Once started, can a reaction sustain itself?

That time, that place

An Italian. After traveling to Stockholm to receive the 1938 Nobel Prize, he did not return home with his Jewish wife but continued to the United States, driven out by Mussolini's racial laws.

Why this question

When uranium splits, it releases not only energy but several neutrons.

Those neutrons split other uranium nuclei, which release more, and if it keeps going the reaction sustains itself. A chain reaction.

The question was whether it could be stopped. A reaction you cannot stop is not an experiment but an explosion.

What was found

Beneath a squash court at the University of Chicago they stacked graphite bricks and uranium in layers.

Graphite slows neutrons, and slow neutrons split uranium more readily. Cadmium rods absorb neutrons and stop the reaction.

On 2 December 1942 they withdrew the cadmium rods a little at a time, listening to the counters. The clicking quickened, and at a certain point it no longer subsided. The reaction was running itself.

They held it for twenty-eight minutes, then pushed the rods back in.

The emergency measure was a man with an axe who would cut a rope and drop a rod. That day they sent a coded message: the Italian navigator has landed in the new world.