The discovery of nuclear energy was not a single moment of realization but a cumulative scientific journey spanning decades, beginning with the investigation of invisible rays and culminating in the controlled release of the atom’s power. This immense power source is the energy released from the nucleus, the dense core of the atom. Uncovering this potential required groundbreaking experiments and theoretical breakthroughs that fundamentally changed the understanding of matter and energy.
Early Understanding of Radioactivity
The foundation for nuclear energy began in 1896 with the discovery of natural radioactivity by French physicist Henri Becquerel. He found that uranium salts spontaneously emitted penetrating, invisible radiation, even without exposure to sunlight. This observation proved that the atom was not the indivisible particle scientists had long believed it to be.
Following this, Marie and Pierre Curie dedicated their work to studying this new phenomenon, which Marie eventually named “radioactivity.” Through the chemical processing of pitchblende, the Curies discovered two new elements in 1898: polonium and radium. Their findings confirmed that radiation emission was an atomic property, demonstrating that unstable atoms could change and release energy.
Ernest Rutherford continued this work by characterizing the different types of radiation emitted. By 1899, he identified two distinct components, alpha and beta radiation, based on their penetrating power; gamma rays were identified shortly after. Rutherford later used alpha particles in his 1911 gold foil experiment, which led him to propose the nuclear model of the atom, concentrating mass and positive charge in a tiny, dense core called the nucleus.
The Theoretical Framework
The potential magnitude of the energy locked within the atomic nucleus was quantified by Albert Einstein in 1905. As part of his work on special relativity, Einstein developed the mass-energy equivalence equation, \(E=mc^2\). This concise formula stated that energy (\(E\)) is equal to mass (\(m\)) multiplied by the speed of light squared (\(c^2\)).
Before this equation, mass and energy were treated as separate properties that were conserved independently. Einstein’s work showed that mass and energy are interchangeable forms of the same fundamental entity. Because the speed of light squared is an enormous number, the equation demonstrated that converting even a minuscule amount of mass into energy would release tremendous power. This framework established the physical possibility of unleashing vast amounts of energy from the atom.
The Discovery of Nuclear Fission
The theoretical possibility of nuclear energy became a practical reality in late 1938 with the discovery of nuclear fission. Chemists Otto Hahn and Fritz Strassmann conducted the experiment in Berlin, bombarding uranium with neutrons, expecting to create heavier elements. Instead, their analysis showed the presence of barium, an element significantly lighter than uranium.
This result was baffling because no known nuclear reaction could account for forming an element half the size of the original atom. Hahn and Strassmann published their findings in January 1939 but were unsure of the physical explanation.
The correct physical explanation was provided shortly after by physicist Lise Meitner and her nephew, Otto Frisch, who were corresponding with Hahn from their exile in Sweden. They applied the concept of the atomic nucleus acting like a liquid drop that could split apart after absorbing a neutron. They calculated this splitting process would release approximately 200 million electron volts of kinetic energy. Frisch coined the term “fission,” borrowing from biological cell division, confirming that the uranium nucleus had split and demonstrating that nuclear energy could be practically released.
Achieving the First Controlled Reaction
While the discovery of fission proved that nuclear energy could be released, harnessing it required achieving a controlled, self-sustaining chain reaction. This next step was accomplished by Italian physicist Enrico Fermi and his team at the University of Chicago. Their goal was to prove that neutrons released during the fission of one uranium atom could cause fission in others, creating a continuous release of energy.
Fermi designed and oversaw the construction of an experimental nuclear reactor known as Chicago Pile-1 (CP-1), built in a squash court beneath the university’s Stagg Field. The reactor was a large, carefully stacked lattice of graphite bricks and uranium slugs, with cadmium control rods inserted to absorb neutrons and regulate the reaction rate.
On December 2, 1942, Fermi and his colleagues slowly withdrew the control rods, allowing the neutron population to increase and the reaction to accelerate. The pile achieved criticality, sustaining the first human-made nuclear chain reaction. This moment marked the transition from a laboratory curiosity to a controlled, usable power source, proving that the immense energy demonstrated by fission could be managed and utilized, ushering in the Nuclear Age.