Physical specifications and measurements of the MIT Graphite Exponential Pile

Abstract

The Massachusetts Institute of Technology's Graphite Exponential Pile (MGEP) presents a unique set of research and pedagogical opportunities mostly unavailable at other institutions that offer degrees in nuclear sciences and engineering. In early reactor physics classes, many students solve simplified transport and diffusion equations on cubic geometries, similar to that of the MGEP. Having a physical reactor to which these analytical solutions can be easily compared can highlight where the assumptions of the equations break down. In addition, introducing Monte-Carlo simulation allows for a more rigorous investigation into the relationship and agreement/disagreement between analytical, stochastic, and experimental analyses. This work has three primary goals. Firstly, to compile, interpret, and condense all historical accounts of the construction of the pile, including recent physical measurements.

This will result in the production of an accurate and detailed physical description of the pile. Secondly, to determine the spatial distribution of neutrons in the MGEP analytically, experimentally, and in Monte Carlo simulation, with the results of each being compared. Lastly, a compilation of specific experiments will be completed with the purpose of serving as reference for future researchers and classes wishing to use the MGEP. The first goal is met by bringing together all historical accounts of the pile and concisely summarizing them. In addition, modern work on the pile, including the cutting and sanding of various pieces to ensure symmetry, has been completed and documented. The second goal is met by first using the classical one-group diffusion theories originally proposed by Fermi to produce an analytical solution for the scalar flux at all positions in the pile.

After computing an analytical solution, experiments using the activation of Indium (In) foils and 3He detectors are conducted to empirically determine the flux shapes in various positions in the pile. As well, the Open MC Monte Carlo code is used to generate high-fidelity, 3-D simulation results for the flux distribution in the pile. The third goal is met by providing example descriptions and procedures for class experiments, each of which is compiled in a single thesis.