It is a bleak day, December 2, 1942. The siege of Stalingrad has arrived at its 100th day. The disastrous Operation Mars, launched by the Red Army west of Moscow, resulted in about 170,000 Russian and near 40,000 dead German soldiers. Afrika Korps German forces under Kesselring are pushing back the Allies at Tebourba, Tunisia. Just a couple of days ago, four British battleships attacked and sunk four Italian commercial carriers at Skerki Banks, near Sicily, killing over 2,200 soldiers and seamen. Mussolini declares to the press: “The last word has not yet been spoken.” The Chicago Tribune reports today that Luftwaffe has already pounded Britain with 200,000 bombs, causing nearly 100,000 injured or dead people. Yesterday, the U.S. Office of Price Administration announced for the first time gasoline rationing in thirty western states, including Greater Chicago Area.
Glenn Seaborg is shivering while walking to his laboratory along Eckhart Hall, University of Chicago campus. The MetLab, in charge of producing the “alloy-49”, that is pure Pu-239, has come under the fire of a team of engineers of MIT appointed by the DuPont company, which attributed less than 1% probability of success to the enterprise. On the side, Chadwick in England had presented some calculations according to which a plutonium bomb could never work, because of the inevitable contamination by Pu-240, whose spontaneous fast fission would make the device uncontrollable. And, cherry on the tart, the new building in Argonne designed to host the experiments is encountering difficulties, and would not be ready before new year. The fear was spreading that, according to major rumors and some scarce information, Hitler was building his own nuclear bomb. In the race against nazi Germany time was more precious than money, that is why Fermi has been able to convince General Groves to immediately start assembling a test pile in the lower hall of the Stagg Field, the football court of the University, while waiting for the Argonne site to become operational. It had been a frantic two weeks since they started working, on November 16. The concern about safety was huge: everybody trusted Fermi’s calculations, but these were the only way to check what was going on in the experiment. Anderson and Zinn were in charge of the loading of the 45,000 graphite bricks, 19,000 of which with a hole to hold uranium metal and oxide pellets. The whole structure was held together by wood lumbers, assembled as a cubic lattice, while the uranium core resembled as close as possible to a spheroidal shape. The loading proceeded steadily, layer by layer, 360 tons of graphite encasing about 50 tons of natural uranium. Seen from the outside, the huge structure of 7.5m large by 5.8m height resembles to an unfinished house.
Day after day the pile has grown toward its final shape. And as the size of the pile increased, so did the nervous tension of the men working on it. By both logic and science, they knew the Chicago pile, or CP-1, would become self-sustaining. It had to. All the measurements indicated that it would. But still the demonstration had to be made. As the eagerly awaited moment drew nearer, the scientists gave the greater attention to the smallest detaiIs, the accuracy of indium foil measurements, the exactness of their construction work. Fermi guided the entire pile construction and design. His friends described him as “completely self-confident, but wholly without conceit.” So exact were Fermi’s calculations, based on the measurements taken from the partially finished pile, that days before its completion he was able to predict the point at which the reactor would go critical, almost to the exact graphite brick number. Yesterday afternoon around 4pm, Zinn and Anderson made several measurements of the activity by indium foil and Geiger counters. They were sure that once the cadmium rods were withdrawn, the reaction would become self-sustaining. Both agreed, however, to not start the operation until Fermi and the rest of the group were present. The control rods were locked in, and further work was postponed until today. December 2.
After my degree in theoretical nuclear physics, I was looking for a postdoc job, which in the mid ‘80s was a rarity to find. Quite reluctantly, I also applied for a permanent position as reactor physicist in ENEA, Rome. I knew next to nothing of reactor physics, so I borrowed the Lamarsh’s Introduction to Nuclear Engineering from the university library (a much simpler source, compared to the revered, but tougher Bell & Glasstone). I still cannot believe it, but that was enough to gain my place to the Neutron Physics Laboratory, and that was also where I first met the famous “four-factor formula” of Fermi. The key to a controlled nuclear chain reaction is contained in a deceivingly simple formula for the multiplication factor, usually denoted by k: it is simply the ratio between the number of neutrons from fission in one generation divided by the number of neutrons absorbed or escaped in the preceding generation. If k is larger than one the chain reaction is self-sustaining, if it is lower it dies off. In a reactor at the critical point, it is just equal to one: one new neutron is injected for each old neutron consumed. How to calculate such a number practically includes all the physics of the first half of the XX century. Fermi derived a quick way to obtain the multiplication factor for an ideal reactor of infinite size, as the product of four quantities, k∞=hfpe. The first and second terms are relatively simple to calculate, as ratios between different cross sections of the fissile and moderator materials; the fourth is also related to cross sections, albeit in a somewhat more complex way. The third one is the meanest guy of the group, the resonance escape probability. Already at Fermi’s time it was becoming clear that neutron cross sections display a vast number of resonances at all energies, and especially close to the thermalization. That makes the life of a neutron inside the reactor an endless nightmare, desperately trying to escape capture while slowing down to thermal energy, before its unlikely, random meeting with one U-235 nucleus.
