Characterization of the MIT Graphite Exponential Pile

Abstract

Subcritical graphite piles were originally built as an important tool for determining diffusion length and criticality in the early days of nuclear engineering. MIT still retains a graphite exponential pile, recently recommissioned back into service, and utilized by students for hands-on demonstration of classroom topics like buckling, subcritical multiplication, and 1 group diffusion. For most effective use, the characteristics of the pile, including flux profile, source-dependence, and fuel slug composition, should be well known. In this thesis neutron radiography was performed on 10 different fuel slugs from the pile to directly measure the thickness of the aluminum cladding and confirm the suspected method of slug fabrication. This sample of fuel slugs had an average clad thickness of 4.66mm±0.03mm on the bottom, 4.10mm±0.02mm on the top, and 1.48mm±0.02mm on the side. Additionally, a detailed neutronic model of the pile was created using MCNP5 software and used to map the spatial source dependence. Experimental measurements were taken to compare to simulated model data and determine the accuracy of the model. The overall flux shapes were similar, but the error between the model prediction and the measured data was greater than 15% for the 6-7 sampling locations that were furthest from the source. The characterization of the slugs and development of a neutronic model will allow for more localized calculations of a wide variety of fuel loading patterns and better utilization of the Pile as a teaching tool..