From discovery to chain reaction
In late 1938 the German chemists Otto Hahn and Fritz Strassmann found that bombarding uranium with neutrons produced lighter elements such as barium. Lise Meitner and her nephew Otto Frisch, who had left Germany, provided the physical explanation soon afterward and used the word fission. The key point was that splitting a heavy nucleus releases a large amount of energy, along with more neutrons, which could in principle split further nuclei in a chain reaction.
Chicago Pile-1
A working chain reaction first had to be proved. A team led by Enrico Fermi built Chicago Pile-1, a stack of graphite blocks and uranium under the stands of a University of Chicago athletic field. On December 2, 1942, it reached a self-sustaining reaction. It was a physics experiment and produced almost no useful power. Graphite acted as a moderator, slowing neutrons so that they were more likely to split uranium-235. Control rods that absorbed neutrons were used to adjust the reaction.
During the war, large reactors followed at Oak Ridge in Tennessee and at Hanford in Washington State, designed mainly to make plutonium for weapons. Their heat was discarded rather than used.
Making electricity
After the war, attention turned to civilian use. A reactor is, at its core, a heat source: the heat boils water, and the steam turns a turbine and generator, much like a coal plant but without the burning. This also connects it to the older story of steam engines.
- 1951: The Experimental Breeder Reactor I in Idaho produced electricity and is reported to have lit a few light bulbs. It was an experiment, not a power station.
- 1954: A small reactor at Obninsk in the Soviet Union began supplying power to a grid. Its output was about five megawatts, by commonly cited figures.
- 1954-55: The submarine USS Nautilus, launched in 1954, showed that a reactor could power a ship for long periods without surfacing, a milestone in submarine history.
- 1956: Calder Hall in England opened as a larger station intended to supply the national grid, though it was also designed to make plutonium.
- 1957: Shippingport in Pennsylvania began generating power as the first full-scale US civilian plant, using a pressurized water design.
Reactor types
Several designs emerged. Pressurized water reactors keep water under high pressure so it does not boil in the core, and pass heat to a separate steam circuit. Boiling water reactors let it boil directly. British gas-cooled reactors used carbon dioxide and graphite, and Canadian designs used heavy water, allowing natural uranium as fuel. The Soviet RBMK used graphite and water. Light-water designs eventually became most common worldwide.
Growth and doubts
In the 1960s and 1970s, many countries ordered reactors, and some expected electricity that would be very cheap. In practice, construction costs, schedules and regulatory requirements often rose. Public concern also grew about accidents, waste and the link to weapons.
The Three Mile Island accident in Pennsylvania in 1979 involved a partial core melt, with limited release of radioactivity outside the plant, but it affected policy in the United States. The Chernobyl accident in 1986 in Ukraine, then part of the Soviet Union, was far more serious and released large amounts of radioactive material. Fukushima Daiichi in Japan was damaged by an earthquake and tsunami in 2011. Detecting and managing radiation exposure, from the early days of radiation detection to current monitoring, is a continuing part of reactor operation, and the question of radiation dangers shaped regulation.
Where it stands
Nuclear plants supply a notable share of electricity in several countries and produce little carbon dioxide while operating. They also leave spent fuel that needs long-term management, and views on whether they should expand differ widely. Newer ideas include smaller modular reactors, though these remain in development and their costs are uncertain. Nuclear power sits next to wind turbines and solar panels in discussions of low-carbon electricity, and the debate is as much about economics and public trust as about engineering.