​Celebrating the Chicago Pile anniversary

On December 2, 1942, Enrico Fermi and his team successfully made the "Chicago Pile" (CP-1), the world's first nuclear reactor, diverge under the West Stands of the St​agg Field in Chicago. The pile had approximately a spherical shape and used a mixture of uranium oxide (about 40 tons) and metal uranium (about 6 tons) as fuel, and graphite (about 385 tons)​ as moderator. The external graphite layers were used as a reflector. No radiation shielding nor cooling systems were used, since CP-1 was conceived as a "proof-of-concept" physics experiment to demonstrate the feasibility of the chain reaction. The neutron chain reaction was maintained in a slightly supercritical state for approximately half an hour, at a peak power of about 0.5 Watt. This historic event marked the birth of the nuclear age.

A drawing of CP-1, from reference E. Fermi et a​l., US Patent 2,708,656​, N​eutronic reactor (1955).

In order to celebrate the eightieth anniversary of the Chicago Pile, we have modeled CP-1 and its successor CP-2 using TRIPOLI-4® and TRIPOLI-5®, the Monte Carlo codes developed at SERMA/LTSD. To the best of our knowledge, a similar modeling effort based on Monte Carlo codes has never been attempted before.

Two-dimensional cut of the TRIPOLI-4 model of CP-1, including the pile itself (the double core with metal lumps in the center and oxide lumps in the periphery is visible) ans the racquets court of the Stagg Field where the reactor was built. Although not visible on this cut, the model includes the control and safety rod​s that were used for the divergence, control and shut-down of the reactor.

​Building a TRIPOLI-4 model of CP-1

Our work has been structured in three parts.

To begin with, we have exhaustively explored the bibliography concerning CP-1. We gratefully acknoweldge the support of many Institutions in the USA that provided access to unpublished reports, drawings and other very useful documents: the Nuclear Engineering Division and the Argonne Research Library of Argonne National Laboratory, the Niels Bohr Library and Archives of the American Institute of Physics, the National Museum of Nuclear Science & History, and the National Archives at Chicago. ​ This investigation has led to a technological database (geometry and material compositions of the fuel elements, moderator, structures, etc.) based on the available technical reports.

We have organized a series of conferences in order to promote the celebration of the anniversary of CP-1 and the "archeological industry" investigation of this astonishing artifact, among which an event at the General Consulate of Italy in Paris (April 2023), a talk at the Departement of Engineering of the Cambridge University (May 2023), and a plenary presentation at CEA/Paris-Saclay center (July 2023).

Then, based on these data we have created a three-dimensional and fully detailed working model ("numerical twin") of the CP-1 reactor: this input file has been used to feed the Monte Carlo codes TRIPOLI-4® and TRIPOLI-5® to simulate the divergence of the reactor and explore the main features of the pile, encompassing the kinetics parameters, the reactivity coefficients, the control rod worth, the reactor period, etc. The obtained results have been validated with respect to experimental measurements provided in the original reports and drawings of the Manhattan Project.

TRIPOLI-4 model of CP-1, displaying the pile, the wood filler and the wood casing, and the graphite pier supporting the slots for the safety and control rods.

Finally, we have perfomed a sensitivity analysis in order to assess the impact of the tecnhological uncertainties (mass and geometry of the components, etc) and of nuclear data libraries (especially thermal scattering laws of graphite) on the developed numerical model. Several Monte Carlo simulations attempting at reproducing the behavior of CP-1 have been performed, and the impact on reactivity, kinetic parameters the other key quantities of the reactor has been quantified.

​From CP-1 to CP2

CP-1 has lived only about three months, and in Febrauary 1943 it was dismantled and rebuilt (in an approximately cubical shape) at the Argonne site, mainly for safety reasons. The new pile was baptised CP-2: it was operated at higher power (1 kW) and included a thick concrete shielding. CP-2 had a longer life and was finally decommissioned in 1954, after having being successfully used for R&D purposes: it contributed extensively to cross section testing for a plethora of materials, mainly thanks to a thermal column device.

A drawing of CP-2, from reference E. Fermi et a​l., US Patent 2,708,656​, Neutronic reactor (1955).​

Prompted by our previous work on CP-1, we have also worked out a three-dimensional and fully detailed Monte Carlo model of CP-2, based on the original reports and drawings of the Manhattan Project​.

Two-dimensional cut of the TRIPOLI-4 model of CP-2, including the pile itself, the concrete shielding and the top graphite/wood shielding, with the thermal column.​