So, at 8:30 of today, December 2, Fermi’s group assembles in the squash court of Stagg Field. The team arrangement is depicted in the funny sketch by Raymond Murray (attached). Norman Hilberry is attending at the safety, or ‘ZIP’, cadmium rod, attached by a rope: in case of problems, he would cut the rope with an axe and drop the rod in the uranium core. The urban legend has that this should be the origin of the term ‘scram rod’, currently used in modern reactors: it should have been the acronym for ‘Safety Control Rod Ax Man’ (a contested etymology). Another group is holding big buckets of cadmium salts solution above the structure, to be poured as a last resort. The side rod, maneuvered by George Weil (on the right of the sketch), is the actual control of the reaction. In fact, the neutrons present in a critical system do not all share the same lifestyle. There are the prompt neutrons, produced in the U-235 fission, and the delayed neutrons, which are produced in the decay of the fission products. About 98% of the neutrons in a reactor are prompt, but bringing the pile to the critical stage using only these neutrons means risking to not be able to keep it under control, because once the k goes above 1 the neutron production grows exponentially. This is a nuclear bomb. Fermi knew they had to focus on that 2% of delayed neutrons that leave enough time for the response of a human control system. The rod operated by hand by Weil intervened on the flux of the delayed neutrons: the calculation was set to arrive at a slightly subcritical stage with just the prompt neutrons, and leave to the delayed neutrons, which come at later times (up to a few minutes) the task to sustain the reaction at k=1.
At 9.45 Fermi orders to withdraw the three cadmium rods attached to an electric motor. The counters start ticking quickly. Shortly after 10, he calls Zinn to pull out the ZIP rod. Zinn pulls the rope by hand until the rod goes to the mark, and ties it to the rail. About half an hour later, without taking his eyes off the instruments, Fermi says quietly: “Pull it to 13 feet, George”. Weil starts pulling out the cadmium rod that controls the delayed neutrons, marked with ticks indicating how many feet it is still inside the core. Fermi checks the paper pen recorders, and makes a few quick calculations. “This is not it. See? It will get to this point and stop.” He indicates a spot on the graph. In a few minutes the pen moves slowly, exactly reaching to that point and then remains constant. Weil keeps pulling out the rod in steps, each time Fermi recalculates the new point, and each time the pen stops exactly at the calculated point. It is about 11.45, when they see that Weil’s rod is completely pulled out, and still the counters go to a constant level. Fermi realizes that the main ZIP rod safety point is not correct, it has been set too low. “I’m hungry” he says with a smile “Let’s go to lunch.” Perhaps, like a football coach, he knew that his men needed a break. At lunch they talk about many things, but do not touch the main subject. Fermi appears confident. By 2pm they are back in the court, the new safety point of the ZIP rod is set, and Weil can start over, pulling out the control rod. Again, Fermi checks instruments and calculates, then at 3.20 he says to Compton: “This is going to do it. From this point – he indicates on the paper – the trace will climb up exponentially.” For a few endless minutes, he moves the dials of his slide rule, then he flips it over and writes a few numbers on the back. Overbeck voices out the neutron count, increasing steadily. Weil keeps his eyes on Fermi, who now smiles and closes his slide rule with a slap. “The reaction is self-sustaining – says calmly – the curve is exponential.”
At 3.42 Watts writes in his notebook “We’re cooking”. The group keeps looking at the counters and monitors, the pens going up without ever leveling off. This was it, the first controlled nuclear chain reaction. At 3.53 Fermi says to Zinn, “OK, zip in.” Zinn moves the ZIP rod down, the counters stop, all the pens go straight. Many sighed with relief. The show has ended. Right after, Wigner pulls out the famous Chianti bottle he’d been hiding for all this time. Fermi is in charge of uncorking it, paper cups appear and all the team drinks to the success. As the group files out from the West Stands, one of the guards asks Zinn: “What’s going on, Doctor, something happen’ in there?” Later, Compton and Greenewalt confirm the success of CP-1 to the committee led by Warren Lewis of MIT. After the committee left, Compton called James Conant at Harvard, by long-distance telephone. Their code was not prearranged. “The Italian navigator has landed in the New World,” said Compton. “Were the natives friendly?” asked Conant. “Everyone landed safe and happy.” replied Compton. Only in the late afternoon Seaborg arrived at the MetLab, still concerned about the visit of the Lewis’ committee, and ran into Greenewalt in the corridor of Eckhart Hall, who was bursting with the good news. “The pile is a success!” Seaborg enthusiastically recorded in his diary.
In February 1943, the MetLab scientists started dismantling CP-1. It was a zero-power reactor, that could not operate at high power levels because of the lack of any security shielding. It actually never went above 200 W. The interest now had quickly shifted to the main goal of mass production of Pu-239 from U-238 (neutron capture followed by two beta decays). The compromise reached between the University of Chicago and the DuPont company was to build the experimental production pile at Clinton, Tennessee, while the separation plants would be located at Hanford, upstate Washington. The MetLab thus concentrated on basic fission research, in the newly established laboratory in the Argonne forest, west of Chicago. By March 20, the materials are reassembled for the new pile, now known as CP-2, in a complete cubic shape, encased in a thick radiation shield of 15 cm of lead, topped with a wood platform. Fermi continued to work in the new ‘Argonne Laboratory’ (at that time called ‘site A’), enjoying with his wife Laura the forest location and its isolation from the big city. As usual, he did all the calculations for the CP-2 reactor, and for the new, heavy-water moderated CP-3 that was going to replace it in the coming months. Soon, Fermi and many others moved to Clinton, Hanford and Los Alamos, to continue the Manhattan Project. The design and construction of the successors of the CP reactors (which would no longer be called a ‘pile’) was now in the hands of Walter Zinn, who would become the first director of the Argonne National Laboratory in 1946. He would design and build the EBR-I reactor, the first plant to ever produce electrical power, lighted up on December 20, 1951.