wore baggy clothes to hide the pregnancy. Zinn apparently never knew or if he knew he never let on. Fermi became convinced he might have to perform midwife duty at the lab some nine months on, given the twenty-seven miles between Argonne and the Chicago hospital where Leona and John planned to have their baby (why exactly he didn’t expect John, who worked there every day, to perform these duties if necessary is not clear). Fermi even researched what he would be required to do. Fortunately for everyone involved, including Laura, it never came to that and the Marshalls had their first child at the hospital.

As she settled into life in Chicago, Laura also devoted time to the children. By this time, Nella was twelve years old and Giulio was almost seven. They were both enrolled in the famous Lab School established in 1896 by American educator John Dewey. The school, founded on progressive principles of child development and education, was somewhat less permissive than Nella’s New York school. Giulio was unhappy, but Nella adjusted well—perhaps because she was older and her temperament was better suited to the more structured program. Also, Giulio was upset that he was taken away from his friends in New Jersey without sufficient explanation. It was a resentment he carried with him throughout his life.

SOME OF THE NEW MEMBERS OF THE ARGONNE TEAM HAD HEARD of Fermi’s extraordinary abilities but were now witnessing them for the first time and were suitably impressed. One of the most important of these was an experimental physicist named Luis Alvarez, a student of Compton at Chicago in the early 1930s. Alvarez’s work in those days was the stuff of Chicago legend by the time Fermi arrived. After getting his degree, Alvarez left Chicago to join Lawrence’s team at Berkeley, doing a series of important experiments at the cyclotron. At the beginning of the war, Alvarez worked in England and at MIT on the development of radar, and in the summer of 1943 he arrived at Argonne, climbing on top of CP-2 and working with Libby to design and build new instruments, something at which he was particularly adept.

Alvarez was never a modest man, so his report of an early encounter with Fermi is particularly instructive. He joined a conversation in the Argonne cafeteria with Fermi, the Marshalls, and Herb Anderson, who were discussing how neutrons might obey a law of refraction similar to those of X-rays. Fermi commented that he could not remember the exact formula for X-ray refraction. Alvarez pointed out that it was contained in the classic textbook on X-ray diffraction written by Compton and Allison. Alvarez had seen a copy of the textbook on a desk next door and offered to get it. Fermi told him not to bother—he would derive it.

Alvarez goes on to describe Fermi’s performance:

As a student of Compton’s I had thought long and deeply about X-rays, but I had never seen the refractive-index formula derived from basic principles. Enrico wrote James Clerk Maxwell’s classic electromagnetic field equations on the blackboard and then in six separate steps derived the formula. The most remarkable aspect of this tour de force was that Enrico worked through his derivation line by line at a constant rate, as if he were copying it out of a book. That night at home I reproduced it and was quite pleased with myself. If one step was easy enough to allow me to go faster than he did, the next was so difficult that I could never have managed it alone. But Enrico worked the difficult steps at exactly the same rate he worked the easy ones.

In time, the two of them became good friends. Alvarez, back at Berkeley after the war, would occasionally call Fermi to ask for recent graduate students for post-doc positions at Berkeley and Fermi would happily oblige.

Increasingly, young American physicists who encountered Fermi for the first time were impressed by his ability to work his way through thorny problems. Many of the brilliant physicists of Alvarez’s generation—about ten years younger than Fermi—had never worked closely with people of Fermi’s European peer group, the Heisenbergs and Diracs of the physics world. The closest many of them had come to this group was Oppenheimer at Berkeley, who in the late 1920s brought European-style theoretical physics back to the United States. To see one of the quantum pioneers at work must have been quite inspirational. Increasingly, young American physicists were exposed directly to Fermi and shared Alvarez’s sense of awe.

That awe was not confined to the younger generation. At Fermi’s memorial service in 1954, Sam Allison recalled that on a train ride to Hanford, Compton decided to make small talk with Fermi. He remarked to Fermi that during his time in the Andes pursuing cosmic-ray studies his watch did not keep good time. “I thought about this considerably,” Compton explained, “and finally came to an explanation that satisfied me. Let’s hear your discourse on the subject.” Fermi immediately pulled pencil, paper, and slide rule from his pocket and after a few minutes came up with an answer that explained the phenomenon and even predicted the amount of the watch’s inaccuracy. Compton’s look of wonder was something that Allison, Compton’s long-time collaborator, would never forget.

