Nuclear energy is a relatively new energy source. It was discovered in the 20th century, unlike fossil fuels, which have been used for millions of years.

1900 – Physicists discover the atom contains large quantities of energy.

1904 – British physicist Ernest Rutherford earns the nickname “the father of nuclear science” because of his contribution to the theory of atomic structure.

1905 – Albert Einstein develops his theory of the relationship between mass and energy. The mathematical formula is E=mc2, or “energy equals mass times the speed of light squared.” This theory was proven almost 35 years later.

Albert Einstein publishes his paper on the photoelectric effect, along with a paper on his theory of relativity.

1934 – Physicist Enrico Fermi conducts experiments in Rome that show neutrons could split into many kinds of atoms. When his team bombarded uranium with neutrons, the surprising results indicated that the elements were much lighter than uranium.

1938 – German chemists Otto Hahn and Fritz Strassmann fire into uranium neutrons from a source containing the elements radium and beryllium. They were surprised to find lighter elements, such as barium, in the leftover materials.

Hahn and Strassman contact Austrian physicists Lise Meitner and Meitner’s nephew, Otto Robert Frisch. Meitner and Frisch thought the barium and other light elements in the leftover material resulted from the uranium splitting — or fission. They added the atomic masses of the fission products and saw that they did not total the uranium’s mass. Meitner used Einstein’s theory to show the lost mass changed to energy. This proved fission occurred and confirmed Einstein’s work.

1942 – Enrico Fermi and Hungarian physicist Leo Szilard, who had both immigrated to the US, create the first manufactured reactor, known as Chicago Pile-1. In addition to uranium and graphite, it contained control rods made of cadmium, a metallic element that absorbs neutrons. When the rods were in the pile, there were fewer neutrons to fission uranium atoms. This slowed the chain reaction. When the rods were pulled out, more neutrons were available to split atoms. The chain reaction sped up.

Chicago Pile-1 is demonstrated on December 2. Control rods were withdrawn a few inches at a time during several hours until finally the nuclear reaction became self-sustaining. Scientific theory became a technological reality.

Most early atomic research focused on developing an effective weapon for use in World War II under the code name the Manhattan Project. 

1951 – Electricity is generated for the first time by a nuclear reactor on December 20 at the EBR-I experimental station near in Idaho, US. About 100kW was initially produced. It became the world’s first electricity-generating nuclear power plant when it produced sufficient electricity to illuminate four 200-watt light bulbs. It subsequently generated sufficient electricity to power its building and continued to be used for experimental purposes until it was decommissioned in 1964.

1953 – US President Dwight Eisenhower’s speech, “Atoms for Peace,” calls for greater international cooperation in the development of atomic energy for peaceful purposes.

1954 – The Soviet Union's Obninsk Nuclear Power Plant became the world’s first nuclear power plant to generate electricity for a power grid, and produced around 5 megawatts of electric power.

1955 – The United Nations’ “First Geneva Conference”, then the world’s largest gathering of scientists and engineers, met to explore nuclear technology.

1956 – The world’s first commercial nuclear power station, Calder Hall in Sellafield, England, opened with an initial capacity of 50 MW (later 200 MW).

1957 – EURATOM, the European Atomic Energy Community, is launched alongside the European Economic Community, which is now known as the European Union.

The International Atomic Energy Agency (IAEA) is launched.

The world’s first large-scale nuclear power plant begins operation in Shippingport, Pennsylvania, US. The plant reaches full power three weeks later and supplies electricity to the Northeast area of the US.

1958 – Construction begins on the world’s first nuclear-powered merchant ship, the N.S. Savannah, in the US. The ship launched July 21, 1959.

1970 – The US, UK, Soviet Union, and 45 other nations ratify the Treaty for Non-Proliferation of Nuclear Weapons.

1974 – The first 1,000-megawatt-electric nuclear power plant goes into service: Commonwealth Edison’s Zion 1 Plant.

1991 – At the end of 1991, 32 countries had nuclear power plants in commercial operation or under construction, indicating worldwide commitment to nuclear power technology.

2018– There are about 440 nuclear power reactors operating in 30 countries plus Taiwan, with a combined capacity of about 400 GWe. In 2018 these provided 2563 TWh, over 10% of the world's electricity. About 55 power reactors are currently being constructed in 15 countries, notably China, India, Russia and the UAE.