Dec 022015


The first self-sustaining nuclear chain reaction was initiated in CP-1 on this date in 1942, under the supervision of Enrico Fermi, who described the apparatus as “a crude pile of black bricks and wooden timbers”. Chicago Pile-1 (CP-1) was the world’s first nuclear reactor to achieve criticality. Its construction was part of the Manhattan Project, the Allied effort to create atomic bombs during World War II. It was built by the Manhattan Project’s Metallurgical Laboratory at the University of Chicago, under the west viewing stands of the original Stagg Field.

The reactor was assembled in November 1942 under the supervision of Fermi, in collaboration with Leo Szilard, discoverer of the chain reaction, and Herbert L. Anderson, Walter Zinn, Martin D. Whitaker, and George Weil. It contained 45,000 graphite blocks weighing 400 short tons (360 t) used as a neutron moderator, and was fueled by 6 short tons (5.4 t) of uranium metal and 50 short tons (45 t) of uranium oxide. In the pile, some of the free neutrons produced by the natural decay of uranium were absorbed by other uranium atoms, causing nuclear fission of those atoms, and the release of additional free neutrons. Unlike most subsequent nuclear reactors, it had no radiation shield or cooling system as it only operated at very low power. The shape of the pile was intended to be roughly spherical, but as work proceeded Fermi calculated that critical mass could be achieved without finishing the entire pile as planned.


In 1943, CP-1 was moved to Red Gate Woods, and reconfigured to become Chicago Pile-2 (CP-2). There, it was operated until 1954, when it was dismantled and buried. The stands at Stagg Field were demolished in August 1957, but the site is now a National Historic Landmark and a Chicago Landmark.

The idea of chemical chain reactions was first put forth in 1913 by the German chemist Max Bodenstein for a situation in which two molecules react to form not just the molecules of the final reaction products, but also some unstable molecules which can further react with the parent molecules to cause more molecules to react. The concept of a nuclear chain reaction was first hypothesized by the Hungarian scientist Leo Szilard on 12 September 1933. Szilard realized that if a nuclear reaction produced neutrons or dineutrons, which then caused further nuclear reactions, the process might be self-perpetuating. Szilard proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts, although he did entertain the possibility of using uranium as a fuel. He filed a patent for his idea of a simple nuclear reactor the following year. The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, followed by its theoretical explanation (and naming) by Lise Meitner and Otto Frisch, opened up the possibility of creating a nuclear chain reaction with uranium or indium, but initial experiments were unsuccessful. Fermi’s experiment marked the dawn of a new era.

I could give you a lot more historical and technical details. It mostly bores me so I cannot imagine what it will do for you. Instead here’s two pieces of relatively unimportant information which amuse me.

First, Fermi christened his apparatus a “pile”. Emilio Segrè later recalled that:

I thought for a while that this term was used to refer to a source of nuclear energy in analogy with Volta’s use of the Italian term pila to denote his own great invention of a source of electrical energy [a battery]. I was disillusioned by Fermi himself, who told me that he simply used the common English word pile as synonymous with heap. To my surprise, Fermi never seemed to have thought of the relationship between his pile and Volta’s.

Pila is still the common Spanish word for a (small disposable) battery. I’m not sure about Italian (I’ll have to ask my students). I know that pila can be used in Italian for a battery, but I am not sure how common it is, or what type of battery. I do remember early on in Buenos Aires asking in a store for batteries for my camera and using batterias only to get blank stares. Batteria in Spanish is used for car batteries and such.


Anyway, Volta used the word pila for his invention for reasons that are obvious when you see a photo of it. It’s a pile of stuff. It’s amazing how dense world-class physicists can be when it comes to a simple matter of etymology.

Second, I now teach at a technical high school in Mantua named after Fermi: Istituto Superiore Enrico Fermi. I’m not sure how much the students know about Fermi; I’ll find out later when I go in.

While writing the appendix for the Italian edition of the book Fundamentals of Einstein Relativity by August Kopff in 1923, Fermi was the first to point out that hidden inside the famous Einstein equation (E = mc2) was an enormous amount of nuclear potential energy to be exploited. “It does not seem possible, at least in the near future”, he wrote, “to find a way to release these dreadful amounts of energy—which is all to the good because the first effect of an explosion of such a dreadful amount of energy would be to smash into smithereens the physicist who had the misfortune to find a way to do it.” Interesting remarks from the man who was, in fact, the first physicist to do it and was not smashed into smithereens. See

Chocolate Pudding in a Glass Dish

Apparently in March 1942 Fermi wrote a recipe for chocolate pudding. The essence is that you melt 30 grams of chocolate with ½ teaspoon of sugar per serving in a double boiler. Remove from the heat and whip the chocolate with one egg yolk per person. Pour into individual cups and chill in the refrigerator for 8 hours. Serve with toasted almonds and/or whipped cream. This recipe appeared in a newspaper article whose provenance I don’t know. No matter. It’s from Fermi and that’s good enough for me.