INCREASINGLY, FERMI’S ATTENTION WAS DRAWN TO PROBLEMS ARISING in the planning and construction of the reactors. The first major plutonium reactor, known as X-10, was rising at Oak Ridge, with three objectives: to determine how fast plutonium could be produced, to develop the chemistry to separate plutonium from the other by-products of controlled uranium fission, and to provide small samples of plutonium to determine its suitability for use in a fission weapon. Fermi’s old friend Emilio Segrè, now at Berkeley, but soon to relocate to Los Alamos, would be one of the key scientists researching this latter issue.

The construction was a helter-skelter affair, with engineers working round the clock from the sketchiest of drawings. Based at Argonne, Fermi provided guidance and advice on scientific and technical questions. Eventually, the enormous cube took shape atop a small hill at Oak Ridge. It looked more like CP-2 than CP-1, but there was a difference. It was built so that, after the reactor had been running for an appropriate length of time, the uranium rods could be easily removed and placed in acid baths to initiate the extraction of plutonium. A large cube of graphite, twenty-four feet on each side, it had 1,260 long channels bored into one face. Into these channels rods of uranium metal would be inserted. Along a perpendicular face, channels were drilled to accept cadmium safety rods, which were attached to a mechanized system to pull them out or reinsert them as conditions required. Designed to run hot, at one thousand kilowatts (one megawatt), to produce enough plutonium to study in a relatively short amount of time, it required a cooling system that neither CP-1 nor CP-2 had. The system chosen was air cooling. Air flow supplied by motorized fans circulated air in and around the channels for the uranium rods. At the intended power level, radiation from the pile became an important safety issue, so engineers encased the pile in concrete shielding seven feet thick, creatively formulated to retain water even after curing so that it would be more effective at slowing down any excess neutrons escaping from the pile.

FIGURE 18.1. The author at the control panel of the X-10 reactor at Oak Ridge. Photo by Susan Schwartz.

The Oak Ridge pile went operational on November 4, 1943, less than a year after the first pile in Chicago. It was a tremendous achievement and, in recognition of this, Compton and Fermi traveled to Oak Ridge to witness the event. They were sleeping in when they were awakened before dawn—apparently with some glee—by the crews loading the uranium fuel, who were ahead of schedule. The pile went critical at five o’clock that morning. Soon it was producing plutonium, which was transported by courier—on a commercial flight—to Los Alamos for study by Segrè. Segrè’s discoveries regarding these samples, which were often just a few milligrams in size, would alter the entire course of the Manhattan Project.

For the time being, however, the success of the Oak Ridge pile must have given great satisfaction to the Chicago physicists and to DuPont’s leadership, especially Crawford Greenewalt, who had come to worship Fermi. Seaborg’s chemical separation unit next door would soon give Groves and the civilian leadership confidence to start an all-out effort to build industrial-scale plutonium production reactors at Hanford.

DURING LATE 1942 AND EARLY 1943, WORK UNDER OPPENHEIMER at Berkeley continued to focus on issues relating to the use of U-235 for a fission bomb; all signs pointed to the conclusion that fast neutrons would cause that particular isotope of uranium to split. This was progress for the project, because fast neutrons would be the only way the energy locked in the uranium nucleus could be released in the short time required to create an explosion. Gifted with an ability to read people and situations quite accurately, Oppenheimer charmed the blunt anti-intellectual Army general. He did not spend a lot of time regaling Groves with meditations on high culture. Instead, he evinced a complete confidence that, with the scientific team at his disposal, he could deliver the goods. Groves agreed and selected Oppenheimer as the scientific director of the effort to design and build the actual weapons once sufficient materials were produced.

The story of how Oppenheimer persuaded Groves to designate a mesa about twenty miles northwest of Santa Fe, New Mexico, the site of a boys’ boarding school called Los Alamos, as the site—code-named Site Y—for the technical effort to design and build the bombs is told elsewhere. Fermi attended the first organizational meetings Oppenheimer held there in April 1943, along with a stellar cast of scientists, including Rabi, Bethe, Segrè, and many other old friends. Fermi’s presence was eagerly anticipated. A senior group of scientists were lunching at Fuller Lodge, the canteen for the high-level scientists, and Teller assured them that Fermi would be arriving within the week. Stanislaw Ulam, a Polish mathematician who in later years was credited with a major breakthrough in the development of the hydrogen bomb, intoned “Annuncio vobis gaudium maximum, papam habemus.” To the bewildered group, John von Neumann, Ulam’s brilliant Hungarian colleague who played a central role at Los Alamos over the next few years, translated the Latin used by the church to announce the election of a new pope: “I announce to you with greatest joy that we have a new Pope.” The group knew of Fermi’s nickname in Rome and burst into applause. They had been looking forward to Fermi’s arrival and loved Ulam’s allusion.

