December 2, 1942, was the coldest day on record in fifty years in Chicago. But on this very day, a group of scientists from the secret Metallurgical laboratory, formed in early 1942, conducted the world's first self-sustaining nuclear chain reaction at the University of Chicago at Stagg Field in the world's first nuclear reactor, whose name is Chicago Woodpile 1 (Chicago Pile-1).
"Woodpile" has earned its "low-tech" name. It was a stack of forty thousand graphite blocks held together in a wooden frame, seven and a half meters wide and six meters high. Inside about half of the blocks were holes containing small amounts of uranium oxide; inside several others were bits of purified uranium metal, the production of which was still a new process.
The fuel, as already mentioned, was unenriched natural uranium: approximately thirty-three tons of UO2 and four - U3O8. Graphite was chosen as the moderator because it turned out to be the only material of the required purity that could be obtained in the required quantities: three hundred and fifty tons of it were used.
The Chicago Woodpile was virtually devoid of security features. The scientists' only protection from radiation was provided by a set of cadmium control rods, designed to be manually inserted and removed. After all, as one government report said, "there were no guidelines to follow, and no prior knowledge to use."
The Woodpile was not an abstract scientific achievement. It was part of a much larger plan, conceived under the auspices of the Manhattan Project, to create a fleet of industrial-sized nuclear reactors - not to generate electricity (much later), but to produce plutonium, the fuel for nuclear weapons.
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The essence of the experiment was as follows: The Woodpile was launched, brought to a critical state (the point at which a nuclear reaction becomes self-sustaining), and then shut down before its growing heat and radioactivity became too dangerous. The reactor worked for 35 minutes. at a power of about 200 watts. The background radiation was 3 R / h in the immediate vicinity of the reactor and about 360 mR / h in the far corners of the hall.
The "metallurgical" laboratory experimented with the reactor for several months before the "Woodpile" was dismantled and, already with a radioactive shielding, reassembled at a site farther from the city, where the reactor was renamed "Chicago Woodpile 2" and operated for more than ten years before it was finally dismantled and buried in the forest.
In memory of Woodpile 1, the University of Chicago erected a Nuclear Energy monument with an inscription that reads: "On December 2, 1942, man achieved the first self-sustaining chain reaction here and thereby initiated the controlled release of nuclear energy." Interestingly, in one sentence, the then director of the university suggested adding a phrase at the end: "for better or worse." The proposal was not accepted.
