The Beginning of the Atomic Age – the first controlled nuclear chain reaction 75 years ago

It was first shown to be possible in Hitler’s Germany: splitting atomic nuclei. Under bombardment with neutrons of appropriate energy the nuclei of uranium atoms burst like drops of water and break into two parts. With 200 million electron volts, the energy of these fragments is much larger than any energy produced in previously known atomic processes.

The discovery of nuclear fission reads like a thriller. Here is the well-known chemist Otto Hahn, who knew much about chemistry, but less about physics. There, the brilliant physicist Lise Meitner, who has to leave Berlin immediately after the successful experiment, because the Protestant Christian woman is of Jewish descent and has to fear for her life in Nazi Germany. In exile she comes up with the decisive idea: The explanation for these enormous energies lies in Einstein’s famous formula E = mc2. For the two nuclei that emerge from the split are in their total slightly lighter than the original uranium nucleus. The difference in mass corresponded exactly to the energy of 200 million electron volts. For the first time, a process had become known in which the equivalence of energy and mass formulated by Einstein finds its direct manifestation. It was thus clear that the atomic nucleus of uranium can be split (Otto Hahn, but not Lise Meitner, received the Nobel Prize for Chemistry in 1944 for this discovery).

But something else had become clear: Unimaginable energies are dormant inside the atom. The physicists referred to those energies as “nuclear energy”. The coincidence was that the neutron-induced fission of a uranium nucleus released three more neutrons, which in turn could split further uranium nuclei. The physicists realized that a huge amount of energy could be released via a chain reaction in a very short time. This quickly brought the possibility of military application into play. The development that followed reads like a second thriller.

Already in 1939, less than a year after Hahn’s and Leitner’s discovery, Otto Frisch, the nephew of Lise Meitner and also a physicist, wrote together with his British colleague Rudolf Peierls a memorandum describing the technical construction of a bomb based on nuclear chain reaction. Now non-physicists listened up.

As the world’s leading nation in research and technology, Hitler’s Germany was predestined to be the first to build up military applications of nuclear energy. A bomb with such tremendous explosive power in Hitler’s hands would have had catastrophic consequences for the world. One of physicists that was horrified by a nuclear power Germany was the Hungarian Leó Szilard, who, like Leitner, had suffered greatly under Nazi Germany. He persuaded the hitherto strict pacifist Albert Einstein to write a letter to US President Franklin D. Roosevelt, suggesting that America proceeds to build a nuclear bomb.

Roosevelt picked up this initiative. Under strictest secrecy, the US government put together a team of senior scientists and technicians. Most of them had come from Europe and were driven by the motivation not to give Hitler sole access to nuclear weapons. The only goal of the project, baptized “Manhattan Project” and hitherto most complex, most expensive and most difficult technical project in history, was the construction of a nuclear bomb.

The first step was to demonstrate that a chain reaction of neutron releases could indeed be initiated and sustained. This was achieved in strict secrecy and separated from the public by Enrico Fermi, who a few years before had emigrated from his home country Italy that was allied with Hitler and had imposed similar racial laws against Jewish people (Fermi’s wife was Jewish). Underneath a sports arena at the University of Chicago, Fermi, one of the few physicists that achieved brilliance both in theoretical as well as experimental physics, constructed the first nuclear reactor in history. There, exactly 75 years ago, on December 2, 1942, the first controlled chain reaction of nuclear fission ran and reached criticality (as the physicists describe the status of a reactor where as many neutrons are released as they disappear through absorption and leakage). The experiment was so risky that Fermi did not even inform the university president about it beforehand. If the experiment had gone bad, probably a large part of Chicago would have been irradiated. As a safety precaution, only a few scientists were prepared with an ax and buckets full of cadmium sulfate, which should have interrupted the chain reaction.

From that day on, the world was different. The atomic age had begun. The eyewitnesses of the historical experiment were surely aware of this: “All of us knew that with the advent of the chain reaction, the world would never be the same again,” wrote physicist Samuel Allison later. The rest of humanity was not to learn until August 6, 1945, that a new era had begun.