Chicago Pile-1

Plate 49 speaks for itself: “Half a watt.”

The Physics

On 2 December 1942, Fermi's team pulled the last control rod out of a graphite-and-uranium pile and the neutron count rose steadily: the first self-sustaining chain reaction.

n(t) = n₀ e^((k − 1) t / ℓ)

• n: neutron population, dimensionless
• n₀: initial population
• k: effective multiplication factor, dimensionless
• t: time, s
• ℓ: mean neutron generation time, s

2 Dec 1942: k just above 1. About ½ W; ~200 W on 12 Dec.

Honest caveat: The simple exponential ignores delayed neutrons, which in practice set how slowly a reactor near k = 1 responds.

Sources

• E. Fermi, Am. J. Phys. 20, 536 (1952).
• US DOE, The First Reactor (DOE/NE-0046).

On the poster

Plate 049, “Chicago Pile-1”: the pixel-art plate with its slogan “Half a watt.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.