When the entire group was assembled at Los Alamos, they discussed a “primer” on bomb design prepared by Oppenheimer student and collaborator Rob Serber, the result of the Berkeley theoretical group’s months of study and debate. Oppenheimer began to plan a division of labor, which he was able to sustain until late summer 1944, when events forced reorganization. This was Fermi’s first exposure to the magnificent wilderness of the New Mexican portion of the Rio Grande valley. Over countless millennia, the river ground out the valleys the scientists now hiked in and explored. The wilderness around Los Alamos was like nothing Fermi had ever experienced, and when Norris Bradbury, the first postwar director of Los Alamos, invited Fermi back for a number of summers after the war, it was not a hard sell for the inveterate outdoorsman.

The April 1943 meetings at Los Alamos were the first time all the senior scientists came together to discuss the design and construction of the bomb. In later years Oppenheimer recalled that Fermi seemed taken aback by the enthusiasm of many in the group. “I believe your people actually want to make a bomb,” Oppenheimer reported Fermi commenting to him. “I remember his voice sounded surprised,” Oppenheimer added. Fermi, like Szilard, understood the gravity of the challenge they were facing and, though he understood it might be necessary, especially because the Germans were known to be pursuing nuclear weapons on their own, he did not share the majority’s enthusiasm.

He was, however, capable of coming up with rather brutal and ruthless ideas for winning the war. A letter from Oppenheimer to Fermi dated mid-1943 survives, commenting on an idea being considered by the two physicists for massive radiation poisoning of the German civilian population. How the idea arose is not clear, because Fermi’s side of the correspondence has never been found, but it is clear that both Oppenheimer and Fermi were open to considering a range of ideas to end the war quickly. Fermi might not have been enthusiastic about these ideas, but he probably considered it an obligation to think them through anyway.

Fermi returned to Chicago after the April 1943 meetings and continued his CP-2 experiments and his consultations with DuPont over the construction of the plutonium production reactors until August 1944, when he finally moved to Los Alamos. In the interim, Oppenheimer and others at Los Alamos continued to consult with him on a variety of problems, and he traveled to Los Alamos on several occasions, but his home base remained Chicago.

WHAT FERMI MADE OF OPPENHEIMER, DURING THOSE INITIAL April 1943 meetings or later, is hard to divine. He made only the occasional, brief comment to close colleagues. Yet even the casual observer would have noted the differences between the two physicists. Oppenheimer was the product of a highly cultured upbringing on Manhattan’s Upper West Side. The son of assimilated and wealthy German-Jewish parents, he attended the exclusive Ethical Culture School and then Harvard, where he distinguished himself in a wide range of disciplines. He then studied at Cambridge University under J. J. Thomson—the man who discovered the electron—and took his doctoral degree with Max Born at Göttingen. Oppenheimer was a man of the world, well read, conversant on art, history, and philosophy. He felt right at home at Göttingen, where a few years previously young Fermi felt an outcast. He was a prodigious theoretician who contributed to the quantum theory of his generation. At Berkeley he developed a reputation as a stern, even cruel, taskmaster, someone who occasionally took delight in ridiculing hapless graduate students or post-docs who were guilty of an error in calculation. One might accept C. P. Snow’s judgment that Oppenheimer would have traded his entire career to have made just one discovery of the magnitude of Fermi’s three great contributions and yet still respect Oppenheimer’s achievements as a physicist of high caliber. Certainly, he had the respect of his colleagues at Berkeley and, increasingly, those of the growing Manhattan Project for which he now found himself in a leadership role.

The contrast with Fermi was striking. Fermi was neither well-rounded nor interested in high culture. He came from a distinctly middle-class background and never felt the need to rise above that. He could be socially charming, but in a more informal way. Everyone who knew him commented on the surprise they first felt when they met the legendary genius and discovered he was extraordinarily approachable, even diffident.

