Chicago Pile 1: 80 Years of Nuclear Fission

Today marks 80 years since the world's first man-made controlled nuclear chain reaction. Here's how it all began

Eighty years ago today forty-nine scientists, led by Nobel Prize winner Enrico Fermi, demonstrated that humans can control nuclei energy.

In fact, that day the world’s first man-made controlled nuclear chain reaction was achieved with the Chicago Pile 1 experiment. That was the notable beginning of the path leading society to harvest energy from this extremely dense source.

As we continue on this path and enhance nuclear fission with our everyday work, you can take a few moments to dwell on this historic moment with us.

The reactor was constructed underneath the football stadium at the University of Chicago in October and November 1942. It consisted of graphite blocks, some of them embedding uranium (metal and oxide).

Graphite is a form of crystalline carbon and in nuclear reactors is used as a moderator. In fact, carbon doesn’t really absorb neutrons and it is effective in moderating them, in other words slowing them down, to lower energies where fissioning of uranium-235 is easier. This is what we call a thermal fission reaction and in most current commercial reactors the same function is fulfilled by water.

These blocks were arranged and stacked in a rough sphere with 57 layers, it was supported on a wooden structure and rested on the floor at its lowest point. This pile had an overall dimension of around 7 meters.

During construction, the geometrical configuration was constantly checked and refined in order to make sure that the critical dimensions could not be reached inadvertently without taking the proper precautions. Two types of detectors were used to measure: a BF3 counter and an indium foil.

On 2 December 1942, the pile was operated by taking out all the cadmium strips but one. Cadmium is a strong neutron absorber; hence it impedes the chain reaction. The last rod is then slowly pulled out of the pile. As the critical conditions were approached, the intensity of the neutrons emitted by the pile began to increase rapidly. After 28 minutes of the reactor sustaining the chain reaction, the cadmium control rod was reinserted – shutting it down.

That afternoon, the achievement was celebrated with a bottle of Chianti brought by Eugene Wigner. The straw wrapping was signed by the group and, 20 years later at a reunion of the CP-1 pioneers, everybody added their signature.

Fermi and some of these pioneers later founded Argonne National Laboratory, which is still an important centre for nuclear technology development.

Enrico Fermi had an extraordinary impact on physics and nuclear phenomena and their application. The Nobel Prize for Physics was awarded to him for his work on slow neutrons, but he foresaw fast reactors would have been the future of nuclear fission.

Our mission is to industrialise this technology, which up to now remained largely untapped.

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