December 2 – Pile On!

Today’s factismal: The world’s first artificial self-sustaining nuclear chain reaction took place in Chicago on December 2, 1942.

Did you ever wonder why a nuclear reactor is sometimes called a “pile”? The answer to that question was built in Chicago some 74 years ago. During World War II, we knew a lot of things. We knew (thanks to Becquerel) that atoms could split into smaller parts in what would come to be known as an atomic reaction. And we knew (thanks to Einstein) that those reactions could release a lot of energy. And we knew (thanks to our spies) that the Germans were working on ways of turning that energy into an explosive. And we knew (thanks to the way the war was going in 1942 {not well}) that if they developed it, they’d use it. So we decided to develop it first. And so was born the Manhattan Project, the most famous secret project ever.

The world's first artificial nuclear reactor. The uranium went into the holes in the graphite bricks. (Image courtesy DoE).

The world’s first artificial nuclear reactor. The uranium went into the holes in the graphite bricks.
(Image courtesy DoE).

The first stage of the Manhattan Project was discovering if we could control the reaction; if we couldn’t then there wasn’t any way to build a weapon. So Enrico Fermi (who discovered how to make small atoms out of big ones by bombarding them with neutrons) and Leó Szilárd (who discovered how to make other atoms do the bombardment in a chain reaction) constructed the world’s first artificial nuclear reactor in a Chicago University racquetball court. (There was a natural nuclear reactor in Africa some two billion years ago, but they didn’t know about it then.) To make it, they placed 771,000 pounds of graphite bricks into a rough cube that was 20 feet high and 25 feet wide; as they built, they filled the bricks with 92,990 pounds of uranium pellets and control rods of indium, cadmium, and silver. The graphite absorbed some of the energy of the neutrons released by the uranium as it decayed; that made it more likely that the neutrons would be absorbed by other uranium atoms causing them to decay. And the control rods would absorb the neutrons completely, stopping the reaction. By sliding the control rods in and out of what Fermi describer as “a crude pile of black bricks and wooden timbers”, they would be able to control the reaction (in theory, at least).

And on December 2, they tested that theory. In front of a group of other physicists who were also working on the Project, they slid the rods out and started the world’s first artificial self-sustaining nuclear chain reaction. Twenty-eight minutes later, they slid the rods back in and stopped the reaction. The test was a success and that meant that the Manhattan Project could go on and we could use it to win the war.

Today, we are still splitting atoms, this time for peace. And we are still trying to learn what happens next. If you’d like to help the physicists at Stanford discover what happens when you make tiny ones out of little ones, then why not contribute the idle time on your computer with LHC@home?

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