Fermi also found Oppenheimer’s style of physics more than a bit foreign. In 1940, he had traveled to Berkeley to give the Hitchcock Lectures, and in his spare time sat in on a lecture by an Oppenheimer acolyte. Later he confided to Segrè:

Emilio, I am getting rusty and old. I cannot follow the highbrow theory developed by Oppenheimer’s pupils anymore. I went to their seminar and was depressed by my inability to understand them. Only the last sentence cheered me up; it was, “and this is Fermi’s theory of beta decay.”

By 1943 Fermi was a legend, even before his colleagues began to appreciate the beta decay paper of 1934. Many things contributed to the legend: his landmark discoveries, his single-minded commitment to physics at the highest level, his astonishing ability to work his way through complex problems, his ability to simplify those problems in ways that facilitated understanding by those less gifted than he. He could be blunt, even dismissive if he believed someone was wrong, and his fondness for teasing those closest to him could be irritating, but he was never deliberately cruel. Physicists presenting new research knew they were in trouble if Fermi interrupted and said, “Excuse me, but there is something here I do not understand.” That was enough to convey his belief that the speaker was wrong. He had already won a Nobel Prize, something about which Oppenheimer may have had a bit of envy. He had also been in on the very start of the project that was now under Oppenheimer’s guidance.

The April 1943 meetings were not the first time the two physicists met. They encountered each other during Fermi’s occasional visits to Berkeley in the 1930s, particularly in 1937 when he spent time learning about Lawrence’s cyclotron and in 1940 when Fermi delivered the Hitchcock Lectures there. Now, though, they would come into increasingly regular contact. In spite of their differences, they developed a deep mutual respect. Oppenheimer respected Fermi’s way of thinking, his way of solving problems, his confidence and the way that confidence inspired others, and of course his enormous contributions to physics prior to the war. Fermi respected Oppenheimer less as a physicist than as the person in whom the US government, to which he now owed total allegiance, had vested authority over the entire Manhattan Project. If he ever disagreed with Oppenheimer, it would never be in public.

In later years, Oppenheimer was subtly dismissive about Fermi: “Not a philosopher. Passion for clarity. He was simply unable to let things be foggy. Since they always are, this kept him pretty active.” And yet he seems to have been profoundly influenced by, and perhaps even obsessed with, his wartime colleague’s intellect and the scale of his contributions to physics. Leona Libby recalls a dinner party in Pasadena after Fermi’s death at which Oppenheimer suggested a parlor game he called “Who do you want to be on your day off?” One would venture a name and the others would try to analyze what the choice signified, in a sort of amateur psychoanalytic fashion. Astonishingly, Oppenheimer chose Fermi. There was silence in the room. No one knew what to ask him. It is significant that Oppenheimer himself chose the game and then chose Fermi, reflecting an inscrutable, deep-seated need to identify with his Manhattan Project colleague.

ONCE OAK RIDGE WAS UP AND RUNNING, ATTENTION TURNED TO the construction of the first major plutonium production reactor at Hanford. Construction at the vast high-security site began in March 1943.

The project was vast, involving the construction of hundreds of buildings, the recruitment of many thousands of laborers, the creation of a virtual state-within-a-state—many times larger in area than Oak Ridge. The centerpiece was three plutonium production reactors, built alongside the river, and a huge chemical separation plant at some distance from the reactors, where spent fuel rods were transported by truck for disintegration and chemical treatment for plutonium extraction. The first reactor to go live was the “B” reactor, followed by the “D” and “F” reactors.

The work was a triumph of organizational collaboration between Groves’s Army Corps of Engineers and Greenewalt’s DuPont engineers. The decision to go ahead and build these behemoths was made after the success of CP-1, but before CP-2 had been completed and well before the Oak Ridge facility went critical in November 1943. Groves was rolling some very heavy dice here, backed by the complete confidence he earned from the civilian leadership of the project and his own faith in his “crackpots” at the Met Lab.

From Chicago, Anderson, the Marshalls, and Wheeler would eventually relocate to Hanford to contribute to the design and construction of the reactors.

