On December 2, 1942, Enrico Fermi and a small band of scientists and engineers demonstrated that a simple construction of graphite bricks and uranium lumps could produce controlled heat. Let’s look back to see how simple that first reactor was.
Behind the Scenes
The space chosen for the reactor was a squash court under the football stadium at the University of Chicago. A squash court is a rectangular room with the following dimensions: 32 feet long, 18.5 feet wide and 20 feet high.
Water Zinn was Fermi’s principal construction superintendent. The crew included a half a dozen scientists, a carpenter and about thirty temporary laborers waiting for their draft notices. Several suppliers, including Mallinckrodt Chemical Works and National Carbon Company provided the raw materials.
Zinn and his crew shaped the graphite into bricks that were 4 inches square in cross section and 16 and a half inches long. About one third of the bricks had two 3 and a half inch deep holes drilled in them to accept lumps of uranium.
The crew built the pile by stacking the bricks in a semi rounded shape. They laid two layers of solid graphite bricks followed by one layer of bricks containing two five pound lumps of uranium. The bricks at the edges of the pile were solid.
The Key to Control
Fermi provided a simple means to control the reactor. He directed Zinn to machine channels into some of the bricks so that he could insert long strips of wood wrapped with cadmium foil. Since cadmium is a neutron absorber, the rods prevented a chain reaction.
The rods were moveable, but locked in place during construction. Some of the rods were designed to be rapidly inserted to snuff out the reaction if it looked like it would get out of control. In many ways, control rods perform the same function as vents for a barbecue grill. By adjusting the vents to control oxygen flow across the coals, backyard chefs control the rate of combustion; by moving rods to control the number of available neutrons, nuclear plant operators can control the rate of fission.
A Calculated Success
On a cold December day, the team carefully pulled out the rods. Instruments plotted the neutron concentration in the reactor as a measure of the power level. The reactor behaved just as Fermi had predicted.
There was no chance of the first pile experiencing a “meltdown”. A rapid power surge would have caused the reactor to heat up. An increased temperature would have caused the pile materials to expand. This expansion would allow a higher percentage of vital neutrons to leave the pile without causing fission.
The pile only contained the minimum amount of material to sustain a chain reaction at its low initial temperature. Even a small temperature increase would have caused the reactor to immediately shut itself down. Since graphite’s melting point is on the order of 3500 degrees Fahrenheit, the pile would have never come close to melting.
Fermi’s pile was just an experiment, never meant for power production. It showed the world, however, that nuclear fission could be used to produce a controlled, self-sustaining, heat producing reaction.