Atomic energy, now usually called nuclear energy, became a gleam in the eye of scientists in the early part of the 20th century. The possibility that the atom held a vast reservoir of energy was suggested by the large kinetic energy of the particles emitted in radioactive decay and the resultant large production of heat. In 1911, 15 years after the discovery of radioactivity, the British nuclear pioneer Ernest Rutherford called attention to the heat produced in the decay of radium, writing:
This evolution of heat is enormous, compared with that emitted in any known chemical reaction… The atoms of matter must consequently be regarded as containing enormous stores of energy which are only released by the disintegration of the atom.
At this time, however, Rutherford had no concrete idea as to the source of this energy. The magnitudes of the energies involved were gradually put on a more quantitative basis as information accumulated on the masses of atoms, but until the discovery of fission in 1938, there could be no real understanding of how this energy might be extracted. In the interim, however, important progress was made in understanding the basic structure of nuclei. Major steps included the discovery by Rutherford in 1911 that an atom has a nucleus, the discovery in 1932 of the neutron as a constituent of the nucleus, and a series of experiments undertaken in the 1930s by Enrico Fermi and his group in Rome on the interactions between neutrons and nuclei, which eventually led to the discovery of fission.
As these developments unfolded, there was general speculation about atomic energy. One of the earliest scientists to have thought seriously about the possibilities was Leo Szilard, who was later active in efforts to initiate the U.S. atomic bomb program and then to limit it. He attributed his first interest in the extraction of atomic energy to reading in 1932 a book by the British novelist H. G. Wells. Writing in 1913, Wells had predicted that artificially induced radioactivity would be discovered in 1933 (he guessed the actual year of discovery correctly!) and also predicted the production of atomic energy for both industrial and military purposes. In Szilard’s account, he at first “didn’t regard it as anything but fiction.” A year later, however, two things caused him to turn to this possibility more seriously: (1) He learned that Rutherford had warned that hopes of power from atomic transmutations were “moonshine” and (2) the French physicist Frederic Joliot discovered artificial radioactivity as predicted by H. G. Wells.
Szilard then hit upon a “practical” scheme of obtaining nuclear energy. At the time, it was thought that the beryllium-9 (9Be) nucleus was unstable and could decay into two alpha particles and a neutron. This was a misconception, based on an incorrect value of the mass of the alpha particle. Actually, 9Be is stable if only by a relatively small margin. In any event, Szilard thought that it might be possible to “tickle” the breakup of 9Be with a neutron and then use the extra neutron released in the breakup to initiate another 9Be reaction. In each stage, there is one neutron in and two neutrons out. This is the basic idea of a chain reaction. This particular chain reaction cannot work, as was soon realized, because too high a neutron energy is required to cause the breakup of 9Be, and even then, there is a net loss of energy in the process, not a gain.
Szilard tried to find other ways to obtain a chain reaction, but his efforts failed. Nonetheless, in the interim, he went so far as to have a patent on neutron-induced chain reactions entrusted to the British Admiralty for secret safekeeping, the military potential of nuclear energy being important in his thinking. However, the key to a realizable chain reaction—fission of heavy elements—eluded Szilard as well as all others.
In these early speculations, there was an awareness of the potential of atomic energy for both military and peaceful applications, and the latter loomed large in the thinking of some scientists. For example, in a document dated July 1934, Szilard explained planned experiments that, if successful, would lead to power production. . .on such a large scale and probably with so little cost that a sort of industrial revolution could be expected; it appears doubtful for instance whether coal mining or oil production could survive after a couple of years.
Along the same lines, Joliot prophesied in his 1935 Nobel Prize acceptance speech: “Scientists, disintegrating or constructing atoms at will, will succeed in obtaining explosive nuclear chain reactions. If such transmutations could propagate in matter, one can conceive of the enormous useful energy that will be liberated.”