Construction on the B reactor began in October 1943, based on a scaled-up version of the Oak Ridge X-10 reactor. The core was significantly larger than the X-10 core, consisting of a graphite block twenty-eight feet wide, thirty-six feet deep, and thirty-six feet high. The face into which fuel rods were fed had channels for 2,004 fuel rods, compared to the X-10’s 1,260 channels. Cadmium and boron safety rods could be controlled automatically in channels running perpendicular to the fuel rods. Fermi and Wigner calculated that in order to produce the volume of plutonium required at the pace demanded by Groves and the team’s leadership, the reactor would have to run very hot indeed—at some 250 megawatts of power, more than some million times the top power achieved by CP-1. This required special treatment of the graphite to cure it against expansion from the ambient heat of the reactions, as well as a more aggressive cooling system to prevent the aluminum cladding of the uranium fuel rods from melting.

The Columbia River provided plentiful fresh water to be pumped through the core’s channels, in the space between each rod and the wall of its channel, and then returned to the river when deemed safe. The creation of 2,004 channels for fuel was an example of overengineering. Neither Fermi nor Wigner thought that so many channels would be necessary, but Greenewalt’s senior engineer, George Graves, in consultation with Wheeler, decided to pack more capacity into the core in case they needed more power. This, it turned out, was perhaps the most fortunate decision made by the DuPont team.

On the occasion of Fermi’s first visit to Hanford, with Wigner in tow, the two were stopped at the gate to the high-security B reactor site. Wigner momentarily forgot that his code name was Gene Wagner and when the guard asked him to identify himself, Wigner said “Wigner—oh, excuse me please, Wagner!” The guard was immediately suspicious. He began to grill the timid, quiet man with the strong Hungarian accent. “Is your name really Wagner?” Fermi immediately intervened. “If his name is not Wagner, my name is not Farmer.” Wigner continues: “And the guard let us pass. That quick self-assurance was so typical of Fermi.”

Fermi arrived at Hanford a week before the B reactor was scheduled to go critical, in mid-September 1944. Greenewalt and Leona Libby escorted him around the vast facility, including the assembly line for the fuel rods that encased the uranium in aluminum/boron sleeves, where technical problems had delayed the start-up of the reactor. Greenewalt never quite got over the excitement of watching Enrico Fermi bring CP-1 to criticality, and Fermi respected the man whose team of engineers could make a large-scale plutonium production plant—reactor, separation facilities, and all the facilities needed to support these—rise from the desert floor in just over a year. They made an incongruous pair, yet the two of them worked well together—so well, in fact, that their partnership lasted long after the war was over.

THE PROBLEMS RELATED TO THE CANNING OF THE URANIUM RODS were finally solved, and by September 26, 1943, the fuel rods were being loaded into the face of the hulking core. Fermi helped in the operation, perched on a scaffold high above the floor, gently easing a rod or two into its channel. The reactor went critical that afternoon and everyone went home to relax as the pile started to generate plutonium. According to Wheeler, the plan was to power the pile up to nine megawatts and then maintain that power level for a while. Then they would run it hotter and hotter until it reached the 250-megawatt level for which it had been designed. Sometime the next morning, however, the reactor began inexplicably to lose power. The operators were puzzled and decided to pull the safety rods further out of the pile to keep the power at a steady nine megawatts, but the power continued to drop. By midafternoon the control rods were almost all the way out of the pile and still the operators were having trouble maintaining nine megawatts. At about four o’clock, Fermi suggested bringing the power down to 400 kilowatts to see if they could hold the reactor there. They couldn’t. By the end of the day, the B reactor—over a thousand tons of graphite and uranium rising thirty-six feet off the ground—was, for all intents and purposes, dead.

FIGURE 18.2. The author at the control panel of B reactor at Hanford. Photo by Susan Schwartz.

Panic now gripped everyone in the reactor building. Over $7 million ($95 million in today’s dollars) had been spent on the reactor alone, not including the staggering cost of the fuel rods. The US government was in the midst of spending what would be $350 million in total, including two other production reactors, separation facilities for the fuel rods, and all the associated facilities for a small city of forty-five thousand people—nearly $5 billion in today’s terms, an unprecedented expenditure by the US government at that time. If this reactor failed to do its job, though, it would not simply be money and time wasted, it would be a crucial logjam in the timeline for production of one of the main materials for the bomb itself. There was enormous pressure to figure out what was going wrong and to do it fast.

