Nuclear Archeology of the Chicago Piles

In 1942, the Chicago Pile-1 experiment achieved a state of self-sustained fission chain reactions known as criticality, inaugurating the nuclear age. Despite its historical significance, most of the physical remnants from CP-1 and its successor CP-2 were dismantled and discarded, leaving many of the operational details unresolved. In this work, we examined a surviving uranium-graphite fuel assembly that was used in both CP-1 and CP-2 to determine trace elemental, isotopic, and anthropogenic isotope contents. Rare earth element signatures indicate Central European uranium provenance, elevated calcium levels are consistent with known uranium metal reduction processes, and graphite analysis revealed extremely low concentrations of neutron poisons. Experimentally determined fission product and plutonium concentrations were interpreted using reactor simulations and used to derive cumulative energy production, local neutron fluence, and the most probable in-core location of this fuel assembly. This study demonstrates that modern radiochemical methods integrated with high-performance reactor simulations can recover operational, material, and spatial information from the world's first nuclear reactor many decades after it was decommissioned.