Wheeler had spent time over the past year worrying about reactor “poisoning.” By his calculations, the fission by-products of the controlled chain reaction, particularly at high power, might prove troublesome if allowed to build up. Those by-products could absorb neutrons out of the chain reaction, slowing or even stopping the reaction from working—”poisoning” the reaction. This is why he and DuPont engineer George Graves had decided to “overengineer” the reactor by installing one-third more fuel rod channels than Wigner thought were required for the reactor to work at 250 megawatts.

In any case, it was not immediately clear just what was causing the loss of power. Fermi and Leona Libby suspected that water had leaked into the fuel rods from the cooling system, but upon inspection the water cooling system was intact. There was also concern about a possible leak in pressurized helium being pumped into the graphite core to replace air that might reduce the reproduction factor, but they could find no such leak.

The reactor itself gave the team an important hint at the cause of the problem. Spontaneously, it started up again. Throughout Wednesday afternoon the power returned to the reactor and by Thursday afternoon, September 28, 1943, the reactor reached nine megawatts again, only to falter once more a few hours later.

Wheeler suspected that some radioactive element had been produced as a by-product of the chain reaction, with an enormous ability to absorb neutrons and a fairly brief half-life, on the order of around eleven hours, after which the chain reaction could revive again at full force. He also suspected that because the drop-off occurred only after the reactor was able to reach the fairly high running power of nine megawatts, the real poison was a product of another radioactive by-product that itself did not absorb neutrons but decayed into something that did. Otherwise, the reactor would not have been able to reach nine megawatts in the first place. Wheeler checked a wall chart that listed isotopes that might be created in fission reactions, along with half-lives, looking for a likely suspect. The only one that really seemed to fit the profile was xenon-135, produced in the decay of iodine-135, a known first-generation by-product of uranium fission. Xenon-135 has a half-life of just over nine hours.

He made some rough calculations of the ability of xenon-135 to absorb neutrons. He found that the culprit was produced in about 6 percent of all fission events. He also discovered, to his and everyone else’s astonishment, that its ability to absorb neutrons in its general vicinity was vastly higher than any element previously studied. When Fermi and his Columbia team studied a list of which elements were particularly good neutron absorbers, they had discovered that cadmium was one of the most potent. They had not tested xenon-135, because the isotope was extremely rare and quite unstable. But Wheeler’s calculation suggested that this form of xenon was one hundred thousand times more potent than cadmium.

Fortunately, with xenon’s half-life of only nine hours, the solution was clear. If the reactor was fully loaded with all 2,004 of the uranium fuel channels filled, the reactivity would swamp the effect of the xenon, and the reactor would be able to operate smoothly at its rated power level.

Wheeler and Graves deserve enormous credit for deciding to overengineer reactor B in the event that fission by-product poisoning required an increase in the power of the reactor. In so doing, they allowed the Manhattan Project to keep to its tight deadlines and salvaged the multi-billion-dollar engineering project on which so much depended. When Groves was apprised of the problem, he was furious and lashed out at Compton, whose somewhat feeble response was that the problem would be studied in greater depth at Argonne, where a new pile, CP-3, had been built to supplement CP-2. Groves did not reproach Fermi, though well he might have.

In Fermi’s brilliant career, he demonstrated his fallibility on only a few occasions, none more dramatically perhaps than this one. His 1934 failure to realize that he had split the uranium atom led later to some embarrassment, but we can take some comfort that his failure deprived the fascist government of a four-year head start on nuclear weapons. The reactor B mistake was far more serious and might have led to the loss of the plutonium side of the project altogether. He never commented on his failure to anticipate xenon poisoning, but he must have been enormously embarrassed. Anticipating it would have saved much time, because the adjustments required to swamp the xenon poisoning delayed the running of the reactor at full power for some five months. The B reactor only achieved full power in February 1945, at which time two other reactors, D and F, were nearing completion. Fermi could have forecast this problem, and also could have Wheeler, who had been concerned about poisoning for some time prior to the completion of the reaction and had also not anticipated the xenon problem. At that moment, though, Fermi had other pressing things on his mind. During the summer of 1944, the work at Los Alamos was running into potentially catastrophic roadblocks. Oppenheimer decided that a total reorganization was required and Fermi wound up with another new role, one that would bring him into residence on the secret mesa northwest of Santa Fe.